209 | What Is Compressor Surge?
When it comes to turbo sizing there’s a lot to be gained in getting it right, but also much to lose if you get it wrong. One potential issue that is possible to run into is compressor surge and while this term is thrown around frequently, it’s often misunderstood. In this webinar we’ll learn what compressor surge is, how to know when it’s happening, and how to avoid it.
- Hey guys, Andre from High Performance Academy, welcome along to another one of our webinars. Now today we're going to be discussing the topic of compressor surge. Something that is a little bit misunderstood I think in turbo tuning circles, and I hear a lot of misinformation about compressor surge, about what it is, about what causes it and about what sort of results we're likely to see if we run our turbocharger into surge. So today we're going to dive in a little bit deep, we're going to find out exactly what compressor surge is, what it is, what it isn't, and we're going to find out what causes it and what we can do if we have a turbocharger that is exhibiting surge. As usual we are going to be having questions and answers at the end of the webinar so if there's anything I talk about or anything related to this topic, please ask those in the chat and the team will transfer those through to me and we'll jump into those at the end.
So let's start I guess with the obvious, is what is compressor surge? And it is a little bit tricky to explain so bear with me and we'll try and do our best. And essentially it is a situation where the turbocharger is pumping out more air than we can easily fit into the engine or more air than the engine can accept. And we get to a situation if that becomes so dramatic, that the air actually backs up and basically it causes the compressor wheel to stall, we get the air come back out through the compressor wheel and it's going to be chopped up. When I say the compressor wheel stalls, we'll jump into this in a bit more detail, I'm not talking about it stopping either, that's a common misconception, it does not stop, it will continue to spin, we've got our turbo potentially doing somewhere in the region of 80 to maybe 140000 RPM, It's not gonna stop but what it does is it aerodynamically stalls and we end up getting the air, the pressurised air coming back out through the turbocharger. Now when that happens, we get the sort of fluttering or chirping sound, I've heard this referred to as a pigeon, as a rooster I think it was, there's a few different terminologies around but it does give a really distinctive fluttering sound that anyone who's spent any time around turbocharged cars will have heard.
One of the classic examples where you do hear this very regularly is if any of you have followed rallying, with rally cars particularly with their anti lag systems, you're going to hear the chirping or fluttering sound of compressor surge every time the driver backs off the throttle. So that's what we've got here and that's the basic principle behind it. We've basically got the turbocharger pumping out more air than it can cram into the engine, some of that air backs up and basically aerodynamically stalls the compressor wheels and we've got that air coming back out through the turbo. So there is a really big distinction here that's important, we're gonna cover this a couple of times. But compressor surge can occur under wide open throttle operation and we're going to find out why that's the case but essentially one of the keys there is it really comes down to turbocharger sizing.
The other option or area where we can run into compressor surge is where we are backing off the throttle which I've just mentioned there with my little rally car analogy. So very very different modes of operation for our engine and our turbocharger. Although essentially we've basically got exactly the same thing. Alright so we'll jump in a little bit deeper into what exactly causes it. And I've sort of mentioned here, essentially we get into a situation where our engine can't consume the amount of air that the turbo is trying to move.
So this results in our compressor wheel stalling aerodynamically. Now we need to sort of understand here, because there is a little bit of confusion around what exactly boost pressure is, and this has a big influence on our compressor surge and where abouts we're operating on a compressor map as well which sort of all ties in hand in hand. And a lot of people sort of think that boost pressure is the be all and end all when we are looking at turbocharger performance. But the reality is that the boost pressure is really just an indication of the restriction to air flow that the engine is providing to the air coming out of the turbocharger. So what I mean by this is that if we had a factory car with a stock turbocharger on it, let's say for example we're dealing with a factory Mitsubishi Evo 9 and we've got a stock TD05 turbocharger fitted to that car and let's say we're running 20 psi of boost.
Now this is just to break down that misconception about boost pressure. What we can now do is run our car on the dyno, let's say for example that we produce 300 horsepower just in round numbers. Now we take that factory turbocharger off, and we replace it with a much larger turbo, let's say we go with something like our BorgWarner EFR 8474 and we run the car on the same dyno at the same 20 psi of boost pressure. Now most people with at least a broad knowledge of tubrocharging would probably be able to figure out that if we go from a stock TD05 to a much bigger 8474 turbocharger, at that same boost pressure, we're going to end up with our engine making more power. So this is just to dispel the myth that that boost pressure is the critical component here, it's really just agian an indication of the restriction to the air flow and for exactly the same reason there, if we stuck our same 8474, we take it off our two litre 4G63 engine and fit it to a much bigger, let's say a GM six litre LS2 V8, the engine is physically much larger, it's going to present much less restriction to the air flow.
So what we're going to be able to do with that turbocharger is make the same power that we got from our two litre engine, on a much lower boost pressure. We're going to be able to move the same amount of air at a much lower boost pressure. Now to try and tie some of those concepts together, let's jump across to my laptop screen and this is one of the most important parts when it comes to choosing a turbocharger and matching it to our engine and this is our compressor flow map. So we've got a couple of bits of information on this, I'm not gonna get too detailed and we're gonna have a look at some information from a data log on one of our in house test cars shorttly and see how this ties in to this compressor map. But on our vertical axis here we've got our pressure ratio.
Now this is the pressure at the outlet to the compressor divided by the pressure at the inlet. Now generally the inlet pressure is going to be pretty close to ambient or barometric air pressure but of course it is quite possible with a factory style intake system that there will be some pressure drop through our air box, our air filter, air flow meter and our plumbing. So quite often, particularly at high air flow values, we can actually see the pressure pre compressor drop quite low. This actually has a dramatic effect on our pressure ratio calculation. So again that's our compressor outlet pressure divided by our compressor inlet pressure.
On our horizontal axis here we've got our corrected mass air flow. Now this, in this particular example it's actually presented in kilograms per second. Most of these compressor maps generally are displayed in pounds per minute. And we can actually see here that that's been added in, the maximum air flow from this particular EFR 6758 is rated at 53 pounds per minute. So then we've got our efficiency islands on this compressor map.
This sort of shows for a given mass air flow and a given pressure ratio, how efficiently the compressor wheel is operating. Not too much importance to us for our webinar today, the key point that we wanna look at is this line on the far left side of our compressor map, this is the one we wanna be taking notice of and this is our surge limit. So basically what we need to be doing, every time we are choosing a compressor wheel, is make sure that we are always operating our engine on the right hand side of that surge line. If we end up running too much boost and we end up somewhere up here in our, the left hand side of that surge line, this is where we're going to press our turbocharger into surge. Alright so we've got the two scenarios there that can basically both occur, we've got our wide open throttle compressor surge where we're operating the engine under full boost pressure and we've got our engine running on the left hand side of that compressor map.
And then the other situation is where we have got the throttle lift off compressor surge. Now I just wanna back up and just mention here a couple of these misconceptions that I quite often hear. And I think a lot of this comes down to the fact that the terminology used can be a little bit confusing. I've used the term stall there and I've tried to clear this up, the fact we're talking about aerodynamic stall of our compressor wheel and you could look at our compressor wheel here and essentially each of the blades on our compressor wheel does act a little bit like a wing on an aircraft and just like a wing on an aircraft, it is possible for that blade on the compressor wheel to aerodynamically stall. And when that happens, basically to put it in layman's terms, it kind of loses grip on the air and it can no longer push the air into the engine.
So when we've got this situation occurring when we've got compressor surge occurring, the turbocharger itself does not stop spinning. Remember I've just mentioned earlier the turbo speed might be anywhere in the region of maybe 80 to 140, 150000 RPM. We've got a lot of mass with both the compressor wheel as well as the turbine wheel. So that's not going to instantly stop and at the same time it's also not going to end up spinning backwards, that's another common misconception I hear. Neither of those things are going to happen.
We've just got the situation where the compressor wheel is aerodynamically stalling, it's slipping the air and we've got our boosted air is exiting out through the turbocharger. Now particularly under wide open throttle operation we've got this situation where this becomes a consistent repetitive process. The turbocharger will stall, some of the boosted air that's built up in the system will escape back out through the compressor wheel, that's where we hear that chuffing or fluttering. And when this happens, it reduces the pressure post turbocharger slightly. We can go back out of surge and that allows the turbocharger to come out of stall and basically build up pressure again and the whole process repeats.
Obviously this is all happening very fast. Now with that, so we'll deal now with our throttle off application and with our throttle off application, this is where we're at full boost pressure and we back off the throttle maybe for a gear shift or maybe just because we need to slow down. So the turbocharger is at high RPM, it's pumping a lot of air into our engine. Obviously we're closing the throttle body and then the air has nowhere to go. So it essentially backs up and we end up with the pressure between the throttle body and turbocharger increasing.
So if we look at our compressor map here we sort of get into a situation where even if we are operating maybe out here in the correct area of our compressor map, out to the right of our surge line, under those conditions, momentarily what happens is our boost pressure basically pre throttle body, we're obviously not measuring that normally because we're measuring our boost pressure in the inlet manifold, our boost pressure may spike up or at least stay the same and momentarily our air flow drops, so we sort of end up moving, or can move to the left of our surge line and our turbocharger begins to surge. Now this normally only lasts for a second or so before the boost pressure drops away and the turbocharger comes back out of surge. And of course this is why we see blow off valves fitted to our turbochargers. Now this is another area where I think there's a little bit of misconceptions around why turbochargers are fitted with blow off valves and these days, particularly with our ball bearing turbochargers, the need for a blow off valve is possibly not quite as dramatic as it was a few decades ago. So basically our blow off valve, we've got one here which is a recirculating blow off valve, it's fitted to the compressor cover of our EFR.
When we lift off the throttle, the blow off valve is going to be referenced to manifold pressure which drops into vacuum, this opens the blow off valve and allows the boost pressure to recirculate back into the compressor inlet. We've got a more conventional style Turbosmart Race Port blow off valve here, which is a vent to atmosphere style, essentially it's achieving exactly the same aim. So the idea with the blow off valve there is to release that pressure and prevent the turbocharger from going into surge. So if you're running a blow off valve on a car, a turbocharged car and you are still hearing that flutter, that telltale surge when you lift off the throttle, then chances are what that means is that either your blow off valve is too small, so it can't physically vent the volume of air that you're actually pushing through with that turbocharger, or alternatively another thing we do see occasionally is the spring in the blow off valve may be a little bit too stiff so it's a little bit too hard for the manifold vacuum to overcome spring pressure and physically open the blow off valve. Now the reality of this though is that a lot of people, I think the main sales pitch for blow off valves is more around the idea that by venting that excess boost pressure, we're going to stop the turbocharger from stalling and generally the way this is explained is more along the lines of that we are actually slowing the turbo speed down.
It's gonna prevent that, it's gonna keep the turbo up to speed, meaning that when we open the throttle, we're going to be back on full boost again quicker. So this is the explanation I hear. And I don't personally buy into that. We're actually intending, once we've got our turbocharged 86 back up and running to do a little bit of experimentation around that and try and gather some data to see exactly what the reality of that situation is, both with and without the blow off valve operating, whether we are seeing any improvement in boost response and a throttle lift when we get back onto the throttle. But my personal opinion is the blow off valve more often than not is actually there as a turbo protection device and the reason for this is that when the turbocharger stalls, or when it goes into compressor surge I should say, we do end up with quite a lot of force being placed on the thrust bearings inside of the turbocharger.
Particularly if you are running an older journal style turbocharger, journal bearing style turbocharger, then the thrust bearings can be a little bit fragile and generally it's not a good idea to run the turbocharger consistently into surge on throttle lifts if you're running a journal bearing turbocharger. We really do wanna check that turbocharger and the blow off valve is great at doing that. On a ball bearing turbocharger such as the two that I've got here, the design of the ball bearing cartridge in the turbocharger is significantly stronger in every way but definitely in thrust. So this is less of an issue and basically that becomes a personal preference I guess but under, for the purposes at least of protecting the turbocharger, then the ball bearing turbocharger makes the blow off valve and optional accessory, we don't necessarily need it. OK the more damaging situation though is where we are running into, where we are running into surge when the turbocharger is at full throttle.
And this can be quite damaging because now we've got the turbocharger producing a lot of air flow, it's running a very high RPM. Particularly if we continue in surge, there is a lot of destructive force being placed on the turbocharger components so we really don't want that occurring. The other aspect there is when we are running into surge under wide open throttle, we're also going to end up impacting on our engine performance. We will find that the engine isn't going to run particularly nicely. So generally it's a situation we want to get away from.
And again if we just jump into our compressor map. Basically that's going to happen any time we are operating out here on the left hand side of the turbo's compressor map, beyond that surge line. So really the key here is how can we get into a situation where compressor surge is happening under wide open throttle operation? And I think it's fair to say, now I've been in the industry for around about 20 years, it's becoming less and less of a problem for mainstream turbocharger users. That's because there is a lot more knowledge around turbochargers, and there's a lot more knowledge around turbocharger sizing. And there's a lot more technology being placed into turbocharger design.
In the early years of my career where we were seeing a lot of people hybrid factory turbochargers where they would what was one of the most common upgrades would be to take a factory turbocharger and replace the factory compressor wheel with a larger compressor wheel. This can work well but this is probably one of the perfect examples for a recipe that can create a turbo sizing mismatch. It can create compressor surge. So the reason this occurs is we've still got a factory exhaust wheel, maybe we've got a factory exhaust housing. So we've got this small turbine wheel that can provide a lot of energy to drive that compressor wheel.
And when we get into the situation where we're asking for a lot of boost pressure, generally this can push that compressor wheel very hard, particularly at low RPM. As again we've got that small tight turbine wheel that can provide all of that energy to the exhaust wheel and basically it creates enough compressor speed that it produces boost pressure, runs the pressure ratio that puts us into surge. So how that will normally transpire is you're going to see this situation occur when you're increasing the boost pressure, probably at lower boost pressure it might be absolutely fine but as you increase the boost pressure, you're going to get to a point, generally it'll happen around about peak torque and as you just increase the boost pressure a little bit further you're going to start getting those first signs of compressor surge. And that just shows you that you're just starting to operate beyond that surge line. What you will find is that if you stay in the throttle, as the RPM increases and the ability of the engine to consume more air from the turbocharger increases, we'll probably fall back beyond that surge line and the compressor will stop surging.
So it can only be momentary, if you're just getting to a point where you're right on the very limit of surge, obviously if you try and increase the boost pressure further, that's only going to make the situation worse. So the other scenario that we do see this situation occurring is when we are in a drag racing application. We're seeing this now mainly with small capacity engines and only really the guys who are really pushing the boundaries and the good example I'm gonna give you here is Colin Willshire from Jett Racing, his Mitsubishi Eclipse drag car, powered by a 4G63 are pretty confident in saying that currently he still holds the world record for the fastest four cylinder outright drag car. I won't try and give you the time but very low six second range at the moment. So the situation he's in is he's basically pushing his turbocharger as hard as he can to try and get as much air flow out of that turbocharger as possible.
His particular engine runs to around about 11, 11500 RPM. Now in drag racing when everything is working perfectly and the car's going down the track like it should with an air shifted five speed transmission, basically the engine is only ever operating between perhaps 9000 and 11, 11500 RPM. It's in a very narrow window and in that window of engine RPM operation, the engine is able to accept or consume all of the air that the turbocharger is pushing. So when the pass is good, when the track is good, everything works out and surge is not an issue. What Colin was finding though is when he was running on a bad track, sometimes get into a situation where the car would get a little bit loose and one of the solutions for getting back on track without needing to lift off the throttle is to just click into a higher gear or short shift the gear.
Now when that happens, we've still got the turbocharger at full boost pressure, putting out a huge amount of boost pressure, in these engines 100 plus psi, but of course when we short shift we drop further down into the engine's RPM range where it cannot consume all of the air coming out of the engine, and this can push the turbocharger into surge. In particular at those sort of power levels and boost levels, the forces involved can be very very destructive. So Colin actually came up with a fairly novel solution to this which I wanna show you here. Now this is actually iteration two of his engine but let's just jump in and have a quick look at my laptop. So this is actually a compound turbocharger setup and one of the reasons that he's gone to the compound turbocharger setup is actually to help go further and get away from the problems, further in terms of boost and power and get away from the problems associated with surge.
So what we can see here is, off the top of my head I think we've got a GT55 mounted up here at the front, GTX55. It's a little bit hard to see down below, in the factory turbocharger location I think we've got a GT4202 mounted down there. So the turbocharger system doesn't really matter. What I wanted to do is show you this little device that he's got fitted up here on his charge air pipe and this is a tiny little drive by wire throttle body. I've got another shot of it which doesn't quite give you the perfect idea but here it is here anyway.
I think this little guy was only around about 28 millimetres. And they've got some custom firmware in their Motec M150 ECU package. And basically by monitoring the pressure ratios as well as the turbo speed, they know when the turbocharger is likely to run into surge and what they can do here is actually bleed off some of that excess air if the turbocharger runs into surge so that Colin or the driver can stay at wide open throttle. That'll allow the turbocharger to come back out of surge, it can accelerate through into an RPM range where the engine can then accept all of the air flow again, drive by wire throttle body can close, and can continue on the path. So obviously not a perfect solution but that is one instance where you can end up running into surge on a system that is set up as optimal as it can be, when you're trying to do things on a very small capacity engine, make the sort of power on a small capacity engine, 2000 plus horsepower sort of things that are really really outside of the range of the turbochargers that are available.
Alright so we're going to move into some questions really shortly so if you do have any, please ask those into the chat. Now what I wanna do first of all is talk about the solutions if we have got compressor surge occurring, how we can go about fixing it. And I wanna show you a little bit of data that we've got off our Toyota 86 test car. So the first one, as I've mentioned, probably one of the main issues with compressor surge is if you've got a mismatch in your turbocharger sizing. So really this comes down to sizing the compressor wheel for your engine in the very first place.
And at least for mainstream applications now, particularly if you're dealing with any cars that are popular for modification out in the performance market, there's a huge plethora of information on what turbocharger systems are available, what they do, what sort of boost pressure, what sort of RPM you're likely to see full boost pressure at and what sort of power they can produce. So we're no longer in the dark ages where we're doing a lot of experimentation and seeing how particular turbos perform. Unless you're really at the cutting edge of pushing the boundaries like the likes of Jett Racing there, you can learn from other's mistakes and find out what works for your car before you even lay down a cent for your turbocharger. The other option you've got, if you are running one of those combinations that I mentioned where you're only right on the limit there of compressor surge, it's not a tragic set up, what you can do is apply a bit of a bandaid here where you are going to purposefully reduce boost pressure around the peak torque area of the engine where it's running into surge and then as the RPM increases and the ability of the engine to swallow all that air flow increases, then you can obviously ramp the boost pressure up. Now it's a bandaid, it's definitely not an elegant solution but it is a way of getting around shelling out for a whole new turbocharger setup.
And the last option there which i just looked at with the Jett Racing application is we can bleed off some of that boost pressue. Now I wouldn't recommend this for anything other than the sort of application where Jett Racing are using it. It's really not an elegant way of doing this for a street car or a circuit car and essentially it's again just a bandaid because we're wasting that excess boost pressure, that excess air flow out of the turbocharger and generally what we're going to be doing is creating a lot more heat and probably producing a lot less power than we could if we were doing exactly the same thing with a correctly sized compressor. Alright so I just want to show you quickly here the compressor map that we had up on my screen. So the reason I've actually chosen this one here is that this is for the Borgwarner EFR 6758 turbocharger and the reason that I've chosen this is because it is the turbo that we've got fitted to our Toyota 86.
And I've just got some data here from our Motec system that I wanted to talk you through so this is from a dyno run where we were gathering a little bit of turbo data. So at the top of the data here we've got our engine RPM, probably nothing particularly out of the ordinary there. Our run starts at about 2000 RPM and we go through to about 7400 RPM. Next we've got our throttle position, obviously you can see we're at full throttle there for the entire ramp run on the dyno. And we've got our boost pressure overlaid there with our exhaust gas back pressure.
Now these are both in kPa so I apologise for those of you who are more familiar with working in PSI. So we're only running about 60 kPa there, it's not a high boost application given that this is a naturally aspirated engine, 12.5:1 What we can see here, not that this is really the topic of today's webinar but the exhaust gas back pressure, our orange trace here, always a good idea to look at this in a performance application and it can give you a lot of information about the sizing of the turbocharger. Essentially this shows how much drive pressure is available from the exhaust gas to actually drive that turbine and provide energy to the turbine and spool up the turbocharger. And we can see here that up to the point we're at about 6400, 6500 RPM, our turbine inlet pressure, our exhaust gas back pressure is actually below our boost pressure. And from my own experience, that's always a great place to be.
We sort of see some really large gains in the performance of the turbocharger package when our exhaust pressure is lower than our inlet pressure. This is a balancing act though because if we go with a very large turbine housing or very large turbine wheel and turbine housing in order to get that back pressure down, we're going to end up with a lazy turbocharger that's not gonna spool up well. So in this application we're actually doing pretty well, we've got full boost by about 3400 RPM and we're only really just starting to exceed the boost pressure in the exhaust manifold by the end of that run there so not a bad thing. Next we've got our turbocharger speed so we can see there at the end of our run there we're getting up to about 95000 RPM. I've just added in here our exhaust gas temperature although not really too relevant for today's demonstration.
And then we've got two more pieces of information. So we've got our air box mass air flow. So I've logged this in kilograms per second so it matches our compressor map. This is not necessarily going to be 100% accurate because this is data from the factory mass air flow sensor. So this really all entails the calibration of that mass air flow sensor being correct.
And as a lot of you will be aware as soon as we start changing components in the inlet system, the chances are that our factory mass air flow sensor calibration is not gonna be absolutely accurate anymore. But we do include that just as a bit of an example there. And lastly we've got our pressure ratio which as I've said is our compressor outlet pressure divided by our inlet pressure. Now I kind of already touched on this, we do need to be a little bit careful with our pressure ratio. Because normally we are going to be measuring boost pressure in the inlet manifold.
And it's quite likely that there's gonna be some pressure drop occur between the outlet of the turbocharger, we'll probably get a bit of a pressure drop through our plumbing, we're going to likely see some small pressure drop through our intercooler core. So what I'm getting at here is the pressure in the inlet manifold is likely to be slightly lower than the pressure coming straight out of the turbocharger and likewise as I mentioned, we would normally take into account barometric air pressure or ambient air pressure as our compressor inlet pressure but that's not always going to be the case, depending on the design of our inlet. However this is what we've got here so we can see that we're operating at a pressure ratio of about 1.6 Now the key, why I wanna just show you this is that it allows us to really accurately see where abouts we are operating on this compressor map. So let's just take this point here, right at the end of our run. We can see that our air flow is around about 0.21 kilograms per second.
Pressure ratio of 1.60 and we've got a turbocharger speed of about 95000. So let's head across to our compressor map and we'll see how that all pans out. So we know that we were at around about 0.21, that's about there. 1.6 is our pressure ratio, so that's about there. And we know that our turbo speed was about 95000 so we're somewhere through here, a little bit difficult to scale our turbospeed.
So we really only need two of these points, three just gives us a little bit of extra correlation so we know exactly where we're running. And from that point we can see where we are on that compressor map and we can see that at least at 7500 RPM we are so far away from our surge line, we don't need to worry about it, in fact we're really not utilising that turbocharger to its potential at all. So that's just how we can use some of that data. if you've got pressure ratio, which is pretty easy to calculate and turbo speed which now is becoming easier and easier to get that information on the mainstream performance turbochargers, or alternatively you've got a mass air flow calibration, calibrated correctly, then we can see where we're operating. Now the other thing I'll point out here is if you do wanna get into a little bit more detail in making sure that you don't get this wrong in the first place, we do have another webinar in our archive that covers analysing turbocharger performance, we do this on the dyno using our turbo Toyota 86, and we've also got another one we did more recently which covers how to read a compressor map as well as how to use Borgwarner's Match-Bot design sheet as well so you can actually make sure you're going to get a turbocharger that's pretty well matched to your application.
What I would say as well with the turbo mismatch, because the current crop of performance turbochargers, they tend to be pretty well matched with compressor and turbine combinations. While it's not impossible to run one of these performance turbochargers into surge on the wrong engine, it is much more difficult to mess this up than it used to be. Alright we'll jump into our questions and we'll see what we've got in there. If you've got any more, please feel free to continue asking them. OK we've got our first question comes from Danny who has asked, not sure what I missed but turbo lag and turbo threshold, can you explain the difference? Yeah absolutely this is another one of those areas where there really is a huge amount of misinformation and misconception in the industry and I think a lot of this comes down to people using the wrong terminology.
So turbo lag often mistaken for turbo threshold. Very very different. So turbo threshold I would probably refer to this as boost threshold. This is the point, if we do a ramp run on our dyno or we do a full throttle acceleration run out on the road, this is the point in the RPM range where we can first achieve full boost or our maximum boost point. So this is where we've finally go enough exhaust gas energy being provided by the engine to actually spool the turbocharger up and achieve our target boost pressure.
OK so that's boost threshold, and this is often confused with lag, often people call that lag and lag is really the time it takes the turbocharger to spool up, once we're beyond that boost threshold. So let me say for example we've got a turbocharger, let's say our Toyota 86 where we know we've just seen, we've got full boost by about 3400 RPM. Let's say for example we're operating at 5000 RPM. We've just changed gear, we've been at full boost, we've got off the throttle and now we're going straight back to full throttle. So the lag is actually the time it takes for our turbocharger to get back to full boost because obviously when we back off the throttle, our boost pressure falls away completely, we're back into vacuum and there's going to be some finite amount of time that it takes for that turbocharger to spool back up.
And this comes down to a big part is the inertia or weight of the components in the turbocharger. So particularly with larger turbochargers with very large diameter turbine and compressor wheels, there's a lot of mass to those components, particularly in the turbine and it takes time to physically get that spinning up again and this is why we've seen a drive recently from both Borgwarner as well as Garrett to come up with special materials, particularly for their turbine wheel that help reduce the mass of those wheels, this reduces the inertia and helps improve the spool up. Craig has asked, where is the best place for a blow off valve in the pressure side of the system? OK so this is another area that I commonly get asked about. I actually did an Instagram about this a while ago and it got into some pretty heated debate and I think the reason for this is the point of it was missed. So hopefully as you've already taken from today, I don't, I'm not a big advocate of a blow off valve for improving the performance of the turbocharger in terms of boost response when we jump back on the throttle.
Really my take is it's there to protect the turbocharger. So there are two areas we can mount the blow off valve. There's heated debate for both options. One is straight off the compressor, basically we can see this is exactly what Borgwarner have done. It's fitted right on the compressor cover itself.
The other argument is that we want to fit the blow off valve at the point where the boost pressure first starts to back up which is right by our throttle body. In all honesty, I've used both locations on numerous cars, we see both locations used in OE application, my take, it just doesn't matter. The bigger issue is probably having a blow off valve on there that's sized correctly for your application and generally this will come down to physically where you've actually got room in the engine bay to neatly mount the blow off valve and for me that's a bigger driver of where I'll put it. Peter has asked, what are the benefits of variable vein turbos in terms of compressor surge? OK so VGT, VNT turbochargers, we don't really see these used too often on a gasoline engine, very very common on diesel engines where exhaust gas temperatures tend to be a little bit colder. That's not to say they're never used but just not as common.
The variable geometry turbochargers have the advantage that effectively we have a variable area ratio, aspect ratio for our turbine housing, we can move veins in the turbocharger to affect the way the exhaust gas is directed to the turbine wheel. So by opening this up, basically it frees up the flow through the turbine housing, providing the exhaust gas, essentially less energy to drive that turbocharger. So in this application what we could do is open up the veins in order to reduce exhaust gas energy to the turbocharger and help move us away from surge. So that's probably how I'd do that. It's actually the same as the wastegating option though.
We're doing exactly the same with the wastegate, we're opening that wastegate to bypass exhaust gas around the turbocharger rather than using that exhaust gas to drive the turbocharger harder. Andy has asked, have you tried running a turbo in vacuum in shifts by having the throttle pre turbocharger? No not something I've ever done. I would see some significant issues with doing so in terms of the volume that the intake system would then include. Unless you wanna get rid of your intercooler which these days would be pretty uncommon. Maybe it'd work if you're running methanol.
By the time you've got an intercooler, intake plumbing and a plenum in that system, you're gonna have a huge volume of air there, you're probably gonna have some issues around that. Don't know if I see any advantages either. Robin has asked, does it help surge to recirculate the blow off valve? OK so this doesn't really have any impact on surge. We can reduce or eliminate surge with either a vent to atmosphere blow off valve or a recirculating blow off valve. As we can see here, our factory OE style blow off valve which is becoming quite common these days is fitted straight to the compressor cover and it does recirculate back to the inlet.
Now the reason for this is predominantly most OE manufacturers are using a mass air flow sensor. And with a mass air flow sensor, they are very sensitive if we are using a vent to atmosphere blow off valve. The reason for this is when that happens, we've got all of the air goes through the mass air flow sensor, it's being measured it's being metered, the ECU is being output a signal from that mass air flow sensor telling it how much air is entering the engine and obviously the ECU is then injecting fuel to account for that air. When we now back off the throttle, and the blow off valve opens and vents that air to atmosphere, it's already gone through the air flow meter and been metered so it's not making its way into the engine and all of a sudden we end up with big problems with the engine running excessively rich. This can cause stalling, particularly on gearshifts, coming up to a set of lights where we're backing off the throttle.
So that's a bit of a problem, recirculating blow off valves, much better fit for a factory mass air flow sensor. Of course a lot of people want the noise of the blow off valve, that's a different argument, probable for a different day. Digital Ashley has asked what's better for performance, a throttle response, et cetera, vent to atmosphere or divert back into the system to help surge? OK pretty much dealt with exactly that same question. Just remember here again, at least in my own personal opinion, the blow off valve really there primarily for a turbocharger reliability aspect, as opposed to response performance. Digital Ashley's also asked, I have diverters and haven't had the blow off valve before, does that mean surge would be worse with diverters? So if you're using the term as I would use it, diverter valve is just essentially another name for a blow off valve.
I see this applied to some European cars so essentially doing exactly the same thing. Feel free to clarify if that's not the angle that you're going with. Manuel has asked, how hard is it to control boost on a compound setup? I can't speak from personal experience because I personally haven't done this. It does seem pretty daunting, we actually had a really in depth talk to Shane T about exactly this on the Mazfix Nissan S15 at PRI last year. If you are interested in learning, 'cause it was really involved and he did get into the boost control aspect, check that out on our Youtube channel, head over, search for our Youtube channel and search for probably Shane T will probably do it or compound turbo, and yeah you'll learn all about that.
Turns out from Shane's perspective, but Shane is a bit of a guru here, let's be honest, he found it was actually much easier than he had expected it to be. Barry's asked, on most of the setups I've seen run the turbo speed sensor, I noticed the wires are never protected or heat shrinked, just twisted and left raw. Is there any reason for this? Yeah OK good question there Barry. I have only used the turbo speed kits as supplied from Borgwarner at this stage. You are right in that I think the wires are bare.
In the Borgwarner kits these have a nice little piece of extruded plastic conduit over it so you don't actually see them. I think in the Garrett systems they are exposed like you're talking about. So the, at least again I'm speaking just purely from my experience with Borgwarner, it's not a case of just lopping off the wires and terminating with your three pin DTM connector though. My understanding is that there is actually some conditioning circuit in the connector that's provided so that's possibly why, it basically makes it a little bit hard to pull that off and put some DR25 over it for example. Elijah's asked, should we be looking at twin scroll or single scroll for low horsepower low volume application street cars and weekend warrior type projects? OK so I mean the argument for twin scroll is that all things being equal, and generally unfortunately they aren't but the theory is that twin scroll turbochargers for a given size exhaust housing are going to be able to provide better response or better boost response.
I think it's been pretty well proven here. So I mean particularly for a street car, I'd take boost response any day of the week. That's definitely going to make your car a lot more enjoyable to drive. However if you are right on the limit of surge, anything you can do to produce boost at a lower RPM where the engine's less likely to be able to take that additional compressed air, that can put you over the edge into surge. Again, probably pretty unlikely if you're dealing with a mainstream performance turbocharger that's pretty well matched to your particular application.
Comparing there, single scroll to twin scroll, you're gonna get an advantage with the twin scroll in terms of boost response. Peter has asked, can one install a blow off valve which is too large or bleed too much boost or is the goal to bleed all of the boost ASAP? Look I don't think it's really possible to bleed off too much boost. So realistically no you can't have a blow off valve that is too large. Probably the downside there would be the fitting, the size of the blow off valve and physically fitting it in your engine bay. But yeah you're not gonna see a disadvantage from having a valve that is too large but it's also not necessary.
It really just needs to be matched to the sort of turbo sizing, the air flow that you're seeing through that engine. And one of the areas where we are seeing this, a couple of years ago I think, Turbosmart brought out, I think off the top of my head, they called it their Big Bubba blow off valve. And this was designed for some of the pro mod turbo and supercharged cars that are running over in the States, where the mainstream blow off valves that are on the market really just weren't able to vent the huge volume of air that those sort of cars are, or engines are moving. Danny has asked, on a compound setup, will the smaller turbo not get over boosted or over spun by the bigger one? I'm not exactly sure how to word that. It's a good question, it really comes down to understanding how the compound turbo system works.
So really what it is a case of having a wastegate pre your primary turbocharger, or your smaller turbocharger. So if you didn't have that, what you're obviously going to be doing is forcing all of the exhaust gas through the turbine side of that smaller turbocharger. And the problem with that is you're going to end up with high exhaust gas back pressure and it's going to over speed the turbocharger. So that is controlled via a wastegate between the exhaust ports and the turbine on that particular turbocharger. It's important though that that exhaust gas energy is not wasted.
That is then run into the exhaust system between the primary and the secondary turbocharger. So it is a complicated system, little bit beyond the scope of today's webinar realistically and I will put my hand up and say I am definitely not an expert in compound turbocharging. Mikey has asked, do you think a diesel engine would benefit from a blow off valve considering they don't have throttle plates? No not really, there's really nothing, as you've just said yourself, there's nothing there for the air to back up against. The only issue you could get with a diesel engine with compressor surge is under that full throttle application if the turbo sizing isn't correct, running into surge under wide open throttle. But other than purposefully opening the blow off valve, to vent that air under wide open throttle, the blow off valve's really not gonna save us there.
So the blow off valve is really a throttle off device, where the throttle is closed, we don't have a throttle body on a diesel engine, no real use. Neil King's asked, Andrew Hawkins wants to know if twins or single is better for a GTR? Yeah I actually got myself in a bit of trouble with one of our webinars a little while ago. Talking about singles versus twin, he's put out a video that got a lot of heated debate, it's actually a great video and it does a really good job of explaining why for the mainstream there, the single is definitely the option for the RB26. I managed to get myself into some hot water there because I managed to mix up my words and said something along the lines of a twin system was gonna be better, even though at the time I actually had a single turbocharger which I'd been talking about for one of our client's cars sitting on the bench. So anyway, Andrew understands I made a mistake there and I think most people probably know what is superior there by now.
Emmanuel has asked, can a compound turbo setup work on a street car project? Look lots of questions here today about compound turbocharging so maybe at some point in the future we might look a little bit deeper into doing a specific webinar on it. Again I am not an expert on compound turbocharging. Probably the reality is I would say on a street car setup, the advantages or the potential advantage with the compound setup probably wouldn't justify the complexity as well the additional cost. We've now got some really really high quality turbochargers available to us that provide the sort of boost response that we couldn't have even dreamed of five or even 10 years ago so unless you're trying to do something really special, the likes of what Jett Racing are trying to do, they're trying to make 2500 horsepower or more from a four cylinder two litre engine. That's the sort of area where compound turbocharging is going to come into its own.
I can't say there would be no advantage but yeah I think you'd really have to seriously question the complexity versus the benefits. Alright team that has brought us to the end of our webinar. As usual if you do have any further questions on that topic, please ask those in the forum, I'll be happy to answer them there. Thanks for joining us and we look forward to seeing you all in our next webinar.