208 | What is Dwell Time
When setting up an aftermarket ECU, one of the common questions we get is regarding correctly setting the dwell time. Getting this wrong can result in insufficient spark energy, or worse still, a damaged coil, however most tuners don’t know what dwell time is or how to decide on the correct values. In this webinar we’ll find out what dwell time is and show you how you can utilise an oscilloscope to find the correct dwell time.
- Hey guys it's Andre from High Peformance Academy, welcome along to another one of our webinars. Now this time we're going to be discussing the topic of dwell time. And I know this is an area we quite often get asked about, we see this on our forums quite regularly, what dwell time should we be running for a particular coil? And I think there's also a lot of misunderstanding about what exactly dwell time is, how it affects the ignition system and what the implications of having this dwell time incorrect may be and when we're talking about having the dwell time incorrect, it can be a problem if we've got too much dwell time or too little. So today we're going to find out a little bit about this. We're going to go through some of the theory of the ignition system, we'll find out what actually influences the dwell time and we're going to look at three techniques of setting our dwell time.
We'll look at how we can calculate the dwell time, how we can measure the dwell time and then we're going to look at the option that I'm gonna guess right now that 99% of you out there are probably going to be using which is the trial and error method and we'll show you here on our Mainline dyno with our Subaru STi and our Haltech Elite 2500 ECU how we can go about doing that. As usual we will be having questions and answers at the end of this webinar so if you do have any questions that you'd like me to discuss in more detail, anything that I talk about that you didn't quite understand, please ask those in the chat, the guys will transfer those through to me. Alright so I think what we want to do is start by discussing what actually happens when we have our dwell time wrong. And you're just actually gonna have to bear with me because I didn't manage to get one of our pictures up. But I have now so let's head across to my laptop screen.
And if we set our dwell time too high, what we're going to end up doing is creating a lot of heat in the coil and this kind of manifests itself with some ugliness that looks a little bit like this. If our dwell time is set too high, we're physically going to put too much heat into the coil and it's going to end up melting the coil. When this happens, your coil is going to fail and that's gonna get quite expensive, particularly if you've got multiple coils on your engine that have all failed simultaneously, generally if your dwell time's too high, you're going to end up smoking all of the coils on the engine pretty quickly. So that's one extend of it, that's what happens when our dwell time is set too high, however the other aspect here is what actually happens to the spark energy. So here on my laptop screen we've got two sparks created from the same coil.
And I'll just be up front here as well, I have grabbed today, quite a few screenshots from the Dtech website, and I'll put a reference in our notes actually a little bit later on, maybe in our forum so you can visit that website yourself and there's a lot more detail than what I'm going to be able to go into today. So I just wanna give credit where credit's due for some of these photos. So basically both of these sparks created with the same coil and on the left here we've got a dwell time that is too short. So basically this has ended up charging the coil up to a point where it's got around about two amps current in it, in the primary winding and we can see we've got a very weak looking spark. On the right hand side we've got the same coil dwelled properly or charged properly and this ends up with around about seven amps in the primary windings and we can see we've got a much larger spark event.
So how that manifests itself when we are running our car is that if we don't have enough dwell time and our coil is not able to produce the voltage that it's capable of, what we're going to end up with is a situation where we are very likely to run into problems with ignition misfires. Now this is going to become more prominent if you are dealing with a turbocharged or supercharged car or anything with a power adder basically where we are creating a lot more cylinder pressure. That cylinder pressure makes it more difficult for the spark to jump across that spark plug gap and if it can't do that, we're going to end up with no spark occurring, we're going to end up with an ignition misfire and at best we're not going to end up with the sort of power our engine's capable of producing. Ultimately we're also going to end up with an engine that performs pretty horribly. Alright so that's the basis of it, what we really need to do now is get into the basics of our coil operation.
And I'm going to keep this relatively high level here. We're not really going to be jumping too deep into it but just enough to give you an idea of how the electronics, the coil igntion system works. I will mention that this is covered in detail in our EFI tuning fundamentals course as well. So if we can just head across to my laptop screen at the moment. And this is a basic diagram of what's happening inside our ignition coil.
And what we've got here is two windings around a common iron core that's shown in the middle there. We've got our primary winding, which is supplied our battery voltage. So in this case, we've got our plus terminal here supplied 12 volts, actually when the engine is operating and our alternator is charging, we're likely to find that that's gonna probably be closer to 14 volts. And then we've got our switching. This can be controlled internally inside of the coil as we're seeing a lot of modern coils now, I'll just show you one that's out of our 1UZFE.
This is what's referred to as an internal ignitor coil. So the ignition, the ignitor module is integral with the coil. Some coils have an external ignitor module and this is essential because the ECU can't directly switch the current that's going to be flowing into these coils, so we're basically using an ignitor module. You could think of it essentially as a relay for our ignition coil. So that the ECU doesn't need to switch the full current going through our primary coil winding.
So regardless whether we're using an internal or external ignitor module, essentially the system is exactly the same. It's closing to complete that circuit and allowing our coil to charge. On the other side here we've got our secondary winding. And this is really the important part to understand here. We can see even in this really rudimentary drawing here that we have a vast difference in the number of windings around our iron core between our primary and our secondary.
So the idea here is that the primary windings, we're using a thicker gauge wire with less windings because we are supporting a lot more current flow. Might be somewhere in the region of five to eight amps going through our primary windings so that's why we need a heavier gauge wire. And our secondary windings, we've got nowhere near the sort of current flow that we're seeing in the primary. But the ratio between the primary windings and the secondary windings is the key and this is what steps up our voltage because if we only had 14 volts available, that's not going to be enough to create a spark at our spark plug gap, particularly under high cylinder pressure that we're likely to see when we are trying to create that spark. So the ratio between the primary and the secondary windings, what this does is it steps up our primary voltage and it's not uncommon to see the secondary voltage at the plug gap, several thousand volts and that's what enables us to create that spark.
So primary to secondary winding ratio, that's really really important. From our perspective though we've got no control over this, this is an aspect of the coil manufacturer. We aren't making these coils, we're simply choosing them off the shelf or whatever the OE manufacturer decided to put into them. And that's what we're actually going to be operating with. So what we're looking at here is more aligned with how we can decide based in the coil that we've got, what our dwell should be.
So what we need to understand as well is what is the dwell time, what's that even mean? So the dwell time probably would make more sense if we actually called it charge time. So when we first close our circuit here and we start charging or supplying voltage and current to our primary windings of our coil, it's actually going to take some time for the magnetic field to be built up in those primary windings. Actually I should reverse back a little bit here because this is important to understand as to how the coil works, let's just jump back to my picture here on my laptop. So when we close our little switch here, we start charging up our primary windings, and what this does is the current flow through the primary windings, this creates a magnetic field and this magnetic field builds up and when we then open the switch there again, the magnetic field collapses very quickly and this induces a current flow in our secondary, a voltage across our secondary windings which is then what jumps across our spark plug gap. So getting back to what I was talking about there, that doesn't all happen instantly, it takes some time for our magnetic field to build up.
And if our charge time or dwell time is too short, that magnetic field isn't going to get a change to build up to its maximum level and in turn what's going to happen is when that magnetic field collapses, we're not going to end up with as much voltage being induced in our secondary windings, we're not going to get that voltage at the plug gap and we're not going to get as much spark energy. On the other hand, if we over charge or over dwell our coil, we're going to end up with excessive current flow in the primary windings, we're going to end up with a lot of heat being produced and that ends up potentially doing that damage to our coil which we already looked at. So the other things that's important to understand here is that the dwell time is gonna be very dependent on the particular coil that you are using. It's also gonna depend on our battery voltage and we'll talk about that and see that in action in our Haltech ECU very shortly. But we can't basically take an across the board number for dwell time or charge time and apply that to every coil and expect perfect results.
And I know this is where people come unstuck. We're installing an ECU on an unknown engine with unknown coils, how do we go about getting the correct data, and that's really the basis of today's discussion. So there are four ways we can go about this. And the first way is to discuss with your ECU manufacturer. Most ECU manufacturers will have a database of dwell time values for a range of popular coils.
So particularly if you are dealing with something really popular, really common, this data is almost inevitably going to be available, if not from your ECU manufacturer, possibly from an enthusiast forum dealing with your particular brand of engine. So that's the first place I would generally start. And when you are dealing with a ECU manufacturer, what you can often find is that they will have already developed base start maps for a range of particular common or popular engines and these will normally be populated with the correct dwell time for that particular coil. So that's the first place to get started and that's the easiest place to get started because it only requires either a big of Google searching on some enthusiast forums, maybe a quick phone call, or at worst and email to your ECU manufacturer. Now if you don't get the information that you're hoping for there, this is where things start to become a little bit more complicated.
And here we've got three options that are available to us. The first of these options is that we can calculate the correct dwell time. Now this also requires some pretty specialised data on our coils. So if you're in a situation where you don't have dwell time data for the coils, I'm going to go out on a limb and say chances are you're probably not going to have the required information to complete this calculation. But just in case you do, let's jump across to my laptop and you can have a better look at this in your leisure.
This is actually from the MegaSquirt manual and this is on determining your dwell time setting. So there is actually a calculation which is laid out here so you can calculate the theoretical dwell time required given a certain inductance, a certain resistance and the target current that you wanna build up in your coil. The problem with this is that it's not particularly easy for most home enthusiasts to get one of these pieces of data which is the inductance of your coils. So unless you've got a spec sheet for your particular coil, this is gonna be a little bit tricky. I still present it because if you've got the information, it's a valid way of going about it.
So you've got the calculation here and just to make things a little bit easier, you can just punch in the required data into this little form here, click compute dwell and it's gonna give you what that dwell time is for a particular set of circumstances. So important to understand but again probably something that you're not going to have access to, getting the data that you're going to need for your particular coil. Now what I should have actually done as well, I've jumped ahead a little bit here, so I just wanna talk about the way the current builds up. I've sort of mentioned it briefly but it's really, this is what we're trying to do with our dwell time. So let's look here at what happens when we apply voltage to a piece of wire.
So this is a graph here from an oscilloscope and on our vertical axis here we've got the current flowing through that wire. On our horizontal axis here we've got our time in milliseconds. So in this particular case, all we've got is a long section of copper wire, 13 volts has been applied to that wire, and using an oscilloscope the current flow through the wire has been logged versus time. So you can see with that piece of wire, we see a very very quick rise in our current and you see that it flattens out there just below six amps. So that's a bare piece of wire on its own however with our coil system we've got a little bit more going on.
So this is the same test here but now that piece of wire has been wrapped or wound into a coil, this one though just hollow, there is no solid core in that coil. So exactly the same deal here, we've got 13 volts being applied and we are looking at the rise time and current. So now with the coil formed, we can see that our rise time and our current actually occurs much slower and we aren't seeing our same peak current being produced until almost four milliseconds. Now let's look at our third and final example here which is exactly the same only this time we have now added a iron core. So essentially this is what we're going to see inside our ignition coil.
And we can see now that our current rise time has actually extended out. We aren't even seeing the full current, even out at 4.5 milliseconds. So this is the sort of aspects that affect our dwell time, our correct dwell time, again really comes back to why we can't hope to apply the same dwell values to all the different coils and expect to get perfect results. What we're actually looking for when we are developing our dwell time tables, is we are looking at the time it takes to reach our saturation current, or our maximum current where we can see here if we look at our graph, our current rises initially quite sharply and then it starts to plateau. So we're looking at a point where we're reaching the maximum current for that particular coil.
Now this again does vary, it might be somewhere in the region of five to eight amps. And if we exceed dwell time required or the charge time required to reach our maximum current then we're not going to end up with any advantage in terms of improved spark energy. But what we are doing is putting a lot more heat into the coil which is what damages the coil. Alright so we have looked at our technique there of calculating, as I've said, probably not a lot of use to most people but I just wanna mention it because it is a valid technique if you do happen to have the correct data. The next technique I'm going to also mention briefly because again most tuners that I know will not be doing this.
If you have an oscilloscope and a current clamp, it is possible using an ECU in test mode, where you're able to test fire a coil, to actually bench test and adjust the dwell time until it is optimised. So basically what you're doing is replicating the current versus time graph that we've just looked at there, for your particular coil. It's also a good idea if you are going to do this, you really want to have a variable power supply so you can vary the power, the voltage I should say being supplied to the coil so that you can actually develop a two dimensional dwell time table based on the battery voltage and that's really important because particularly at cranking, our battery voltage is going to drop quite low and as our battery voltage drops low, we need to increase the dwell time, it takes longer to physically charge the coil and this can be a big problem. Particularly if you've got a battery that is a little bit sub optimal, you can find that at cranking voltages, the coils really aren't charging enough to produce good solid spark energy and this can be really problematic with getting the engine to initially fire up, if you haven't extended that dwell time at low voltages. So I'm not gonna get too deep into this because again most tuners that I know probably aren't developing their own dwell time tables.
If there's enough demand for it, we may go through a practical demonstration at a later point using a picoscope and actually seeing how we can do this. But essentially what we're doing is using the current clamp around the primary power supply to our coil, we're varying that power supply so that we can develop that dwell time table versus each of the break points in our dwell time voltage table. The last technique that we're going to look at is the way that most tuners are probably going to deal with this and it might seem a little bit rough and ready, but the reality is that sometimes when we are tuning professionally, we are in a situation where we may be time limited and we need to use a technique that is going to get us the results with a minimal amount of effort. Obviously we don't want to risk either misfires or damaging the ignition system, but as we'll see, it's actually relatively quick and easy to get us in the ballpark. We'll also see that for some systems, the ignition system actually isn't that sensitive to our dwell time.
And before we go into this demonstration, I will mention that we are gonna move into questions and answers shortly after this so if you do have any quesions, please ask those in the chat and the team will transfer those through to me. For now, let's head across to my laptop screen and we'll just have a quick look through the ignition system settings here. So actually we'll go back into our main settings here and if we go to engine, we've got our ignition system settings, these are our main settings. In particular we have our edge for our trigger system, for our ignition system. Now for the majority of engines, this basically just defines the way the output from the ECU to our ignition system will switch and in this case the spark will occur on a falling edge.
This is pretty typical for probably 95% of ignition systems. You do want to check this actually as one of your primary setups though because there are some systems, some Ford and some Honda which are rising edge. If you don't have that right, you're actually going to end up damaging the ignition system very very quickly, irrespective of your dwell settings. We've also got our dwell mode which we can, in this case, select between constant charge and constant duty. We are using constant charge here which again would be the typical setting for most of our systems.
So with those basic settings out of the way, we can move right down to the bottom of our ECU navigator and we're under ignition system and dwell time. So here we have got set up a three dimensional table here. This is reasonably typical in most of the ECU, the aftermarket ECUs that I deal with but the reality is that three dimensional dwell time tables aren't actually that critical. Our main primary driver for our dwell time us our battery voltage. So we could easily get away here with a two dimensional table where we are using only battery voltage.
So what we can see here is if we just take a quick slice through this table at let's say a fixed 2000 RPM, and we can see that, I've just drawn straight through our battery voltage axis, that doesn't really make things any easier, we can see that down at six volts here we've got our dwell time extended out to 4.5 milliseconds. Add our operating voltage which should always be pretty close to 14 volts, we are operating with a dwell time of 2.5 milliseconds. So we're in a situation here, let's say for example where we've got no idea of the correct dwell time for our coils. So the situation we want to go through here is first of all, understanding what a typical dwell time is likely to be. And in most instances, at 14 volts, we're going to probably find that the dwell time will fall somewhere in the two to perhaps four millisecond range.
Generally I'm probably going to expect it to be more in the line of 2.5 through to 3.5 milliseconds. So if we're dealing with an unknown ignition system, I'd wanna start at the bottom end of that scale. Basically on the bottom end of the scale, our real risk there is that we may end up with problems with an ignition misfire. But that's not really gonna damage our engine. Of course as we've already discussed, if our dwell time is too high, we are likely to end up with problems where we actually melt the coil.
We're not gonna come back from that, that's gonna be expensive. So we wanna minimise our chance of doing any damage. And what we wanna do is basically find out at what point in terms of dwell time we're going to end up with problems with an ignition misfire. So that's relatively easy, what we wanna do here is create some load on our engine. So I'm just gonna get us up and running here in fourth gear and we've got a turbocharged engine, we're gonna go to, let's say about 3000 RPM, a point where we know we can comfortably get just about our maximum boost which in this case, we're running around about 1.5 bar.
OK so let's actually start by setting our dwell time really low, let's start by setting our dwell time to 0.3 milliseconds and in this particular instance, on our Subaru, we actually find that the ignition system is not particularly sensitive to the dwell time. So we've gone down there to 0.3 milliseconds. I've still actually got no ignition misfire. But I haven't given it much throttle yet, we're not on boost so let's bring our boost up. OK so with a boost pressure there sitting at around about 1.2 bar, 1.3 bar, we are getting an ignition misfire, actually we just managed to pull through that.
Let's just drop that down to 0.2 OK so I'm not sure if you're gonna be able to hear that but I'm sitting there now with one bar of boost and we've got a consistent misfire. Probably no big surprise for 0.2 milliseconds so I'm just gonna jump that up. At 0.3 I see that that gets a little bit better. 0.5 I'm just now a little bit cleaner but it's still a little bit messy there. Put a little bit more throttle in and we end up with a misfire at 1.4 bar.
So of course at the moment with a dwell time only at 0.5 milliseconds, that's really really short. So what we're going to do there is basically start with a dwell time that's pretty short. I've gone to extremes there just because this ignition system doesn't tend to be too prone to ignition misfires. Generally I would suggest starting somewhere around about 1.0 to 1.5 milliseconds. Get to a point where you are under full load and test and see if you've got an ignition misfire.
And if you have, I'd back off and step the dwell time up in perhaps 0.1 to 0.2 vicinity and test again. Basically you wanna just step that up to the point where the misfire goes away. At the point where the misfire goes away we don't want to be right on that ragged edge there where we've only just got enough spark energy to consistently create a good spark because the spark energy will also be affected by our coil temperature so what we wanna do is give ourselves a bit of a safety margin there. So around about 0.6 milliseconds in this particular instance, I know that that gave me just barely just on the brink of an ignition misfire. So I'd probably in this instance wanna step that up to at least 2 milliseconds.
Because I know for this particular coil system, I know two milliseconds is pretty safe. But let's just go and do a ramp run test here. We'll step that back up to where I'm actually running at 2.5 milliseconds. We'll head across to our dyno screen, and we're gonna go through and do a full throttle ramp run here. And this will give us our power on the bottom and our air fuel ratio on the top, let's do this run and see what we get.
OK so that run there, 2.5 milliseconds, probably a little bit difficult for you to tell at home watching this webinar, but we had a nice clean run there, 180 kilowatts at the wheels and we've got no sign of misfire. So what we're looking for when we are looking for an ignition based misfire, we're looking for a couple of things. We are listening to the sound of the engine, we're going to hear that, it's going to start to sound rough, it's going to break down, we're gonna audibly hear that quite clearly. That's also going to represent on our dyno graph in a couple of ways. First of all we're going to see an irregular or spiky shape to our power, and most likely what we're going to see is that our power curve will actually drop down quite sharply.
As soon as we've got an ignition misfire of course, we aren't correctly lighting off that fuel air charge, so we've got no chance of making the power from it, so we'll see that drop away. Now the other aspect is with our lambda plot or our air fuel ratio plot, when we do have an ignition based misfire, we are going to see that jump up, it's actually going to read lean. And the reason for this is because we've got unburned fuel and air passing through our cylinder. So let's just call that 2.5 milliseconds, we'll leave this up on the screen. And what I wanna do is just reduce this down to a point where we do get an ignition misfire just so you can see what that looks like.
Before we do try and create the misfire though, let's just drop this down to a point where I'm hoping we should still get a clean run, we'll jump back into my laptop screen. So we'll highlight the, actually let's make sure that we're not getting any interpolation as well, we'll highlight the entire area that we're likely to be running in between 12 volts and 16 volts and we'll set it down to 1.5 milliseconds. Let's head back across to our dyno screen and we'll perform a second run here. The other thing to point out is when we are expecting an ignition based misfire, on a turbocharged car, or actually on any car for that, any engine for that matter, we're most likely to have that misfire create or occur around the peak torque area, this is where our cylinder pressure is highest. So let's get our second run underway here.
OK so that second run there, we've gone from 2.5 milliseconds down to 1.5 milliseconds. And the point of that demonstration is you can see that we made exactly the same power. So I just wanted to dispel a myth here and this is something that I see a lot of tuners doing, they'll have an engine that is running cleanly and they'll go and increase the dwell time thinking that a more powerful spark is going to make more power. Essentially in my own experience, from what I've seen is that as long as we are getting a spark that's adequately lighting off our fuel and air charge, creating a bigger spark is not gonna burn the fuel air charge more. Basically once we've got that fuel air charge lit, we create a flame front that propagates out through the combustion chamber.
As long as we're doing this adequately where we're actually lighting off that fuel/air charge properly, then the remainder is self sustaining. So I don't tend to see any advantage in adding dwell beyond the point where we've got a nice clean run there, and hopefully that just demonstrates this. Now I will point out here though of course we haven't got a five gas analyser on this engine and it is possible that when we are starting to get a little bit marginal before we see the effects in terms of power and torque, we may potentially have some problems with the emissions output from the engine so that's a potential area, I certainly haven't examined that myself, because we live in a country where emissions compliance is less of an issue. Lastly, let's just drop our dwell time, now we'll jump back into my laptop screen. Let's try at 0.5 milliseconds and what we'll do is we'll do another run there and just see if 0.5 milliseconds is now enough to generate a problem.
So we'll save our run there, we'll call that 1.5 milliseconds. Let's got back to full throttle and we'll get our third run underway. OK perfect that's exactly what I was hoping would happen. And of course when we've got a dwell so short at 0.5 milliseconds, we don't really have a lot of chance of just about any coil creating enough spark energy to properly light off our fuel air charge. So it's done exactly what I said it would so.
Actually what I'm going to do, I'm gonna get rid of this and we'll just do that run one more time and I'll leave it live 'cause it's just a little bit easier to see it in real time. We don't have the smoothing of the dyno affecting our results so let's just do that little run again. OK we'll back off there. So what we've got is exactly what I said, we can see that right up until the point where our misfire occurred, our power line for all intents and purposes, is tracking perfectly with our last run at 1.5 milliseconds dwell. As soon as we start getting enough cylinder pressure in there, we can't get enough spark energy to light off that fuel air charge, so we see our power drop away dramatically, so exactly what I said would happen there.
And it's really easy and really clear to see that on our dyno graph. I again can feel and hear that the engine is misfiring so it's very clear to me. We've also got the backup data here with this spiky shape to our air fuel ratio plot and again you'll see it's done exactly what I said, it's gone lean there. So this is indicating that we have got an ignition misfire, I've got that unburned fuel and oxygen actually is going through the tail pipe and creating the lean condition there. So obviously we've looked at some pretty extreme numbers there.
0.5 milliseconds, we know that we've got a misfire there. 1.5 we are clean, we're right on the limit there, probably at that point, 1.5 milliseconds. Obviously we can do a bunch more runs there, try 1.0 millisecond, 1.2, 1.3 et cetera to find the point where we are just getting a clean run there. I know that from my testing prior to this webinar, 1.5 milliseconds, that's about where we're first getting a clean run there. However I'm generally going to want to step that up as I mentioned and I'd be more inclined to be up around the two, in this case 2.5 milliseconds is where we are running.
So we're basically trying to find the minimum viable dwell time where we can get a clean run out of the engine, we've got no problems with misfires. But we're not over dwelling the coil and risking damaging it. So we're basically applying a conservative approach starting with as low a dwell as we can, working up to the point where we're just starting to get the engine running cleanly, and providing ourselves a little safety buffer there. Now the caveat that I'm gonna add in here which makes things a little bit tricky is that what we've just looked at here works perfectly well for an engine that is mildly tuned, we're not running a lot of boost on this particular STi and it is all in stock condition. Same could probably be said for the majority of naturally aspirated engines.
However in some instances if we are dealing with an ignition system that is in poor condition, maybe you've got spark plugs with excessive plug gap or spark plugs that are old and really need to be replaced. Then you can of course have an ignition system which doesn't respond to dwell and basically no amount of dwell time is going to fix your ignition break down or ignition misfire. So this is the tricky part, we need to spot that and understand where this is becoming a problem. So symptoms for this would be where we have an ignition misfire occurring around peak torque but changes to our dwell don't seem to have any effect on it. So let's say we were at 2.5 milliseconds, and we've got an ignition misfire that's pretty tragic, pretty severe, if we tried stepping up our dwell time to 3 milliseconds, and it made absolutely no difference at all to our misfire, then I'd be starting to be suspicious that the problem was actually elsewhere outside of our coils or our dwell time for our coils, at least it might be a faulty coil, spark plugs, as I've already mentioned.
So if we, on the other hand, we're having an ignition misfire that was dwell related, we're much more likely to see that as we step up our dwell in maybe 0.2 or half a millisecond step, we may still have a misfire but we should notice a significant improvement, that misfire should become a lot less obvious. So that's an indication that we are on the right track and we could be dwell related. The other point that I will make here is that with our ignition misfires or our dwell problems, we do need to also make sure that our tune is on point. So if our ignition timing is massively retarded or our air fuel ratio is excessively rich or excessively lean, these problems can also create misfires that aren't actually a result of our ignition system. So just bear in mind there, we wanna make sure that our air fuel ratio is at least in the ballpark and that our ignition timing is again, is sort of in the ballpark of where we'd expect it to be.
If we've got all of those conditions correct, then we're going to be looking at dwell time or other aspects of our ignition system that are going to be giving us problems. Alright lastly let's just jump back into my laptop screen here. So at the moment we're still in a situation where we've got all of those zones set to half a millisecond, I've just jumped them back up to where I'd normally be running them, 2.5 milliseconds. And we can see that as we move down to our lower voltages here, six and eight volts, you can see that I'm extending that dwell time out. So again this is a little bit tricky if you can't bench test.
What we're looking for here is clean starting. So this can be again a little bit of a trial and error method. As a bit of a rule of thumb, as I go down to half of my normal operating voltage, let's say that's around about seven volts, I'd be expecting my dwell time to approximately double. So basically what I'm saying here is if I've got 2.5 milliseconds at 14 volts, I would probably be expecting down around seven volts to be somewhere around about five milliseconds. Now that's just a broad rule of thumb but generally it's going to be a pretty good starting point.
You can see that at seven volts we're not quite there, we're actually at about 4.25 milliseconds. But again as long as we're getting good clean starting at cranking voltages, this is generally going to be sufficient, we shouldn't be in those areas for too long, as long as we can get the engine to start up relatively quickly, our voltage should jump straight back to our running voltage. Alright let's jump across and we'll have a look at our questions. If you've got any further questions, please feel free to continue asking them. Barry has asked, I know you said that adding dwell doesn't increase power if the ignition system's up to par, what about coils that have longer spark duration? Do you think there are gains to be had from coils with this sort of spark duration? Look Barry it's not something that I have personally tested so it's a little difficult for me to say.
What I do know is that there are a lot of companies out there preying on the enthusiast market, selling products with claimed advantages where I've personally not been so convinced that those advantages actually exist. Now I'll just give you one example that I actually have personally tested. This was with my old drag car, so a Mitsubishi Lancer Evo 3 for those who aren't aware. Back in the day, this is during its first build, we were running on VP Racing C16 fuel, boost pressure around about 35 psi and we were probably in the region of about 600 maybe 650 wheel horsepower. Now with that particular build, I had anticipated problems with ignition energy and we had fitted an MSD DIS 2 CDI ignition system.
Now this little unit had the advantage that it could be uninstalled and jumpered out of the system, so basically it got wired in and you could fit the MSD CDI ignition system or you could remove it and plug In a jumper and it would run a factory ignition system. Now at that 650 wheel horsepower mark, or wherever we happened to be at that particular boost level, we did back to back runs on the dyno because it only took about 10 seconds to change between the two systems, and there was no difference other than the normal expected run to run variation between CDI and non CDI. Despite the fact that the CDI provided a larger spark energy. So this kind of backs up what I was saying. In my opinion, my experience, I guess probably more to the point, is that provided that we can light off the fuel air charge, once it's lit, once it's combusting, we can't make it combust more.
Now again, coming back to your specific question Barry, I haven't tested just a single change of going from a coil to one that provides a longer spark duration but I'd be pretty much picking the same sort of situation exists there. Provided that the spake energy is there to initially light off the fuel air charge, I think once we've probably got it burning, we're probably not gonna see any advantage from a longer spark duration but I have not tested that so I'll add that caveat there. Mitchell Newman has asked, how does a misfire cause the air fuel ratio to go lean, wouldn't it be rich? Good question there Mitchell and this is actually a common misconception and it comes down to just learning about how the air fuel ratio meter works and what we're expecting to see. And I see this time and time again with novice tuners in particular, they've got an ignition misfire, they're looking at the air fuel ratio plot thinking ah well the engine is clearly lean, they're adding more fuel, often this is going to make the situation worse. So first of all, looking at the air fuel ratio and analysing that plot, what we wanna do is look at the air fuel ratio just prior to the ignition misfire event.
The reason we wanna look at this is as soon as that ignition misfire event occurs, we can't take any reading or any value from our air fuel ratio data. So that's always a little bit of a hint, look at whether we are a little rich or a little lean just prior to the misfire, that's what we wanna fix. Now in terms of your actual question there, why does the air fuel ratio go lean? That's because the O2 sensor is actually responding to unburned oxygen in the exhaust system, it's not actually responding to unburned fuel passing through the cylinder and into the exhaust system, it's unburned oxygen. And of course as soon as we've got a misfire, you've got a full charge of unburned oxygen and fuel passing out into the exhaust and the O2 sensor responds to that oxygen. Mitchell has asked, would you want to set a slightly higher dwell time to compensate for any carbon build up or age? No look I don't think so.
What we need to understand is if we are in the ballpark with our dwell time, if we're there or thereabouts, we should have sufficient spark energy or as much spark energy as that ignition system or coil can provide. So if we start to get a little bit marginal, if the coil's aging, adding additional dwell time is actually not going to improve our spark energy. If you're in a position where you're at the limit of that coil, probably the solution really is to step up to a coil that can provide higher spark energy, rather than trying to add a bandaid with larger dwell time. As we've sort of talked about in the webinar today, you're actually not going to see any improvement when increasing the dwell time beyond the correct value for that coil. All you're gonna do is pour additional heat into the coil.
Barry's asked, the dwell time for some coils is not always given as a 2D table format, but are given as duty cycle percentage at peak dwell time, how is that interpreted? OK so this is something I'll probably get into for you there Barry in the forum. We do have, in some instances the ability for our ECUs to present dwell time as a percentage or a millisecond value as a time based value. So in general, with most of the aftermarket ECU tuning we're doing, we're going to be presenting this in a millisecond time as opposed to a percentage. So give me a little bit more detail on that outside of this webinar, just so I make sure that I actually give you the right information, it's not something that I am normally dealing with. Alright guys that brings us to the end of our webinar.
Thanks for joining us everyone, hopefully that's given you a practical way that you can apply to actually finding suitable dwell time for your next tuning job if you're faced with a set of coils with no data. Remember if you do have any other questions after this webinar has aired, please ask those in the forum and I'll be happy to answer them there. Thanks for joining us and I hope to see you all in our next webinar.