280 | Selecting The Correct Size Throttle Body
The correct sized throttle body is critical to ensure you aren’t placing a restriction in the inlet airflow that will limit power, however it’s more common for people to fit a throttle body that’s too large which then provides very poor control of airflow, particularly at small throttle openings. This can make it difficult to control the engine torque, making it difficult to drive the car. In this webinar we’ll discuss what you need to know to get the right sized throttle body the first time.
0:00 - Intro
1:00 - Not a black and white answer
1:35 - Why do we need a throttle body?
2:45 - How does throttle body size affect torque modulation?
5:15 - Tricks for dealing with a large throttle body
6:30 - Why larger throttle bodies don't always have any effect
9:20 - Cross sectional area is important, not diameter
10:00 - Butterfly and cross bar still cause some restriction at WOT
10:30 - Things to consider when choosing throttle body size
13:10 - Effect of forced induction
13:50 - Individual vs single throttle bodies
16:40 - ITB sizing
20:00 - Trumpet/runner length
21:50 - LS3 example, 92 vs 102 mm throttle body
24:00 - 4G63 example
28:30 - Rough rule of thumb for ITB sizing
30:25 - Checking for restriction
34:45 - Questions
- Hey team, Andre from High Performance Academy here, welcome along to another one of our webinars and this time we're going to be discussing the topic of throttle body sizing. And this is an area that I know there's a lot of confusion about and I tend to see most people err on the side of going too large on their throttle body. And the idea here is they're thinking that a larger throttle body is going to reduce restrictions and hence is going to give them more power. And yes there's obviously some truth in that but once we go past a certain point, going to a larger throttle body is going to give you no improvement in power and what it can actually introduce is some problems with throttle modulation or airflow modulation which actually will make the car harder to drive. Particularly at part throttle so this is something that's very easy to overlook due to the way the airflow goes through a throttle body.
So we're going to dive into that topic, as usual we will have some questions and answers at the end. Unfortunately what you are going to find here is that while I can give you some general guides, some general rules of thumb, there isn't a lot of black and white here because as you could probably understand, there are so many variables here depending on the type of engine, the modifications to that engine that will really end up defining what the ideal throttle body size for that particular application is. But we'll give you some guidelines and then we'll show you as well how you can actually test and find out if your throttle body is giving you a restriction. Let's start though with the basics and this should go without saying but we're going to say it anyway, we need to talk about what the throttle body is there for and on a petrol/gasoline engine, the throttle body is there for our torque modulation and what we're doing, unlike a diesel engine where we don't need a throttle body, with the gasoline engine we need that throttle body to modulate the amount of airflow into the engine and by modulating the airflow into the engine we're therefore controlling the torque and this is really important, the key here is that if we've got good modulation of our airflow, good modulation of our torque, it gives the driver the ability to very accurately control the amount of power and torque the engine's delivering and particularly for a race application, this is really critical if we're talking about road racing, getting torque modulation through corners, particularly in a car that is traction limited so it's got more power than it can put to the racetrack through a corner. This is really important to be able to get the most out of that car.
So a small throttle body is going to improve our torque modulation, our airflow modulation, large throttle body makes that a little bit trickier. What we need to understand is why that's the case. So we've got a couple of little sample throttle bodies here. So we are going to be talking a little bit about ITBs, we've got a set of individual throttle bodies here of a black top Toyota 4AGE 20 valve. I've also got the stock Toyota throttle body here off a 3UZ 4.3 litre V8.
So all I want to do is just start by getting our throttle body under the overhead here. And the part that's easy to overlook is obviously we've got our butterfly in the middle of the throttle body and as we open that butterfly, it's going to open a path through the throttle body for the airflow. The bit that's easy to overlook though is that the relationship between the butterfly angle and the airflow through the throttle body is very non linear. What I mean by this is that when the throttle body is completely closed, we get a pretty large increase in airflow for the first few percent of throttle opening. So when we first initially crack that throttle open, we see a really large increase in the airflow through the throttle body as we first start moving the throttle butterfly.
But again if we just go to our overhead here, this should be pretty easy to understand. As we crack that throttle body open, it gets easier to understand, although the lighting here makes it a little bit harder to see. Once we're past around about 70 or 80% throttle, from about this position here to wide open, we don't actually see a significant increase in our airflow and you can kind of see that, again I'll apologise for our poor lighting. At the moment the amount of the cross sectional area that the butterfly is blocking off is pretty limited from this point 'til it's 90° open. And that's the problem we're facing here, we need to have good control over that airflow with relation to our throttle position in order to be able to control the torque modulation of the engine.
Now as we get to a larger throttle body, we see that increase in airflow for the first few percent of throttle opening gets significantly larger so basically we get less granularity or less resolution in our airflow versus throttle position compared to a smaller throttle body and that's the balancing act we're trying to find here. We want a throttle body that's going to give us maximum airflow with minimal restriction but also we don't want to be adversely affecting our torque modulation so we don't want to make the car harder to drive. Now there are a couple of tricks that we can employ to help us with this situation with a large throttle body. Even with a stock cable throttle body, even on a completely factory car, what can be quite easy to overlook is the fact that there will be a linkage which means that we generally don't have 1:1 relationship between our throttle pedal movement and the throttle butterfly movement. Basically there is a mechanical linkage that means that we're slowing down that movement over the first maybe 10 or 15% throttle opening and then as we get further through the throttle movement it will increase the relationship of the throttle butterfly opening to our pedal position so that's just artificially getting this, trying to improve the resolution in our torque modulation using a mechanical linkage.
These days with the likes of drive by wire throttle bodies which this one is, we can do this with a table inside of our ECU which makes our life a little bit easier. But electronic or mechanical tricks aside there is still a limit to what we can achieve here so when it comes to throttle body sizing, it's not, it's definitely not a case of bigger is always going to be better. Now in researching for this webinar, I've used a variety of sources, some interesting information from Jenvey in the UK who are a manufacturer of aftermarket throttle bodies so I just wanted to give you a couple of examples here because this would probably go outside of what most people think would be possible. So one of their examples is the older 2 litre European Formula 3 engine. These were a 2 litre engine producing, 4 cylinder producing around about 240 horsepower.
Those were mandated to breathe through a 26 mm restrictor. So essentially regardless of what size the throttle body is, that 26 mm restrictor, that becomes the limiting factor for how much airflow we can get into our engine so really once the throttle body's much bigger than that, we see no real advantage. Put that into perspective there, that's about a 530 square mm area that we've got to ingest air into that engine, making 240 brake horsepower. Now a lot of people talk about putting larger throttle bodies on engines and I know one of the popular sizes I see talked about a lot is 75 mm throttle body. That is 8 times larger than the cross sectional area of that 26 mm restrictor.
So that takes us out to 4400 square mm in round numbers. So you can understand there that if you've got a naturally aspirated engine producing maybe 200 to 300 horsepower, with a single throttle body on it then 75 mm is complete overkill, no need to be anywhere near there. And one of the guides, we'll talk about this as we go through it a little further, is that we can use what OEs do on various different engines to give us a little bit of insight into suitable throttle body sizing for a particular application engine and power level. Now the OEs don't always get it right and they certainly aren't always focusing solely on the aspect of trying to make maximum power. But at the same time it does at least get you into the ballpark and as we'll find out as we go through this webinar, a lot of this does come down to some trial and error testing to find exactly what's going to be perfect for your application.
Some other interesting information there from Jenvey, this was on a 270 horsepower touring car engine, again 4 cylinder and naturally aspirated. And this was running individual throttle bodies. The throttle body size was reduced down to 44 mm before there was actually any noticeable drop off in power so again that would generally be smaller than most people would consider to be required in order to support that sort of power. Now I've talked a little bit about cross sectional area here and again this is one of the bits that is very easy to overlook. It's not actually the diameter of our throttle body that's the important part, it's the cross sectional area.
And that of course is pretty easy to calculate, it's just pi times our radius squared. So as long as we know the diameter of our throttle body which is pretty easy to measure, half of that is of course the radius squared multiplied by pi, that gives us our cross sectional area, so what you'll see is that as you step up just a small increase in throttle body diameter, that actually does give us quite a sizeable step up in our cross sectional area. We do also need to consider that there are some restrictions here. So again if we just jump to our overhead, so while we've got, I think this is a 76 mm throttle body for our 3UZ, even in the wide open position, we can see that there is going to be some restriction from the throttle butterfly and more specifically the cross bar that supports that throttle butterfly. So important to factor that into your calculation, you can reduce that to know what your actual full cross sectional area is.
Alright so now we've got a bit of an understanding there we'll go over some of our considerations and we'll start here with our single throttle and plenum, we'll move onto individual throttle bodies shortly. So the considerations here are obviously this one should go without saying, if the throttle body is too small, it can produce a restriction in airflow which will hamper power. We're also going to see that by a pressure drop across the throttle body. So this is what we'll talk about a little bit later on, measuring that pressure drop across the throttle body, this is actually going to give us some insight into whether or not the throttle body is in fact causing a drop in our power. If we go too large, as I've mentioned, we're going to end up with problems with torque modulation and throttle control so what we really want is the smallest throttle body that we can get away with that's not going to give us an unreasonable airflow restriction.
Sounds simple but of course getting to that point is a little bit more complex. And there's also, this is another part that's really easy to overlook. There's not a lot of advantage going to an increased throttle body size if you've got another part of your inlet track that is smaller than the throttle butterfly or the cross sectional area of that. So this is a trap I see a lot of people moving into where they'll be fitting an aftermarket larger throttle body to a factory plenum chamber or intake manifold. It's not a big surprise, usually the factory intake manifold is sized so that the hole that mates up to the factory throttle body is the same size as the throttle body.
That becomes your limiting factor. There's a couple of ways of going about this, I have seen people in the past simply bolt a bigger throttle body straight to the inlet manifold with no port matching or anything. Generally that's going to result in a step which will probably actually adversely affect power. The other alternatives is to use an adaptor flange which actually tapers between the larger throttle body and the plenum chamber. Generally that sort of modification you're almost certainly going to see little to no gain because you're still limited by the size into the inlet manifold, that's going to limit your ultimate flow.
There are of course options around porting it but it's just important to mention here that if you are going to fit a larger throttle body, but you've got another part of your inlet track, either pre or post throttle body that's going to be smaller, then yeah you're probably not going to see any reasonable gain with the exception that going slightly larger in the throttle body could end up getting around the blanking effect of that cross bar and butterfly when the throttle is at wide open. Just talk quickly about the effect of forced induction as well. So I mean anything that causes a restriction here, naturally aspirated or forced induction, it's a restriction there, it's obviously going to cost us some power. With forced induction, we are increasing the airflow but also the pressure. And you could think of this a little bit like an injector, a fuel injector.
If we increase the fuel pressure we can actually get more flow out of a given size injector for a given pulse width so it's a little bit the same with a throttle body. We can force more air through the throttle body but that doesn't necessarily mean that we're not going to benefit from moving to a larger throttle body. We'll talk a little bit here about ITBs versus a single throttle and I mean generally here, the advantage is we do see individual throttle bodies used primarily or a lot I should say at least on highly developed race engines. There's a few forced induction applications that also use individual throttle bodies. They do give us a little bit more freedom in terms of tuning the inlet system on a naturally aspirated engine.
And this isn't just about the throttle body size, this is also about the length of the inlet track, makes it very easy with an ITB setup like our 20 valve setup here. In this case, we've got the flange, we'll go to our overhead shot here. We've got the flange where the throttle bodies will bolt to the cylinder head. And if I just flip these over again, we've got our throttle butterflies inside here, these happen to be 45 mm. And then we have the ability to bolt trumpets onto the end of the inlet manifold, individual throttle bodies.
So basically by controlling the length of the inlet track from the centre of the intake valve out to the end of that bell mouth, we can achieve a tuning effect here which gives us a slight ram effect basically improving our engine's volumetric efficiency at a specific rev range. By taking advantage of that we can improve our engine performance a little bit. So that's the two aspects there with ITBs. In terms of benefits compared to a single throttle body, we also have generally improved throttle response. And there's obviously a little bit more complexity in terms of the implementation of ITBs as a negative there as well.
A little bit more complexity around the tuning, so the tuning aspect of the ITBs in terms of their size and their overall length is both a pro and a con. We can get an advantage there but it can be time consuming and it can also be expensive, you can chew through a lot of dyno time trying to get the perfect results there in terms of whatever powerband you're trying to achieve. In terms of individual throttle bodies as well, I've talked about the fact that even at wide open throttle, a conventional butterfly style throttle body will still provide some restriction to airflow at wide open throttle due to the cross bar. There are designs for racing individual throttle bodies which are a barrel style where there is zero restriction at wide open, also slide style throttle bodies where again once the throttle is in the wide open position, there is zero restriction so couple of options there that we don't tend to see in ITBs. So we want to talk a little bit about sizing of the ITB, the individual throttle bodies, so this is specific to ITBs.
We'll jump across to my laptop screen for a moment and we can get a little bit of guidance here from the likes of what carburettor tuners have been doing for an eternity. So with the carburettor, can be a little bit misleading as well because we've actually got the throttle body size in a carburettor but we've also got a venturi and it's important, the venturi has to be smaller than the throttle body size to create a pressure differential, otherwise we don't get the fuel being atomised and drawn in so it's very easy when you look at something like a 50 mm side draught Weber for example, it may be a 50 mm throttle body but it will have a venturi which is smaller, generally somewhere around about 0.8, 0.9 times the diameter of the throttle body itself to get that venturi effect. So this is a chart that is used for Weber sizing. So on the vertical here we've got the main venturi size in mm, so we can use this as a bit of a guide to sort of figure out what ballpark we're likely to be in. We've got the engine capacity here which is a driver on this and then we've got the maximum rev range that we're going to be going out to.
So for example here if we had a 2 litre engine that revved out to 7000 RPM, we would probably want to be somewhere in about the 40, 41 mm size for our venturi. So that just gives you a bit of guidance there. Looking at what has been done with carburettor sizing is obviously not a bad place to at least get started with ITB sizing. What we need to understand here, it's again the fact that going larger in a throttle body size for individual throttle bodies is definitely not going to give you more power necessarily and this is actually more dramatic, the effect here is more dramatic than we see with a single throttle body and plenum. The reason for this is that when we go larger in an individual throttle body size, what we're then doing is affecting the airflow but also the air velocity.
So when we go larger in our throttle body size, we're actually reducing the air velocity for a given airflow so what this can end up doing is hampering our low RPM performance so going larger in an individual throttle body size can give you a small improvement potentially in power but quite often that's going to be at the detriment and a significant detriment to the torque at low RPM. So it's all about understanding what we're trying to achieve here and factoring that in and actually going through and getting a, testing it to find a result that's actually going to be workable. Particularly for a road car engine, generally, sacrificing a couple of horsepower at 7500 8000 RPM will be more than worthwhile for picking up a 10 or 15% improvement in our torque between 2000 and maybe 4000 RPM because the engine is going to be working in that power range a lot more frequently. Alright I'll just head back over to my notes now. While not specifically about today's lesson here, I just will quickly factor in or talk about the length here of the intake system.
So that's from the centre of the intake valve out to the end of the trumpet. So specific to individual throttle bodies here. There is a factor of this that comes into a single throttle plenum arrangement but we don't get that same tuning effect that we do with ITBs. Rule of thumb here is that longer runners will favour low RPM performance and vice versa. Sometimes we're going to be compromised in terms of what we can physically fit into the engine bay so it might not always be possible to get the ideal solution, again just some information here that came from Jenvey.
If we've got an engine that's revving to 9000 RPM or making peak power at 9000 RPM, their general guide is that the inlet length there should be around about 350 mm. This works on the basis though as well, if we double that rev range and we go out to 18,000 peak power, we'd actually be halving the length of the inlet track, so we'd go from 350 mm down to 175 mm. So just a bit of a rule of thumb there. The reality here is, and I've done this myself with testing on the dyno with naturally aspirated race engines, you can do your calculations, you can know approximately where you're likely to be and then it's still going to be a case of testing multiple trumpet lengths to find actually what does work in the real world on your specific engine so the math, the theory is only going to get you so close and then it really does come down to some actual testing to see what's going to work under real world conditions. Now just to talk about a few other key engines that we can sort of understand what the OE manufacturers were doing in terms of engine size versus the throttle body size.
If we look at a stock LS2, the 6 litre V8 there, stock throttle body size, around about 90 mm and I did actually find quite an interesting dyno comparison here. This one, slightly different engine, LS3 so 6.2 litre and a little bit more modified than stock but I apologise for the quality of the dyno sheet that I've got here but we've got 2 lines here, blue line here is a stock 92 mm throttle body on the LS3. The red line there is a 102 mm throttle body and hopefully what you can see there is there's almost no difference. There's a few horsepower here and there but a lot of this would almost come down to the run to run variation and particularly if we look at peak power here, there is almost nothing. Actually a bit of a bump here interestingly in power and torque where the larger throttle body at 4500 RPM does tend to show a bit of promise but yeah just to show you just bolting on a 10 mm larger throttle body with no other modifications, not necessarily going to give you any increase in power so you want to be mindful of what you're trying to achieve there.
A couple of caveats for this. On that particular engine there, if the engine was further modified, maybe a more aggressive cam was fitted, maybe the compression ratio was raised, then volumetric efficiency of the engine was improved to the point where the 92mm throttle body may have in fact posed a restriction, you may have then found that the larger 102 mm throttle body would actually start to show some merit. But on a stock engine or relatively lightly modified engine at this point, no real clear winner there for the 102 mm throttle body. Alright we'll just head back over to my notes here again for a second. Now just want to also talk about 2 litre turbocharged 4 cylinder engines, pretty popular combination across a lot of different platforms, Japanese and European.
Just to give you one example here, the Mitsubishi 4G63 which is an engine I'm pretty well familiar with, in stock form the likes of the Evo 8, 9 throttle bodies, they're 65 mm which sounds quite small considering what a lot of people want to run on these engines. In stock form that engine, it's around 300 horsepower depending on the exact specification. With that exact same throttle body, we've made somewhere in the vicinity of about 600 horsepower with just bolt on modifications to other aspects of the engine such as the turbocharger, exhaust and the rest of the inlet system so still breathing through that stock throttle body. Now we've doubled the power, probably sufficed to say or safe to say at that point, if we're doubling the power from 300 up to 600 horsepower, I would expect at that point going to a larger throttle body would probably show some merit terms of an improvement in performance but it just goes to show what can be achieved there and it's where you want to focus your attention when you are modifying your engine. Everything obviously costs money so do you want to go to a larger inlet manifold and throttle body or do you want to go to a larger turbocharger, you can't do both, probably the chances are you're going to get more bang for your performance dollar initially doing the turbo upgrade and understanding that you're probably compromising peak power potential through that smaller throttle body, you can always upgrade your throttle body at a later point.
To give you two examples with some fairly heavily modified vehicles, my old drag car, my old 4G63 Evo drag car, we'll jump over to my laptop screen for a moment. So this still used a 2 litre 4G63 and when we retired this it was making 1166 horsepower at the wheels running 54 psi of boost. I'm sorry, I don't have a better shot of it but you can see in there just hiding on the end of the plenum chamber there, we had a Infinity Q45 throttle body which is an 80 mm throttle body. That in and of itself actually was problematic because the opening of that throttle body, we had very, or the size of the throttle body I should say, we did have very poor modulation of the throttle. However it's a drag car so some things we can get away with there.
But to give you another example here as well, this is, have I got another, no, that one there, this is a Mitsubishi Evo 9 that we also built after we'd retired my old drag car. This used a Hypertune plenum and throttle body and again sorry we don't have a better photo here, it is hiding down there in the engine bay a little bit but there is 2 options I think Hypertune offer in terms of the throttle body, this was the 90 mm variant and we're making 1001 wheel horsepower with that 90 mm throttle body so just to give you some idea on, with the 2 litre 4 cylinder engine, what sort of power we can expect through, first of all a factory throttle body and then if you're stepping up to 1000 plus horsepower, probably somewhere in the region of 80 to 90 mm is fine. In terms of most people modifying these engines for street use, generally find that probably going to be somewhere in the region of 500 to 600 horsepower, that's a fairly healthy combination that's fairly easy to achieve without breaking the bank and generally in that instance, somewhere between 70 and 75 mm for the throttle body would be completely fine, probably even larger than you need to. As I've already said, 600 horsepower's achievable on the stock 65 mm throttle body but it will be producing a little bit of a restriction so a bit of a bump up there to somewhere in the region of 70 mm, probably still going to give you a nice compromise between the drivability there, throttle modulation, without giving you too much restriction in terms of airflow. Right so I've got another couple of approximations here as well.
Just let me find this so I can show you through it. OK so again I'm sorry, I probably haven't really made this as clear as I could but this is a rough guide that you can use here for getting your throttle body diameter for individual throttle bodies this time, so not for a single throttle. So our throttle diameter here is 0.8 multiplied by V times RPM. And then to the power of 0.5. So in this case V is our cylinder volume in litres so not our engine volume, this is our cylinder volume.
So I'll just go through this calculation here. So for a, let's say our little 20 valve 4AGE here. So we've got a 1.6 litre engine, so if we divide 1.6 by 4 because we have 4 cylinders, that gives us 0.4, multiply that by 7500 RPM which there or thereabouts is about the rev limit for that engine. And then what we want to do is put that to the power of 0.5, giving us a result of 54.7, let's call that 55. And then we want to multiply that by about 0.8, there's a bit of wiggle room in here but that gives us an outcome of 44 mm rounding up.
So pretty well right on the money of what Toyota came up with there with a 45 mm throttle body. Just interestingly as well, the silver top which came before that engine, same individual throttle body arrangement, they used a slightly smaller 43 mm butterfly so it at least gets you in the ballpark. As I've mentioned here already you do need to still do some testing to actually find what's going to be optimal for your particular application. Right I'll just head back to my notes here for a second again. And we're going to move into questions and answers in a second so this is a good time to ask any questions you may have and I'll do my best to answer those.
Before we get into that though, just talking about the fact that there isn't a black and white number that we can use in terms of what's going to be the ultimate throttle body size and a lot of this does come down to testing to the obvious question is how do we go about doing this testing? What data do we actually need to know if our throttle body is giving us a restriction? So what we want to really do here is look at the pressure drop across the throttle body and we can use this same strategy essentially for measuring the restriction of any part of the intake system. That is a consideration here, we do need to factor in that it's not just the throttle body, we've got the inlet track before the throttle body, we've got potentially an air box, an air filter, maybe the other side of the air box and the inlet to that. Each step of that process of that inlet system can be giving you more and more reductions in airflow there or basically adding up to an overall restriction so we do need to factor that in. So how do we do this? First of all, with a naturally aspirated engine, what we can look at is the pressure, the air pressure in the inlet manifold. So we'll just jump across to my laptop screen for a second, this is a dyno ramp run that I did on a naturally aspirated 5 litre Grand Am engine.
So we've got our engine RPM up here in red, so we start that run at about 2700 RPM and the class I was running in was restricted to 6800 RPM. Alright unfortunately technology is not working too well for us today so I can't show you the datalog but I can talk you through it because really it is quite straightforward. What we want to do is look at our engine RPM and our inlet manifold pressure. And what you'll naturally find with most engines is that if there is some restriction in the intake system, we'll see hopefully at low RPM, we should have atmospheric pressure and then as we move further through the rev range you'll start to see that that pressure in the inlet manifold may just drop down to a slight amount of vacuum. So this is pretty normal but obviously the less restriction we have, the less pressure drop we're going to see.
In the particular example I had on the screen which I now can't show you, we started the run at 100 kPa and by the time we got out to 6800, 7000 RPM we're down to 95 kPa which I'd class as probably not too bad, about 5 kPa, that's about the most I'd like to see in terms of a pressure drop. That works out to be about 3/4 of a psi so obviously here, the less restriction, the less pressure drop we've got, the better the throttle body's doing. But again it is important to just mention here that it's not just the throttle body we're measuring here if we're looking at just the inlet manifold pressure, that's everything pre throttle body as well. So there's a pretty good chance for that that it was actually the intake airbox or maybe the air filter that was giving that pressure drop. So a better way of doing this and this is how we'd want to do this with force induction, we want to look at the boost pressure pre and post throttle body.
So this may require you to add another sensor and again hopefully we've got some data here but I can't show you. Or I have got some data here that I can't show you. But we did this on our SR20, SR86, basically put a pressure sensor pre throttle body and then we've already got the pressure at the inlet manifold and at a run on 18 psi we're seeing basically less than 0.5 psi variation between 2 sensors which is pretty good, that's exactly what we're aiming for. But by doing this, by looking at the pressure pre and post throttle body, you are eliminating the rest of the inlet track as an error there or a restriction so we're really focusing down straight onto the throttle body itself. So this is the best way of going about actually measuring the restriction and knowing whether or not it's worthwhile.
How much should you be aiming for, as I've mentioned, I'd like to see less than a 5 kPa drop, I've seen 10 to 15 kPa on a variety of different engines and that's definitely, in this scenario, where there's some worthwhile gains to be had in fixing that and again, if you prefer working in psi, we're looking at 3/4 of a psi or less drop. Right we'll jump into some questions now and I'll see what we've got here. Ken has asked, If upgrading an RB25DET series 2 to around 400 horsepower, would the stock throttle body be fine or do you recommend upgrading to something larger? Look at that point with a relatively mild upgrade there to 400 horsepower, you'd absolutely be fine with the stock throttle body. I know this for a fact on account of I've done that sort of a combination on an RB25 numerous times so yeah I would probably save that to the point, the throttle body upgrade to the point where you're looking at doing maybe a forward facing plenum on it. 95ej's asked, if you are naturally aspirated but getting into sustained positive boost pressure, in my case 0.5 to 1psi, on your MAP sensor at wide open throttle, is this an indication that a larger throttle body is warranted? That's an interesting situation you've got yourself in there.
I'm not quite sure what you're getting to see 1 psi positive pressure on a naturally aspirated engine because that is quite a feat to achieve. Possibly some sort of pulsation that is causing that but that's not necessarily an indication that a larger throttle body is warranted. What you'd want to do there as I've just mentioned really is look at the pressure cross the throttle plate. Interesting, I'm not 100% sure how you're getting to that situation, it's not a situation I typically see unfortunately so hard for me to answer that accurately I'm afraid. Rynomoto's asked, should I just make restrictors for the tops of the bores for at least the front 4500 bores that will be used for driving around part throttle? I have a 4500 dominator flanged manifold and there weren’t low flow, 1,000 cfm, throttle bodies available, is the reason for the, I'm not sure, it looks like I've got half of this question here so hopefully the boys can send me the rest of it 'cause I sort of come in here half way through I'm sorry.
Dave's question is, what's your estimate on throttle body size for a 6 litre LS turning 6800 RPM? So I mean we've got our own in house L98 which is for all intents and purposes the 6 litre LS with an upgraded cam that makes 425 kW flywheel and that runs to 6800 RPM, still running with the stock 90 mm throttle body I think that is. So at that point, you could possibly start to see an improvement in performance with a step up in throttle body size maybe to 102 mm throttle body but it's definitely not something that you absolutely would need to do. Important there if you are looking at stepping up the throttle body sizes, to make sure just as I was talking about during the webinar, make sure that you're taking into account the adaption of the throttle to the inlet manifold itself as well so there isn't a restriction there. Brown 3000's asked, so from what's been said today, I'm assuming an 82 mm drive by wire Bosch throttle body for a ported RB25NEO with Greddy manifold will be too big for my 500-600 wheel horsepower targets? It's probably bigger than is necessary. I would probably be somewhere in the region of about 76 mm on the Bosch drive by wire range there, that's going to be absolutely fine.
The 82 mm though will still give you good control, good modulation. I definitely wouldn't want to be much bigger than that. The advantage with the drive by wire as I mentioned, is it does give you the ability to set the relationship between our throttle pedal position and the throttle body opening. So you can actually work around the airflow modulation problems that a larger throttle body gives. So what I would say is if you already have an 82 mm throttle body, I'd probably run with it, understanding those implications, I'll try and get the words out, implications and being a little bit smart with your throttle body mapping in the ECU, you'll be absolutely fine but if you are on the market still and you haven't purchased, I'd probably step down to 76 mm there personally.
Suhas has asked, I bought a 52 mm ITB for 4G93 NA. Can I fabricate and use an RB26 style intake manifold with mass airflow scaling, sensing I assume you mean there. How should we vary the trumpet lengths in this kind of setup? Does use of a MAF sensor impact in a negative way? How so how did Nissan get over this on the RB? OK bunch of questions in there, let's see what I can do. Probably 52 mm, unless it's a pretty wild 4G93, I would say is probably bigger than would be ideal, it'll probably still work but you may find that dropping down to 50 or 48 mm might actually give you an improvement in your power and torque curve everything so again just bigger is not always better. So using the RB26 style intake manifold with a mass airflow sensor, I think is probably not going to be the way I'd suggest going about this.
You're going to kind of eliminate going back to a plenum with the mass airflow sensor on an individual throttle body engine, you're going to really sort of get away from some of the advantages in terms of the trumpet length tunability that you would get with ITBs without that plenum. The plenum does allow you the ability to use the mass airflow sensor for tuning but the reality is we don't need it, we just run on alpha N off the throttle position and you're going to get absolutely perfect results. The reason that Nissan did this is the RB26 of course is a turbocharged engine, running twin turbos and individual throttle bodies so that's why they've got that system, you need a way of connecting the turbochargers to the individual throttle bodies which requires the plenum and the mass airflow sensing is a nice clean way of monitoring the airflow for that but we don't need that with a naturally aspirated ITB engine. HDR Engine Development's asked, how does throttle body size affect intake plenum resonance tuning? Can certain throttle body sizes work with plenum volume to increase or decrease torque at certain RPM? OK that's getting probably a little bit beyond my level of testing on this and the reality is that yeah probably there is going to be some effect on your plenum resonance with the throttle body size but it's probably also going to be much more impacted on by the actual volume of the plenum chamber itself which is a whole different discussion for a different day. It's important to make sure if you are running a single throttle body and a plenum chamber, the plenum volume versus the capacity of the engine also versus the RPM range that you're expecting the engine to work in, is another big factor that comes into this.
So it's another one of those areas where we can't necessarily look at one aspect in isolation, we need to understand that all of these factors do play into each other and one will affect the other. But yeah to sum up there, haven't done this testing myself, my suggestion here would be that while yes the throttle body may play an aspect in it, I'd say it'd be much more minor compared to the actual volume of the plenum itself. Adam's asked, is there anything to be said for intake piping sizing or shape pre throttle body? I'm thinking of the whale penis intakes that we encounter on Hondas. Yeah no those are great, I love those. I will actually give you a little bit of insight which to this day still sits in the back of my mind as completely mind blowing, if I had not seen this, I would not have believed it.
I worked for a client who was building a Mach 2 Escort rally car. And the customer didn't really want to spring for a genuine BDA engine so they actually used a Honda S2000 F20C. Completely stock engine with a set of fabricated headers and due to the fitment in the engine bay, the intake to the factory throttle body on the F20 was quite tight. So what they'd done is they'd fabricated basically a 90° straight off the throttle body into a pod style air filter that sat behind the grill. Looked like it would be pretty free flowing so we tuned the whole engine and got pretty good results, I can't remember the specific numbers now but I think somewhere in the region of about 145, 150 kW at the wheels.
Once we'd finished with the tune, the person who built it wanted to test a different air filter which wouldn't easily fit in the grill so temporarily we basically ran a section of 2 foot straight alloy piping to the throttle body so we could try this other filter. We picked up somewhere in the region of about 12-13 kW and that for a naturally aspirated engine, is massive, anyone who's tuned naturally aspirated engines would probably know. And I literally thought there was a problem there so we reverted back, the power went back to exactly where it was, back to back tested this and we could consistently prove a 12 kW gain. We sort of thought this was down to the air filter but that's a massive gain for an air filter, we tried the original air filter just on the extended pipe, same gain there. So it was basically we concluded it was the fact that that F20C was very sensitive about a bend straight off the throttle body and it really wanted a section of straight there.
Unfortunately the packaging in the Escort engine bay made that impossible which is a bit frustrating when you're giving away 12 odd kW but just to show you the sensitivity of the whole intake system. In terms of the whale penis intake for the Hondas, yeah I mean basically, I don't know how this came to pass as a popular intake system. Maybe someone did some computational fluid dynamics, I kind of probably doubt it but basically back to back testing I did find that on the average Honda B series engine, these generally did offer some improvement. Dmitriy's asked, what was the formula that you used to calculate throttle body size? So remembering again this was for individual throttle bodies so the throttle diameter should be approximately 0.8 mutliplied by our single cylinder capacity in litres, multiplied by the RPM ceiling of the engine to the power of 0.5 Actually I'll put that a different way, let's jump over to my laptop screen so you can actually see it 'cause it is quite important, the way that you do this calculation. So it's our single cylinder volume multiplied by the RPM ceiling then we want to have that to the power of 0.5 and then finally multiply that by 0.8.
So if you don't do that in the correct order, you're not going to get the right result. Right we'll head back to our questions. Nicholas has asked, any pressure variance when using methanol on a naturally aspirated application? I'm not quite sure what you're referring to in terms of pressure variance, I wouldn't be sizing the throttle body specifically on the fuel that I'm using. All other things being equal, moving from pump gas to methanol fuel, we would expect a small improvement in engine performance but we'd be there or thereabouts in terms of the throttle body. Grainsauce has asked so a stock SR20DET's throttle body is all you need unless going past 500 - 600hp mark? Off the top of my head now I can't actually remember the stock Nissan SR20 throttle body size but I do seem to recall them being a little bit smaller than 65 mm.
They'd be absolutely fine, I've taken stock SR20 easily up to 400 plus wheel horsepower with no issue. Probably if my memory does serve correct and it is smaller than 65 mm, once you're getting up sort of towards maybe 500 horsepower then yeah there's probably going to be an advantage moving larger than that so yeah sorry I don't have a resource in the back of my mind of factory throttle body sizes for every single engine, I do do my best though. AUSBen's asked, In my experience on my RB30DET with the stock 60mm OEM throttle body which is mapped via Alpha-N with MAP compensation I am finding that my VE map is relatively flat from 70-80% throttle position on up depending on the RPM. I am currently updating the engine management and wiring package and in the process will be swapping to a Bosch 68mm throttle body. My working theory is if the throttle body is approximately sized to meet the naturally aspirated air flow requirements it will probably be pretty close to correct.
My understanding is that when under boost the increase in mass flow comes from an increase in air density with the volume of air in the cylinder unchanging being a function of engine displacement and VE. I intend on validating this by measuring pressure pre and post throttle body. OK so just need to try and process that there Ben but yeah basically your theory there is pretty much on point. Particularly if we take the throttle body out of the equation completely for a moment, what we find, a lot of people think when they're tuning a VE based fuel model that as we add boost pressure, we're just going to keep seeing our volumetric efficiency numbers increase but that's, as you've pointed out, that's not actually the case, we will generally to a point where the volumetric efficiency does flatline and people are sort of thinking well if my volumetric efficiency's staying the same and I'm increasing the boost pressure, then how are we seeing more power but as you've mentioned there it's the air density that's increasing there. So we don't necessarily see the VE of the engine continually increase and in fact what we will find is that as we start going higher and higher in the boost pressure, exhaust back pressure starts to increase, we'll actually see that VE map start to drop away as well, the efficiency of the engine starts to drop away but it's the air density that's the important point there.
I would say with what you're doing there, you haven't really talked about boost pressure or power targets but if you're at a 60 mm OEM throttle body and you're going to 68, that's probably not going to be a bad option. Obviously I've given you some sort of guidelines there on some of the different sized throttle bodies that we see in OE applications so you can kind of use this as a bit of a guidance but really comes down to how much power you are ultimately trying to make. And yes, you're spot on there, if you do end up measuring the pressure pre and post throttle body, you're going to know exactly what sort of restriction the throttle body poses. The advantage with a 68 mm throttle body is that you're going to get really really good control over the airflow. So you will absolutely no problems there.
Allen has asked I didnt get to see the entire webinar, but is there a correlation between size selection and tip in throttle response? Not specifically. You may find that you will need to adjust your acceleration or transient enrichment settings for a different sized throttle body. But if you're doing a really sharp throttle input, essentially regardless whether you've got a smaller throttle body or a larger throttle body, you're still seeing that instant increase from basically closed throttle to all of the airflow available so I wouldn't say that necessarily you're not going to be able to get crisp tip in response with a larger throttle body, it's just going to require some tweaking to your settings in order to get you there. Bananaman's asked, have you ever had any experience tuning idle with a throttle body which doesn't have a vacum based bypass for idle? My SR20 seems to only have the ability to adjust the butterfly opening at idle. No so the SR20, the airflow, the idle speed control is not done through the throttle body itself, it's got a idle air solenoid elsewhere on the plenum chamber depending on exactly the generation of SR20 you're dealing with so that's how that's done.
There is from memory, 2 parts to that, there is a ECU controlled air bypass and there is also a wax pellet which is coolant based so it provides an idle up when the engine is cold. I have had trouble with some of them in the SR20s and the RBs, I think it comes down to maybe the age of them at this point. But yeah there is definitely a system there that should be workable, there's a note there that it's an aftermarket throttle body not factory but either way you should still be able to use the factory air bypass as long as that's not blocked up, to control your idle speed there. Probably also worth just checking, I've seen a lot of SR20s and RBs come in where that's not been functional simply because it's been overlooked and hasn't been wired up so obviously you need to make sure it actually is functional. Right I think I'm going to call it there sorry guys, we've got a few more questions but we are just running a little bit long here so I do apologise to those whose questions I haven't been able to get through to but for our members if you do have any further questions, please feel free to ask those in the forum and I'll be happy to answer them in there.
Thanks for joining us and hopefully we can see you again next time.