243 | Choosing The Right Injectors For Your Engine
The fuel injectors are one of the most important aspects of your fuel system and getting the right injector will provide a good idle quality and great drivability, while keeping your engine’s fuel demands met at high load and high rpm. These days there are a lot of options when it comes to injectors and we’ll discuss what you should be looking at. In particular we’ll cover what you need to know if you’re upgrading the injectors with a factory PCM.
- Hey guys, Andre here from High Performance Academy, welcome along to this week's webinar where we're going to be discussing some of the aspects that you should be keeping in mind when it comes to selecting injectors for your next project. Now I think most people consider that this is pretty straightforward and to a degree it is however there are a few intricacies that you do need to understand if you want to be able to choose an injector that's going to, first of all work for your application and support the sort of fuel flow that you're going to be needing. At the other end of the challenge here is also getting the right data for your particular ECU and this can make the process of tuning so much easier. It's particularly getting more and more critical on factory ECUs when we are reflashing them. It's no longer a case of being able to fit any old injectors we come across and then trying to get the engine to run.
There's a lot of dedicated tables in a factory ECU which we're going to touch on during the lesson that you need to understand and you need to have the correct data to drop into those tables if you want to get the best possible results with a minimal amount of effort. So as usual we're going to have questions and answers, if there's anything that I talk about, please feel free to ask questions in the chat and we'll jump into those at the end. Now we'll go through some of this reasonably quickly and a lot of this is pretty self explanatory. The first thing is when we are choosing a set of aftermarket injectors we're going to need to make sure that the injectors can flow sufficient fuel to keep the engine's fuel demands under control. And this comes down to of course, as we're modifying engines, we're trying to make more power, it's all about trying to get more airflow into the engine.
As we get more airflow into the engine, we obviously need to add more fuel in order to keep our air/fuel ratio under control, that's essential if we first of all want to make the power that that additional airflow can provide and of course at the same time, want to keep our engine operating safely, we don't want our engine running dangerously lean, particularly if we're running with increased boost pressure. So that's the first thing, getting an injector that can actually support our airflow or fuel requirements. The other aspect which is just as important and often these two aspects are at odds to each other, is that we want an injector where we can get really good control of the fuel delivery, the fuel mass that the injector is providing, down at the small pulse widths that we're likely to see at idle, and even at cruise. This can be the difference between an injector that provides a good control of the fuel delivery to get a stoichiometric air/fuel ratio at idle and cruise and one that ends up constantly with the engine stalling or fouling spark plugs because we're running pig rich at idle and there's absolutely nothing we can do about it. Now I'm going to jump a little bit out of schedule here, or a little bit out of order, so we'll jump across to my laptop screen and I want to just show you a perfect example of an injector I was unfortunate enough to spend a lot of time in my early days using and pulling my hair out, which was the Bosch Indy Blue or Bosch 1600 cc injector.
That injector, if I can find it, let me just get this back down to a normal size and I should be able to do this. No I can't. That's helpful. There we go, right got there in the end. So this is, they do come in a couple of different ranges but essentially this is what's referred to as the Bosch Indy Blue, so an old EV1 style, we can see they've got the old square or rectangular junior timer connector on it.
And this is an injector that was one of the few options we had to really support relatively high fuel demands back in the day when I started getting involved in drag racing. It's an injector that did a half decent job of flowing a lot of fuel but it came with a lot of downsides. Particularly if we wanted to use these injectors in an application where we also needed good control at idle so particularly for heavily modified street cars. So now that we know what that injector looks like, let's get this back up in a size that we can see it. So this is an overlay here from Injector Dynamics between their ID2000, something that's basically superseded now and the Bosch 1600.
So really don't care here about the ID2000, I'm really more interested in showing you the problematic areas of control with the Bosch Indy Blue. So in a perfect world, what our ECU really wants to consider or expects from our injector I should say, is that the relationship between injector flow and pulse width is a linear one. In other words if the ECU provides double pulse width to the injector, it's expecting that it's going get double the volume of fuel delivered and that's actually not always the case. So as we can see here, that would sort of look something like, if I draw a line through it, something like this. And we can see there's a couple of areas where it really falls off the rails in a big way.
Particularly at high duty cycle out here, we can see that it gets really really non linear, it's got this ugly little bump up here around about eight milliseconds and then from a bit over nine milliseconds it's essentially the same as being wide open. So we don't have a lot of control if we're running them up about 90% injector duty cycle. One of the reasons why, for that particular injector, not a great idea to be up there anyway but if you don't have accurate data of what the injector's actually doing, it can be really quite confusing from a tuner's perspective. Why all of a sudden so we go up another couple of percent in injector duty cycle but all of a sudden our engine's gone pig rich, it just doesn't make a lot of sense, we end up with some really ugly stuff baked into the fuel table to try and correct this. A bigger problem for us is actually down in this low flow area.
So what we can see is that there's, I'll try and sort of highlight here so it makes a little bit more sense. We can see there's a big dip in our injector flow versus pulse width. And for anyone who has tuned on one of these Bosch Indy Blue's, what you typically find is that you end up with them idling with an injector pulse width of around about 1.5 milliseconds. Which as you can see, just happens to be right smack bang in the bottom of that dip. And the interesting aspect is that as you, if you're idling at 1.5 millisecond and you reduce the injector pulse width because you're too rich, so obviously it would make sense, you'll reduce the injector pulse width to try and reduce the fuelling, you'd actually find with these Indy Blues that the air/fuel ratio goes rich.
Which doesn't really make a lot of sense, again until you can see this sort of weirdness that we've got here. Of course if you increase the pulse width from 1.5 milliseconds, we get exactly the same result, we end up going rich. So essentially you end up pulling your hair out, settling for an injector pulse width at idle of 1.5 milliseconds and basically the air/fuel ratio is whatever it's got, we really don't have a lot of control over that. So this is an injector that really is what's referred to as poor linear response. It's not very linear in its relationship between injector pulse width and injector flow.
And this creates some problems for us. Fortunately that injector now is very very old technology and even though I said I wasn't going to talk too much about the Injector Dynamics ID2000, straight away you can see that we've got an injector that is much more linear. At least from around about 1.5 milliseconds up to around about 9.5 milliseconds. We've got a reasonably linear approximation there as I run that line through it. What we do find is that with just about every injector there is this non linear region down at low pulse width which we're going to explain a little bit more about because this is where we've seen advances in the way OEs characterise the injectors to get better control of the injector and we've also seen the aftermarket suppliers of injectors such as Injector Dynamics come to the party and give us more tools in our tool belt so that we can do a better job when we are programming these ECUs as well.
And the better we can do, the better the results are going to be. With that ID2000 we also get this non linear area right up the top which again we're not generally as worried about. So there's a quick introduction there, sort of jumped right in the deep end but don't worry, we're going to back up and come back to all of these aspects as well. So we've talked about it, getting an injector that's big enough to support the required flow but also an injector that can still control the fuel delivery accurately, give us the small pulse widths, the small, sorry volumes of fuel that we need to get good control of our air/fuel ratio at idle. The other aspect we need to consider is that the injectors need to be compatible both electrically as well as physically with our engine, in other words, we need them to actually be able to physically fit into our engine and be compatible with the electrical system with our ECU control.
And that goes a little bit further than just being able to potentially swap the injector plugs as we'll find out. So now that we've covered the basics there, fuel flow as well as compatibility with our engine, we're going to dive back and we'll talk about each of those in a little bit more detail. And we're going to start with probably the most obvious and most critical which is our fuel demands. So one of the questions that obviously comes up here is how do we know what injector size we actually need for our engine? And I've seen this numerous times where people, back with my old professional workshop where we were tuning basically every day for a living, people would build a project car up over time and they'd buy parts along the way as they could afford them, pretty normal progression with these projects and then they'd finally get to the day where it was time to put the car on the dyno and see the result of all of that money that they've spent as well as their blood, sweat and tears they've poured into that project car and it's really frustrating to get to a situation where the car goes on the dyno and you find that the injectors that they've already spent good money on, actually aren't big enough to support their demands. And this is just simply a case where they haven't had the right information to be able to choose the correct product at the time or they've relied on bad information and it's really something that should be incredibly easy to avoid and we'll show you basically how to do that.
There are various injector calculators online these days, that goes some way towards helping but some of the injector calculators I find are simplistic at best and won't necessarily work for some of the more out there combinations that people are building. Particularly a lot of the online injector calculators completely ignore aspects such as the engine RPM range that we're going to be operating at. And that can be a really big factor as well as the fuel pressure and these days also the types of fuel, given that if we are running an ethanol blend, E85 for all intents and purposes, to make the same power, as we were making on pump gasoline, we need around about 35 to 40% more fuel by volume in order to support the airflow so that's a really easy thing to overlook. A lot of the injector calculators that we see online use BSFC or brake specific fuel consumption in order to calculate the required fuel flow. BSFC, it's not actually a term that most people use, it's not really that critical but essentially it talks about how efficiently the engine is able to convert fuel into power.
There are a couple of problems with using BSFC. First of all, for us mere mortals it's pretty much impossible to get accurate data on engine brake specific fuel consumption. There might be some data available from some engines that we may be able to use but whether or not that's accurate once we start modifying the engine, it's very difficult to really decide on. And it requires some specialist equipment and generally an engine dyno cell to get accurate BSFC data. So we're probably not going to have that.
The other aspect though is that the brake specific fuel consumption is heavily influenced by aspects such as the specific fuel we're running on and I'm not talking here about the likes of pump gas versus alcohol based fuels, I'm talking about the octane rating of a gasoline based fuel in itself. And what I mean here is if we're running a high boost turbocharged engine, on a average octane pump fuel you're almost certainly going to be heavily knock limited. That means that you've got to pull a lot of timing out in order to prevent detonation and protect the engine. Now that's obviously going to have a big impact on the final power that we end up reading. on our dyno. Now if we make no other changes but switching to a higher octane race fuel, still gasoline based so we've still go the same stoichiometric air/fuel ratio for the fuel, we'll find that now we are not knock limited, we can add more timing in, we're going to make a lot more power so just for one example, we've only really changed the octane of the fuel but the BSFC has been significantly affected by that.
So just to give you a really rough guide, one of the ways I always work this out, just as a quick rule of thumb, is to work on the basis that we're going to need, for pump gasoline, approximately one litre of fuel flow per minute for every 200 horsepower we want. Now this is very rough, I'm going to explain this in a bit more detail but for a quick example, if we took a four cylinder engine or basically it doesn't matter how many cylinders the engine has, if we've got an engine that has four injectors in it, and we wanted to make 400 horsepower, using that rule of thumb 400 divided by our 200 horsepower per litre means that we need to have approximately two litres of fuel flow per minute total. So if we've got four injectors, we divide that two litres by four, that gives us 500 cc per minute for each of our injectors. So that's about where we're going to need to be. That would have the injectors running at 100% injector duty cycle.
So the sensible option there would be to look at our options in terms of injector size above 500 cc and that will be pretty close to the ballpark. Now as I said, it is a rough guide at best and in particular, that example I gave you, the 200 horsepower per litre, that is only really something that's going to work for pump fuel so as soon as we go to an alcohol based fuel, that's out the window, we need to add a multiplication factor in there. Of course if we are on E85 we can use what I just mentioned, around about a 35% to 40% increase in that fuel flow for the same power. If we're going to go to methanol, generally I'll work on a multiplication factor of about 2.5 times more fuel compared to pump gasoline. The other aspect though is that this doesn't take into account our fuel pressure, so as we raise the fuel pressure, we are essentially creating a higher differential pressure across the injector and within reason, that does increase the flow that the injector can have for a given pulse width, doesn't work infinitely though, there is a range that that will work with and it also impacts other aspects of the injector's operation such as the injector offset of dead time.
I haven't also taken into account there the engine RPM as well as our air/fuel ratio or lambda target. And that's important because of course the amount of fuel that we need to target, maybe 0.82 lambda, is quite dramatically different compared to if we want to target 0.75 lambda so that does need to be factored in. So at the moment you may be thinking well that's all well and good so what so we do? Well the rough guide I've given you still is exactly that, a rough guide that's useful for getting you in the ballpark. My own personal preference though, I really do like the fuel flow calculator that Injector Dynamics have provided and I'll just mention at this point, because I've now used the term Injector Dynamics probably four or five times, I've got a set of Injector Dymanics injectors sitting here, no we are not sponsored by Injector Dynamics, this is not a paid advert for Injector Dynamics, I just happen to believe personally that Injector Dynamics have kind of set the bar really high in the aftermarket injector world. Certainly, they are now not the only option, providing a good quality reliable product but they were one of the first that really wanted to add data into the aftermarket injector selection process and give the tuner a lot more information around what the injector was doing.
And that's in turn helped lift the performance of the entire injector aftermarket. So personally I like to support Injector Dynamics and Paul Yaw and Tony Palo for the work that they've put in. One of the reasons I'm talking about them now though is because they have developed this injector flow or injector selector which we'll jump over to my laptop screen and we can see that. You will find that at the Injector Dynamics website and you can click on fuel flow calculator. And if you scroll down, I've already got some data in here so you can basically fill in this information out here on the left hand side, doesn't need a lot of data, it's not that difficult, engine displacement, number of cylinders, number of valves, our target boost pressure, what our lambda target will be at wide open throttle and idle, ethanol percentage if you are running an ethanol based fuel, obviously that makes a big difference.
Also, the fuel pressure that we are going to be running. So we can take that into account, if you're sort of getting a little bit marginal, you can see what bumping the fuel pressure up maybe 10 or 15 psi is going to achieve. You can also select it here, your type of fuel system and this is something... ...different fuel system styles which will have a big impact on your injector sizing. Older cars always tend to run what I refer to as a inlet manifold pressure referenced fuel pressure.
So what this means is there is a fuel pressure regulator, reference to manifold vacuum or boost and it raises or lowers the fuel pressure in relation to the manifold pressure. The idea here with that style system is it keeps a constant differential pressure across the injector, meaning that for a given pulse width, the injector always delivers the same volume of fuel into the engine. With later model cars now it's much more common to find what is referred to as a returnless style fuel system or constant pressure which is what Injector Dynamics have referred to it as. So this runs a fixed fuel pressure, generally but not always it's four bar or 58 psi. And the problem with this is that as the manifold pressure increases, particularly if you add boost pressure, you're going to find that the differential pressure across the injector starts to decrease so the more we raise the boost, the less the differential pressure across the injector is.
And it's the differential pressure across the injector that ultimately defines how much fuel will flow so if we think about it in a worst case scenario, here's out injector here, we've got fuel, let's get it around the right way, we've got fuel going into the top of the injector, we've got our manifold pressure on the other side. So if we get to a point where let's say we had 45 psi of fuel pressure at the top and let's say we've cranked the boost and we've got 45 psi of manifold pressure on the other side. When the injector opens, the two pressures are equal so actually no fuel will flow. Now obviously that's an extreme situation but it does highlight the fact that particularly when we add a turbo or supercharger kit to a late model constant fuel pressure system and we start raising the boost pressure, we're starting to erode some of that differential pressure across the injector and we do need to keep that in mind. Anyway, on this graph here we've now got out on the right hand side, everything that we need to know in order to size the injector.
For a start down here the little white crosses show the fuel flow requirements total and then we've got the duty cycle that we're going to see for each of the different Injector Dynamics injectors at various RPM so if we look at 7000 RPM here, we can see that if we chose the ID725 then we'd only be at 85% injector duty cycle which is absolutely great, nothing wrong with that, that gives us a little bit of headroom, probably pretty well sized for our application. If on the other hand the particular engine that we were about to tune was actually going to run out to 10,000 RPM, we can see all of a sudden, in order to get sufficient headroom we'd actually have to jump up from the 725 up to the ID1000. So that would be at 89% injector duty cycle so this is just to show you, to illustrate how the RPM range that the engine is going to be operating at really does impact on our injector sizing. It's something that a lot of these injector calculators simply overlook. And if you can't get your head around why that is, we've got obviously a relatively narrow range of time in which we can inject the fuel.
As the RPM increases, the engine cycles happen faster, there's less time available for the fuel to be delivered into the engine so that's simply that in effect. Now I will mention you can use this data to get a feel for what size injector you're going to need and of course 1000 cc injector we can purchase from a variety of different manufacturers however just to be fair to Injector Dynamics, if you do want to use their injector flow calculator, they've put the hard work in and provided us with an incredible tool so it'd only be fair that if you're going to use that calulator, try and support the people who have put the hard work into developing it. OK so that's how we're going to choose a suitable injector for our application. However as I've mentioned already, the minimum flow is just as important, trying to get good control over the short pulse width performance of our injectors so that we can get the low volumes of fuel that we're going to need when the engine is idling. So we've already looked at that graph of the Injector Dynamics, sorry the Indy Blue, yeah there we go.
So I just want to jump across to another graph that I've politely stolen from Paul Yaw and Injector Dynamics again which shows you the sort of performance of a variety of their different injectors. So on the vertical axis here we've got volumetric flow rate. So basically the fuel volume that's going to be delivered by these injectors and then we've got the actual pulse width that's being delivered out to the injectors. And what we can see here is for an example, if we chose the Injector Dynamics ID2000 which is in blue, this one up here and we're obviously only interested down in the lower range, we get down to about one millisecond and the flow kind of falls off a cliff. So hopefully you can understand there, if we're going to try and drive that injector below one millisecond, you're going to have very poor control over the fuel volume.
The slope of that graph, that line is very very steep and we're really only going between maybe about 0.8 milliseconds and maybe 1.1 milliseconds so you've got very very small adjustments in the injector pulse width are going to end up having a big variation, we're sort of between about 15 micro litres per injection, and obviously zero across that range so it's going to be difficult with a small capacity engine to get that injector to give really really good results. However if we look at something like the ID, let's say the ID725, so I'll just try and draw in here, we, just draw a line through that. We get down to about 1.3 milliseconds before we start falling off that cliff but at that point we're already down at around about two to three micro litres per injection pulse. So this just shows you the importance of the flow characteristics at both ends of the scale there, we can't really consider one without the other. Now in some instances, you could be in a situation where you've got no option because you simply need the fuel flow and if you're maybe running a drag engine that's really only going to idle in the staging lanes and at the start line and everything else is going to be at wide open throttle, perhaps under those conditions you can ignore a slightly rich idle situation, it doesn't really matter that much the idle quality.
However for a street car, something that's going to be daily driven, that's a much bigger consideration, you're going to want really good control over your air/fuel ratio at idle, particularly if emissions compliance is important. So the option there, if you're going to need a huge amount of fuel flow but the idle quality is still important to you is to consider staged injection. And again we'll jump across to my laptop screen, this is a shot of the Plazmaman inlet manifold for our SR20 VE engine and we can see here that this design uses twin injectors per cylinder. So the idea here, they don't need to be this same size injector, we can use one larger injector for fuel flow capabilities and a smaller injector down in the lower range but the idea here is that we use one injector at idle and cruise and then as the RPM and load increases and the fuel demands on the engine increase then we can start staging in the secondary injector. Under those conditions we aren't running both of the injectors simultaneously and obviously that does require an ECU that offers staged injection control and even with staged injection control we tend to find that not all ECUs are created equal there.
Some of them the staging is quite coarse and you will actually struggle to get really good control of the air/fuel ratio as the second injector stages in. More modern ECUs, we tend to find they've got the functionality in there and the sophistication to make this really really easy and basically you don't even notice when that secondary injector starts to come in. Alright so we've covered the fuel flow requirements of the injector, obviously the most critical aspect but we do need to consider the physical fitment to the engine. And what we find is that a lot of the aftermarket injectors that are now being produced or delivered into the industry are based on the Bosch EV14 style injector. So we'll just get that actual injector under our overhead.
So we've got the full injector here, this is what's going to be going into our SR20, so these are the actual injectors you saw in that photo and then beside it this is the base injector that Injector Dynamics are modifying. So Injector Dynamics, it is important to mention here, before I get roasted, Injector Dynamics do not make this injector, this is a Bosch product. So yes you will find a lot of other injector manufacturers out there using the same base injector. Injector Dynamics are a little bit unique in that they've got a close working relationship with Bosch Motorsport. So some of their injectors now do have bespoke components inside that are unique to Injector Dynamics so that is an intricacy there but essentially the outside housing of the injector, we're basically talking about the same thing.
So what all of these aftermarket injector suppliers are now doing is machining up a component to allow these injectors to be adapted to a wide range of different engines so again if we jump to our overhead, this is the cap that is designed to suit. These are available in different o ring diameters as well as different lengths. So by installing that over the top of the injector, basically gets us to where we want to be. There is also the potential to fit filter baskets into the top of these adapters. Once they are all assembled there's a little horseshoe clip that goes around it, makes sure that nothing's going to fall out although it should also be located pretty accurately by your fuel rail and your engine.
Bottom o ring is another consideration there, need to obviously make sure that that's going to fit properly into the inlet port on the cylinder head or the inlet manifold or wherever that's going to be located. So that's the first thing and most of these manufacturers again there'll be a variety of different adapters available to suit most of the common engines so you're going to probably find, unless you're doing something pretty unique and pretty weird, that there will be support for you. The other aspect there is the electrical design. So the connector itself is nothing particularly tricky, they run a simple two pin USCAR connector, these will be supplied by the injector supplier so it's as simple as cutting off your existing connector and reterminating it into that USCAR plug. Job done, well maybe not quite so fast.
While it's less of an issue on late model cars, the other consideration we need to keep in mind is that there are two styles of injector, there are saturated drive injectors and there are peak and hold injectors. Peak and hold injectors, not something we see so much these days but they were common back a decade or so ago. Peak and hold injectors work on a different operating principle, they use a low impedance coil inside of the injector, generally in the region of about one to three ohms. You can test the impedance simply using a multimeter across the two terminals of the injector. High impedance saturated drive injectors on the other hand, we'll generally find will be between about nine and maybe 12 ohms impedance.
The important thing here is that a lot of the OEMs that were using these peak and hold injectors, the ECUs they were using didn't actually have peak and hold injector drives. I don't want to get too down in the weeds with this but the peak and hold injector driver basically provides a very high or relatively high peak current to quickly open the injector, somewhere around about 4 amps to quickly open the injector and get the valve open and off its seat so the fuel can start flowing. But once the injector was actually open and the fuel was flowing, you didn't need anything like the amount of current needed to open it so it would drop down to a holding current of maybe around about 1 amp. Hence the name peak and hold. So this required special injector drivers in the ECU to be able to provide that variable current, the peak current followed by the holding current.
A lot of the OE ECUs despite using peak and hold injectors, they use saturated drives which rely on the impedance in the injector to control the current. So if we used a saturated drive injector driver with a low impedance peak and hold injector, the current draw would be too great and you'd end up potentially burning and damaging the injector drives. So to combat this, and make things nice and easy, a lot of the OEs include what's called an injector ballast resistor pack. And that ballast resistor is essentially there just to bump up the impedance in the circuit to make sure that the current flow is under control. Problem being that if you are dealing with one of these cars, just two that jump to mind would be the Mitsubishi Evo all the way through from the one through to the nine.
Also the Nissan Skyline R32, 33, 34 GTRs just to name a couple but these were really common. All use ballast resistor packs. So if you swap to an aftermarket injector based on the high impedance EV14, you're going to have problems getting control of those injectors and stable fuel delivery if you don't also remove the ballast pack. So let's jump across to my laptop screen and we'll see what that all looks like. So I think this particular photo is actually an Evo ballast resistor pack, yeah it's got the little Mitsubishi symbol on it.
So this will be bolted somewhere on the firewall and basically there is a single 12V feed coming into this and then out of the ballast resistor pack we've got four wires which is the power feeds to the individual injectors. There's a variety of ways of dealing with this, the nastiest and quickest and probably the easiest is simply to cut, which has been done here, strip all of the wires on this particular piece here, join them all together, crimp them all together and insulate them and basically you've just joined them all together by bypassing the ballast resistor pack. There are also some slightly nicer options where for popular cars there will be basically a mating connector, which is just this part here, which is all terminated internally so it just does a nice neat job of getting rid of that ballast resistor pack. But definitely if you are dealing with an older car that used peak and hold injectors just consider that ballast resistor pack, I know it's something that is really easy to overlook. Alright so now we're going to get a little bit more down into the technical aspect of what your ECU needs to know in order to be able to do a good job of controlling the injectors.
Now back when I first started getting involved with EFI tuning, the ECUs that we had access to were relatively straightforward and what they did was often just delivered a pulse width to the injectors that we programmed into the fuel table. And that ignores a big aspect of the injector operation which is the injector dead time. So this essentially takes into account, I don't want to get too technical in here but a simple way of explaining the dead time is it's the difference between the pulse width that's delivered to the injector and the amount of time the injector's actually open and flowing fuel. And this comes down to the mechanical latency of the system. It takes time for the injector to physically open against that fuel pressure and start providing fuel flow.
Likewise when we switch the injector pulse width off, the injector doesn't instantly close so there is quite a significant difference between the amount of time that the pulse width is delivered to the injector and the amount of time fuel is actually flowing. So as ECUs became a little bit more advanced, we started accounting for the injector dead time into that. The injector dead time varies depending on the fuel pressure, the more the fuel pressure, the harder it is to open the injector, the dead time increases. It also varies depending on the battery voltage. As our battery voltage drops, there's less electrical energy available to open the injectors so again our injector dead time increases.
So while it was possible to do a half reasonable job of tuning with an ECU that completely ignored injector dead time, it's definitely nice if we can account for it, particularly a lot of the background compensation that the ECU uses won't work properly if we aren't correctly accounting for dead time. So that was the first thing, getting some solid data on injector dead time. As injectors again and ECUs became more sophisticated, we started seeing the manufacturers wanting to more thoroughly characterise the way the injectors operated and particularly in that non linear area which we looked at where we've got that knee in the flow characteristics on the graph, they wanted to more accurately represent how that affected and then we need more data in order to do that so let's have a quick look at how this is dealt with in a few different ECUs. We're going to start with one of the ECUs that I kind of got involved with fairly early on in my career which is the old MoTeC 100 series ECU. I say old, and they are, I think they're around 20 years old now.
They're actually still a very powerful ECU and still do a great job. But they worked on a relatively simple strategy, if we can go across to our fuel table here, so the fuel table that we see represented off to the right, and the numbers here, this is what I refer to as an injector time based ECU. So we're essentially just directly telling the ECU how long to open the injectors for. Now the numbers here though, they're a percentage, they have to be a percentage of something and they are a percentage of a master injector pulse width if you like. So we'll just see where that comes from, we'll head down to our general setup, down to our fuel and to our setup here.
And it is this parameter here, our injector scaling, in MoTeC lingo, IJPU. So this essentially defines the pulse width relating to a number of 100% in our fuel table at 100 kPa before any background compensations are accounted for so really really simple there. The MoTeC also included what they call battery comp which is dead time or offset and we'll have a quick look at that. So we've got a simple two dimensional table here, battery voltage on the horizontal axis and we've got our injector dead time which is again how long it takes for the injector to actually respond so we can see here at 14 volts here our injector dead time is 940 microseconds. As the battery voltage drops as we move to the left, if we look down here, at seven volts it's out to 2500 microseconds so that's just reinforcing what I was saying.
So this was quite common with a lot of our earlier ECUs. These days a lot of ECUs now have decided to, even in the aftermarket, have decided to more thoroughly characterise the injector flow. So for example if we head across to my laptop, the Link G4+ system. And this is, if you want to set one of these up on their modelled or voumetric efficiency based fuel strategy, we've got our injector set up here and there's a little bit of information required, we can set our dead time or offset table up, it's two dimensional or three dimensional. We've got our injector flow at rated pressure, so this is the important aspect here, what's our actual injector flow so we can see there 1135 cc per minute at a rated flow or rated pressure I should say of 380 kPa.
And then we've got our injector dead time, I'll just double click on that, so we've got a 3D table here, differential pressure on the vertical axis and on the horizontal axis we've got our battery voltage. The reason that we've got differential pressure included in here is that this was on our Nissan 350z which runs a constant fuel pressure so as we vary our throttle position, the manifold pressure varies, our differential pressure varies so where abouts we're operating in this table also varies. So important if you are running a fixed fuel pressure to account for that inside of your ECU. We'll come back and the other aspect is this little parameter here which is our injector short pulse width adder table. So this defines essentially how much the fuel flow is varying from a assumed linear relationship between injector pulse width and flow.
And again in that short pulse width area, it tends to drop off, there's a little bit of a knee there and this table helps define that, we'll just quickly have a look over at that and we can see that the axis of this table is injector pulse width, or effective pulse width and this table just defines how much will be added to the final injector pulse width to get a linear fuel delivery. So this sort of technique doesn't require a lot more effort but we do need that information and where we get that information is really important. We'll go one step further here and we'll have a look at one of the more recent ECUs which is the MoTeC M1 ECU and they've really gone to town on this, there's quite a lot of involved information you will need. In particular down here we've got our injector data, we've got first of all our linearisation, I'll come back to that in a second. We've got our minimum volume so this is the minimum volume of fuel that the injector can reliably and consistently deliver, in this case five microlitres, we've got our reference flow so essentially this is the reference flow value of the injector at a reference pressure which in this case is the next parameter down at four bar.
So 20.98 ml per second at 400 kPa so that stuff is pretty easy to get. The linearisation data however is a little bit more involved. We can see that here, we'll just bring that up to full screen and we'll get rid of our graph for a second. So this is actually a four dimensional table and on the fourth axis here we've got our battery voltage. So we've actually got one of these tables here at each of these battery voltage break points.
The table itself we've got our differential pressure across the injector and the fuel volume here inside of the table itself, our injector pulse width. So you might be thinking well how on earth am I going to get this data and that's a really good question. MoTeC have made this pretty easy because they know we don't have flow benches, we can't get data this accurately at home. So for the majority of popular injectors, this can actually be populated for your injector from a dropdown menu so that's pretty easy. In some instances as well if you're dealing with an OE engine where the injectors are unusual, they're not characterised, you can potentially send your injectors to MoTeC via a MoTeC dealer to have them characterised, there may be a cost for that, I can't exactly tell you.
So lastly let's have a look at how an OE deals with this. So we'll go across to the VCM editor software which we've got here our GM LS table or map is opened up and we're looking here at the moment under our fuel tab and we are under our general tab. And there's a lot of information here but essentially all these parameters here cover our injector control. So you can see here, just purely by the number of tables and parameters as I scroll down, there's a lot to get right here and hopefully you can understand that your ability to guess at these numbers and do a half good job of the characterisation's just not likely. In particular here, one of the key ones we're going to need to have is our flow rate versus pressure so we'll open that up.
We can see we've got a two dimensional table defining the injector flow in grams per second of fuel versus our pressure delta, so the differential pressure across the injector. We've also got aspects down here for our offset as well, injector dead time, whoops not that one, let me find the correct one, no. That's injector tip temperature. Try and find it, there we go. So we need to have all of this data so again we're not really going to have much chance of being able to come up with this off our own back.
So again this is where the aftermarket has come in and again, I am going to be using Injector Dynamics for an example here but a lot of the aftermarket injector suppliers now are giving you the same data. It's important, particularly if you are dealing with a later model OE ECU to make sure though before you purchase your injectors that you are getting this data. If you're left high and dry again it's going to really affect your ability to do a good job of the calibration and just to sort of understand how this all works, if you've got the correct data, you can copy and paste it straight into these tables, swap the injector, flash that into the ECU and basically you can get your engine up and running, the air/fuel ratio will be exactly as it was prior to switching the injectors, it's that good, works that well. So let's have a look at where that data comes from. So again if we head across to Injector Dynamics with one of their individual injectors, the information is supplied in a variety of different ways so first of all for a basic ECU, we'll be able to get away with just our slope and offset versus pressure.
And this just defines for a specific differential pressure over here on the left hand side, what the flow is and the offset so let's say we were running at 55 psi differential pressure. So the first pieces of data here that I'll just highlight for you, this is our injector dead time or offset value at 8, 10, 12, 14 and 16 volts. So you'll be able to just copy and paste these essentially into your dead time table and you're done. The other piece of information here is that at 55 psi differential pressure, 100% injector duty cycle, we should have an injector flow of 1205 cc per minute so that's the data that you'll need for most of the older ECUs. However with these factory ECUs, that's where this plug and play data really comes into play and you can find that data by clicking.
Let's just go to some GM characterisation data that I've already looked at. So this is available for GM, Ford, and a range of different other options COBB for example in there and the idea is that it's a plug and play chop and paste data that you can copy into the relevant areas of your ECU map and the job is done. So for example here if we want to look at the ID725 for GM in HPTuners format, you can download that file, load that up in Excel and this will first of all give you notes about the injector that you're dealing with, then if we come across here we've got our short pulse width adder, we'll click on that, gives you aspects such as the minimum pulse width, default pulse width and the short pulse limit and then this data is just, in a table format you can choose one that suits your particular model of ECU in the GM world, copy and paste that straight into those tables in the VCM Editor, job done. If we click over to the next one we've got the flow rate as well, sorry the offset table which we looked at as well. So really really easy and it makes our life so simple and again, you can just take five seconds, copy that data across, flash it into the ECU and you'll be up and running.
So again if you are going to be looking at an aftermarket injector for a late model factory ECU, just make sure that you're getting this data. Suddenly a really great deal that you're getting on eBay on a set of high flow injectors might not look so flash when you're wasting hours chasing your tail trying to get the engine to idle with an acceptable air/fuel ratio. And potentially if these maps are out, if these tables are out, you're trying to put a bandaid on other areas of the calibration you're just going to end up making your life so much more difficult so it really is becoming more and more critical to have that correct data right from the get go. Alright we have gone a little bit long here so it is a topic that requires a little bit of understanding around how it works and I needed to explain it thoroughly so that you've got the right information but for now we will jump into our questions and if you do have any more, keep 'em coming. First question comes Bjorn who's asked, currently struggling with not being able to access stock injector data for my 2JZ GE stock 312 cc injectors.
Tuning on an EMU Black, startup noise is a pain, injector calibration may not even be accurate as it's what came from the base map. Alright so, while I've just sat here and I've told you how important getting the right characterisation data is for our OE ECUs, and all of that is absolutely correct, we do also need to be a little bit practical here. What's happening these days is that it is becoming more and more common for aftermarket standalone ECUs to use the volumetric efficiency based tuning model and as we looked at in the Link G4+ example there it does require a little bit more information for us to be able to set the ECU up properly. Ideally we want our injector flow and in the perfect world we also want our injector dead time. Getting that for a stock injector sometimes is an exercise in frustration, often it's not going to be possible.
So that doesn't necessarily mean that we simply can't tune the engine. First of all, getting the injector flow at a reference pressure, that part shouldn't be too difficult, as long as you're dealing with an engine that's been around and is popular, the 2JZ definitely ticks the box there. That shouldn't be too difficult. Other than that, doing a static flow test is not impossible, a lot of injector cleaning flow benches will be able to provide that so that'll be your injector flow, in this case 312 cc per minute. I can't tell you off the top of my head if that sounds right or wrong, it's probably in the ballpark though.
The injector offset or dead time data is usually the question I get asked about, what can we do, how can we do this? And the reality is you're probably not going to be able to find someone who will do a characterisation of that injector for you and get accurate data. That doesn't necessarily mean that we're screwed and there's nothing we can do. Generally in that situation what I would do is I would simply start with the default map for our injector dead time that's in the ECU, it's not going to be right, I guarantee you that but let's say we might end up finding our injector dead time table will be maybe 0.85 milliseconds and 14 volts, that's probably there or thereabouts for a stock injector, it's not going to be right but again it's not absolutely critical because it's important to again reiterate I've tuned many ECUs where they completely ignored injector dead time and I could still tune the ECU. Yes, some more fluctuation in the air/fuel ratio than I'd like but you can still do it. So while we've got that 0.85 milliseconds, let's just say as a round number, we know that that's at 14 volts, we can tune the engine to get a good idle air/fuel ratio at that particular point at 14 volts and then we can do some variations to our voltage and see how the air/fuel ratio fluctuates.
Let's disable the alternator, pull the alternator fuse, stop that charging and our voltage will probably drop down to 12 volts, that'll give you another point in that dead time table. And what we want to do then is adjust the dead time table until we're keeping our air/fuel ratio constant. Now again this is not giving you the correct dead time, it's just giving you the correct different between the different voltage points. But while it might not be perfect, that's still going to give you a perfectly tunable ECU. And the other thing to bear in mind is the injector dead time has a much bigger impact on our tuning at idle than it does at wide open throttle, that's because the dead time value is a much larger percentage of the injector pulse width total at idle compared to wide open throttle.
So long story short there, your start up knock noise is almost certainly not going to be a factor if your injector dead time, something else is going on there. I would be looking to start with for your triggering, I would have a guess with what you're explaining without a lot more information, that you're probably finding that the ECU is not triggering correctly on initial crank and it's firing spark at the incorrect point. Another question from Bjorn who's also asked, curious, on injector duty cycle when choosing injectors accounting for duty cycle, example the ID1050x using a Walbro 450 fuel pump, let's say low boost 14 psi, that leaves a lot of head room as the injector's not operating at its higher end of the duty cycle. Is it better this way to account for bigger injectors than your needs for the event of when you need to upgrade? OK so basically long story short there, how big can we go, should we build head room into the injectors? So this is really where the modern crop of injectors have made our lives easier. We're getting a lot more control of the small pulse widths delivering small volumes of fuel that we need at idle which we've discussed, with injectors that can still flow massive amounts of fuel.
So usually there's not necessarily anything wrong with choosing an injector to suit your ultimate aims, maybe you're going to be upgrading the turbo and you want some injector head room for later on. And you're probably still within reason going to be able to get good idle control. It's only really in the very high end drag engines where you're looking at a small displacement motor making maybe 900 to 1000 maybe 1200, 1400 horsepower on ethanol based where you need to start looking at staged injection, you're probably not going to get a single injector to do a great job there. Jess has asked, I've got a naturally aspirated four cylinder GSR engine making 200 horsepower using 100 shot of nitrous, do I size the injector for the naturally aspirated 200 horsepower or the 300 horsepower in nitrous? It's a direct port wet shot system. OK so I was going to say depends on what style of nitrous you are using.
With a wet system, the nitrous is supplying its own fuel so you don't need to actually add the fuel through the fuel injectors, you only need to size the injectors to suit the power the engine is making naturally aspirated before your nitrous is injected. However what you do need to consider is that the fuel pump needs to be able to support the entire fuel demands of the engine so 300 horsepower there give or take. Newbie has asked, why do people use 200 cc injectors on a boosted four cylinder, wondering they never even go 80% injector duty cycle, isn't that detrimental or dumb in that manner? OK so yeah you do want to size your injectors sensibly, there's no point putting a set of injectors on an engine that are never going to go past like 35% injector duty cycle at 8000 RPM wide open throttle. That's just making you use a very narrow range of the injector's flow range. However there are actually some factors that can be overlooked as well.
Specifically around the ability to choose your injection timing. So this really relates the injection event to when the intake valves are opening. And sometimes there can be subtle but worthwhile improvements in the engine performance by accurately timing them. Now within an injector that is sized so that at wide open throttle and high RPM it's running at 80 or 90% injector duty cycle, basically what that means is that the injector's almost open for the entire combustion, for the entire engine cycle anyway so hopefully you can understand that with such as small window to move the injection timing around, there's not a lot of potential for improvement. We saw a lot of particularly naturally aspirated highly strung race engines where they were trying to get every last horsepower out of the engine, we'd purposefully use an injector that resulted in a relatively low peak injector duty cycle.
Giving them a lot more room to move that injection timing around and chasing power so as long as you can get good control of your idle and cruise mixtures, there's not really too many downsides these days on going too large in the injector flow. Barry has asked, I noticed a trend especially in the drag racing scene where guys are running their injectors at sometimes 100 plus psi base pressure. What are your views on this? I'd assume that this would put excessive load on the injector driver circuitry? So yeah it's pretty common a lot of the time with mechanical fuel pumps, the norm was to run 80 psi base pressure plus you've got boost on top of that. So it doesn't really affect the electrical system driving the injector so much, that's not really a huge consideration, the injector, the current flow from that system is really defined by the impedance in the coil and the voltage being supplied to it, it's that simple. What it will do as we increase the base fuel pressure is that it does increase the injector dead time or offset, it's harder to open the injector against that increased fuel pressure so that is a consideration but drag racing is a fairly narrow focus sport, quite often you'll find that the cars running these sorts of systems run a 16 volt total loss battery system as well so they're running with a higher voltage to drive the injector open and really they're obviously not concerned about idle quality or cruise or fuel economy and emissions, it's really all about getting fuel in there at high RPM and high load.
By running that higher base fuel pressure, it can increase the flow capability of a given injector. There are also some arguments around increasing the base fuel pressure, improving the atomisation of the injector, I personally haven't done any back to back testing where I could really confirm one way or another around that. Barry's also asked, have you played around with injectors with different spray angles? I've experienced a case where I ran an injector with a wide angle and I always have wide open throttle, even though the air/fuel ratios were good, switched to a narrow spray pattern injection and the problem went away. Actually Barry this was something I had intended to discuss so thanks for bringing that up. 'Cause you are right, what you'll find is that a lot of OE manufacturers, because they've got the potential to do so and get really deep into their injector design, they will develop an injector that has a focused spray, often it's a twin spray pattern designed to actually target the back of the intake valve.
And of course sometimes when we go to an aftermarket injector we don't get the benefit of being able to choose that. So it's all about how the injector will spray the fuel and where it's sitting in the port, whether that fuel's going to end up on the back of the valve or wetting out the port walls so I know 100% what you're talking about. Yes, I have seen that exactly myself. Unfortunately again we just don't often have a huge selection of injector spray angles that we can experiment with so sometimes we are a little bit limited by what is actually available in the aftermarket there. James has asked, supercharged engines consume more fuel than a turbocharged engine for the same boost or same output, is there a good industry standard coefficient to use when selecting an injector for a supercharger? I.e.
if a turbo four cylinder needs 500 cc to make 400 horsepower, supercharged needs 600? Honestly James while my experience is predominantly with turbocharged engines in high output applications, as opposed to superchargers, I have still dealt with my fair share of supercharged engines and I don't honestly, I haven't actually seen that requirement. There will be different fuel requirements from one engine type to another and that really comes down to the brake specific fuel consumption that I spoke about earlier. But in rough terms, superchargers, particularly if you're talking positive displacement they're not as efficient as a turbocharger, the compressor efficiency is much lower so pound for pound of boost you're getting less power but in terms of the fuel demands, I haven't personally seen a requirement for a dramatic change from one style of forced induction to another. Andre's asked, have you tested different injector placements, conventional back of the valves versus inlet trumpets like they used to do on the naturally aspirated F1 engines? Yeah I have, I can't say I've done back to back testing and this is a problem that we face in the aftermarket. Doing this testing that they did at OE level, and F1 level is incredibly expensive and time consuming and it's very rare that the sort of projects that I get involved with allow the flexibility to make just one change like that.
But I have tuned naturally aspirated race engines with injectors mounted outside of the inlet trumpets and in particular one application, I can't even remember the engine type, I think it was BMW now, used staged injection with a set of primaries down by the intake valves and the secondaries outside of the inlet trumpets and there was a small but worthwhile improvement in engine performance by staging in the secondaries at higher RPM. Also did exactly the same on a methanol powered jet sprint boat engine where we were running twin turbos on a VK56 and the injectors were above the butterflies. Caused some problems around cold start performance and idle control but at wide open throttle it worked really well. Underscore's asked, does the fuel pump matter on what size injectors to use? Example using bigger injectors on a stock fuel pump or stock injectors in a higher rated aftermarket fuel pump? OK so the first thing to understand is that the size of the injector will not matter if the fuel pump can't support the flow required. So the fuel pump must be able to support the fuel flow requirements of the engine and in fact we always want a slight over supply, somewhere around about maybe 25 to 30% additional fuel flow to make sure that we don't end up with our fuel pressure starting to drop away if we're getting very close.
So yeah just putting a larger set of injectors if your fuel pump isn't up to task, not going to fix your problem. Using a higher flowing pump though can give you the benefit of being able to raise the fuel pressure which can get a little bit of additional flow out of a stock injector. So this will work maybe if you needed let's say 10 or 15% more injector flow, might save you the cost of a full set of injectors. However if you're starting to need dramatic changes in the fuel volume that might not work so well for you. Next question comes from ...
who's asked, I'm having difficulty finding an injector that MoTeC have a calibration for that's readily available, fits and is small enough. This has been pushing me towards some ID1050 cc injectors because I know the quality and quantity, pity they stopped the 725 cc injectors, they're way too big, 40% duty cycle based off their calculator on E85 and 30% for E98. Car is still road driven, do you think they are too big? They probably are bigger than is ideal. Now I will give you a story with a customer of mine back through my old shop who had a Honda B18C. We had it on the dyno, ran out of injector on the stock injectors, this was still naturally aspirated and the only option we had to get him up and running which was the injectors that we had in the shop at the time was a set of ID1000s.
Absolute complete overkill for a naturally aspirated 1800 cc engine. However again given that they are a high quality injector with a linear response, we ended up I think running to 37% injector duty cycle peak and you'd never know that they were such a big injector. Gives you again the benefit of being able to play around with the injector timing which I mentioned in a previous question. So specifically I'd say they're bigger than I would normally choose but what I would probably do is go back to your MoTeC dealer and see what the situation is for getting some of your injectors sent into MoTeC to characterise. I can't speak for MoTeC because I haven't talked to them about this for a while but I know that they were building up a bit of a base map of different injectors, popular injectors out there, some of these they were doing for free through their dealer network, sometimes they may charge, I don't know but there are potential options in the M1 series to get your injectors characterised for you.
Chris has asked, what are the advantages or disadvantages of raising the base fuel pressure, also disadvantages and advantages of running two small pumps with one on a hobbs switch? So I think I've really covered this, increasing the base fuel pressure can give you the potential to get a little bit of additional flow out of an injector, it will also increase your injector offset or dead time. Two small pumps with a hobbs pressure switch, probably find that you're going to end up getting an instantaneous bump in the fuel pressure when the secondary pump comes in, you're probably not going to find that it's going to be possible to maintain a constant absolutely rock solid stable fuel pressure so yeah I probably wouldn't switch a fuel pump in like that, there's a lot of fuel pumps these days that can be pulse width modulated to get the effect that you're looking at. Mammal's friends has asked, is a good E85 startup and idle with peak and hold injectors possible? Yes absolutely, the start up and idle control with E85 really comes down to cold start performance, E85 as a fuel can be problematic particularly in low temperatures. It's less volatile than gasoline which makes it harder to light off which is why we see that pump E85 does get diluted in terms of its ethanol content in areas of the world that get very cold winter temperatures. Dennis has asked, what is decapping injectors and how does it affect the performance and efficiency of the injector? So you'll find a lot of modern injectors have a diffuser plate on the injector cap or the tip of the injector.
And decapping is a process where essentially that diffuser plate is ground off and what that does is it can create a dramatic increase in the fuel flow for a relatively modest cost. However it will also affect basically all of the aspects of the injector performance, it will increase the fuel flow but it will also affect the injector dead time, the spray pattern can suffer and it can also affect the linearity of the injector flow so lots of things to consider there but it is a very common option in the aftermarket. I think we're going to have to cut it off here I'm sorry guys, 'cause we are just running a little bit long here so I've got one more question that I will answer there. Which comes from SoulGT's asked, do you take what spark plug you're running into consideration with what injector you want? Absolutely not, the spark plug really is a completely different aspect of consideration, no influence really on our injector sizing. The injector sizing, purely around making sure that you meet the engine's fuel demands.
Alright sorry I can't get through all those questions there, there were some great ones in there though, we're just really out of time there. So thanks everyone for joining us and if you do have any further questions after this webinar has aired, please ask those in the forum and I'll be happy to answer them there. Thanks everyone for joining us and hopefully we'll see you online again next week. Now for those who are watching on YouTube today, this is just some insight into what we put on every week for our HPA gold members. Our gold members get to review these webinars in our archive at their leisure where we've currently got over 240 hours of existing content covering topics on engine building, engine tuning and wiring.
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