256 | Do You Need a Blow Off Valve?
Blow off valves and turbochargers have gone hand in hand for almost as long as the turbocharger has been in existence. There’s various theories about the requirements for the blow off valve, including the ability to reduce the thrust load on the turbocharger and prevent expensive failures, and releasing the pressurised inlet charge air when the throttle is closed to help reduce lag or improve spool when the driver opens the throttle. In this webinar we’ll do some investigation of our own and analyse what the data shows us using our Mazda Rx-7 fitted with a Borg Warner EFR8474.
00:50 - How the data was gathered
1:10 - RX7 overview
3:55 - SR86 overview
6:55 - What does the blow off valve do?
13:35 - Ball bearing turbos
14:45 - Applications without blow off valves
15:30 - Data gathering demo
19:05 - Testing issues/limitations
21:15 - Reviewing the data - RX7
26:10 - Reviewing the data - SR86
29:45 - Conclusions
32:05 - Questions
- It's Andre from High Performance Academy, welcome along to another one of our member's webinars. In this webinar we're going to be diving in deep on whether or not you need a blow off valve, what is the blow off valve or recirculating valve there for? Does it aid spool, does it make your turbocharger spool up quicker on a gearshift, which is often the claimed performance benefit of a blow off valve. On the other hand, is the blow off valve there to actually add protection to your expensive turbocharger? What's the deal there, what's actually going on? So we're going to go through a bit of a preamble on what the blow off valve does, how it functions and a lot of that probably is going to be pretty self explanatory anyway, I imagine most people have been around turbocharging for a while, probably already have a reasonable understanding of what the blow off valve is added for. Now we're going to have a look at a bit of a live demonstration here on our FD RX7 of gathering some data and what we've done with our FD RX7 is we've gathered some data already with the blow off valve blanked off and then we've gathered some data with the blow off valve open. Right now there is a blow off valve fitted to the system and we're going to have a look at a comparison of that data.
Broadly for this demonstration, the FD RX7, if you haven't followed the build of that car, it runs a stock standard Mazda 13B engine. It is fitted with a Turblown turbocharger kit so this is equipped with a Borgwarner EFR 8474 turbocharger which is a ball bearing turbo. Now we've got quite a bit of instrumentation on this engine and this vehicle which is why it works quite well for this particular demonstration. In particular we've got turbo speed so we can see exactly what the turbocharger is doing. One of the common things I hear discussed when there are arguments about the pros and cons of blow off valves is that without a blow off valve, the back pressure will build up, or the pressure will build up and actually stall the turbo and stop it spinning.
Now I already had an opinion on that as do probably most people but we've actually got some data so we'll see what goes on there. So the other thing we've got is some data from our other SR86 racecar which has a little bit more data in some other ways, we've got a pressure sensor pre throttle body there so we can see exactly what's going on. Let's have a quick look first of all, if we jump across to my laptop screen. This is a shot of the turbocharger fitted to the RX7. So the Borgwarner EFR 8474 actually uses a built in recirculation valve which we see with a lot of late model factory turbocharged engines.
It equips the blow off valve directly in the compressor cover. It's nice and compact, keeps everything neat and tidy. This actually also comes to another part of the argument with blow off valves is blow of valve location. There's as many people arguing for the fact that the turbocharger, the blow off valve should be fitted as close to the turbocharger as possible, as we have people arguing the other side of that, that the blow off valve should be fitted as close to the throttle body as possible. And honestly, if we look at the OE applications out there, we kind of see about a 50/50 mix.
The Borgwarner, obviously their turbos, they've decided that on the compressor cover is the neatest solution but believe me that's definitely not the only solution. I'll just mention while we are looking at that as well, this isn't the conventional Borgwarner recirculation valve, this is the Turbosmart version, it's a Turbosmart Kompact recirc valve. It's made for the Borgwarner turbocharger. It just replaces the factory Borgwarner plastic recirc valve but the reality is, this is a little bit more robust and it's aesthetically a little bit more pleasing but performance wise, it's really not going to give you a significant advantage over the factory Borgwarner product. Now on the other hand, with the SR86 that we're going to have a look at a little bit of data from, I just want to quickly show you that as well so we'll dive over to, this was while it was during the build but just so you can get a bit of an idea, we've gone completely the opposite way with this.
So we've got our Garrett GTX3076 Gen 2 turbo over here and it's a little bit hard to see, the blow off valve is actually fitted pre throttle body so there we've got both options. Now again, there are some heated debates on the pros and cons of this. One of the arguments for fitting the blow off valve right at the turbocharger is that if we vent the air pre throttle body then it's already been cooled by the intercooler so we're needlessly adding heat soak to the intercooler. Fair enough, I can't argue with that. The other aspect where people argue the pros of adding the blow off valve closer to the throttle body is that's the place where pressure is initially going to start building up.
So getting it right there is going to allow us to vent that air as quickly as possible, preventing pressure building up. And again, fair enough that might make sense although pressure in a system is always going to be equal so yeah there's arguments for and against. My personal opinion has always been that the reality, it's probably just about a coin toss. I tend to go with where it's most convenient to mount it but that's just me. Now we've actually got that section of pipe off the SR86 at the moment so I just wanted to show you that.
So this runs up to the throttle body, this comes from the intercooler. So we've got our Turbosmart RacePort blow off valve there running AN fittings as well just for some added reliability and we've got an HV Electronics four bar pressure sensor as well pre throttle. So this is interesting because we see this with a few ECUs. With the MoTeC M150 that we're running it actually does run, or it gives you the option if you run a boost pressure sensor as opposed to a manifold pressure sensor, to run the boost control strategy on the pre throttle body boost pressure as opposed to what we've got in the inlet manifold. Getting a little bit away from our target discussion but there can be some advantages in that, particularly when we are at part throttle as well, controlling that boost pressure pre throttle plate.
Still of course relies in the manifold absolute pressure sensor for the fuel and ignition control there but just thought I'd show you that. Actually one other aspect 'cause we will be looking at this data as well, not a usual sensor but you can see this little guy here on the top of the RacePort blow off valve, so this is a blow off valve position sensor so this is a contactless hall effect sensor. Yeah not particularly common that we run them on a blow off valve but these happen to use the same caps as the wastegates so it's really easy to integrate that and we thought why not, because as we'll see it gives us a little bit more data to see exactly what's going on there. OK so now that we've got a bit of an understanding of the back story there, what we're going to be looking at, we really want to talk about what the blow off valve is there to do. So there are again two quite separate angles that we hear argued for the blow off valve, or maybe not argued but let's say discussed anyway.
The first is that the blow off valve is there to aid the boost response on a gear shift and the idea there, or the general concept is that if we didn't have a blow off valve and we close the throttle body, the turbocharger's still spinning at thousands and thousands of RPM, moving air, it's a centrifugal compressor of course, moving air, suddenly that air's got nowhere to go, we see the pressure in the intercooler plumbing pre throttle body spike. That air's got nowhere to go, it's going to come back through the compressor, you actually hear this as a chuffing sound, basically this is compressor surge that we're hearing. And this can significantly slow down the compressor, the turbine wheel and in turn we've actually heard arguments that it'll spin backwards or completely stop, we'll see that that's not actually the case but anyway, that's the argument, it's going to slow the turbocharger down so that when we get back on the throttle, it's actually going to take longer for the turbo to spool up. Now, interestingly the complete other angle of this is that if we vent all of that pressurised air with a blow off valve, then once we get back on the throttle, the turbocharger's got to spool back up and it's got to re pressurise the intercooler, the intercooler plumbing and all of that pipe work up to the throttle body before we really get back to where we were. So again not a lot of clear direction on this, arguments on both side of the fence.
Now my general, actually I'm just going to plug my laptop in before we go flat here which would end up being a pretty poor end to our webinar so just bear with me for a second there. Yeah so my general sort of idea behind this was probably I favoured more that second option that the blow off valve probably didn't actually aid us a lot on boost response and I kind of figured that it was probably just about a coin toss there but I was never a real big proponent for the whole aiding boost response with a blow off valve. I certainly didn't buy into the fact that the turbocharger was going to stop or spin backwards. It's got a lot of intertia when it's doing 70 to maybe 100,000 plus RPM. Stopping it in an instant is probably pretty wishful thinking.
OK so that's one area, the performance advantage in our boost response when we get on and off the throttle. The other aspect though that I think is these days much overlooked is that the blow off valve also serves a purpose to help protect our turbocharger. And this is from the thrust loading in the turbocharger. So we'll jump across to my laptop screen, I'm sorry, I've managed to probably find the lowest quality images on the internet but hopefully we'll still get the idea. So this is a conventional journal bearing turbocharger.
So obviously we've got our compressor wheel over here on the left, we've got our turbine wheel on the right hand side of the screen. So when most people think about the bearings, they think about the actual journal bearings themselves so we've got two bearings that support the turbocharger shaft and these are really key to the reliability of the turbocharger, they're working to very fine tolerances there and as I said, these turbochargers are spinning well in excess of 100,000 RPM so you want to know that everything's right there. The easy part that's easy to overlook though is there are also axial forces in play here which are these forces here that sort of go in and out with the turbocharger. So as the turbocharger produces boost, the compressor wheel is trying to essentially climb out of the turbocharger so we need a thrust bearing to help support the turbocharger and prevent that so that's what this thing in here is, this is a thrust bearing. So again, this is a conventional journal bearing turbo.
These days we're seeing a lot more ball bearing turbos, so a lot of this now is less relevant. So if we have a look at a couple of options for a thrust bearing. What we saw with a lot of lower performance turbochargers from OE manufacturers was the use of a horseshoe or 180° thrust bearing like this. So the actual surface that does the thrust support is this surface in here. So this kind of slips around the shaft, it'll be secured into the turbocharger core and there will be a surface on the shaft, if we come back here.
We can sort of see it here, there is a flat surface on the shaft that will operate against the thrust bearing. So that's all well and good, the problem is with the horseshoe or 180° thrust bearing is we're clearly missing quite a large area that could also be helping to support the turbocharger under thrust or axial movement. So these tend to wear quite quickly, particularly if you take a factory turbocharger like this and you start increasing the boost pressure. There simply isn't enough thrust support for the increased boost pressure and all of a sudden the turbocharger develops end float, then the compressor wheel starts to contact the compressor cover, turbine wheel contacts the turbine housing and basically it all wears pretty quickly. So at this point when journal bearing really was the only option or ball bearing turbochargers were prohibitively expensive, one of the options was to move to a 360° thrust bearing like this.
So involves a little bit more complexity in terms of how the turbocharger is assembled, obviously we can't just slide this over the top of the turbo shaft. But this does give us, understandably a lot more surface area for thrust support. However, there is still a limit realistically to how much support can be offered or how strong these thrust bearings are. And using a blow off valve, the argument there is that the blow off valve helps reduce the thrust loading that occurs during surge when the air is basically backed up and goes back out through the compressor cover and that can be quite destructive. Particularly if we actually run a compressor wheel into surge which is off to the left hand side of the surge line under wide open throttle or full power operation, because of the higher loading that occurs at that point, that can very quickly do a lot of serious damage to your turbocharger but without a blow off valve we are still increasing the thrust loading in the turbo.
So ball bearing turbos, why is this important? When we move to a ball bearing turbo, and I'm sorry I don't have a nice simple little graphic of this, the way the ball bearing braces work is that they actually provide axial support for the turbocharger as well so they basically do away with the need for a thrust bearing and they provide the thrust support as well as supporting the shaft radially. They're also significantly stronger. Now I'm not a turbo engineer here, quoting some numbers that I recall from some time ago from Garrett on ball bearing turbochargers, their claim, and I'm not sure if this still stands the test of time was that they were around about 40 times stronger in thrust compared to a journal bearing turbo. Now again take that with a grain of salt but that gives you some idea, even if it was the magnitude of half of that, significantly stronger in thrust. So this comes back to this argument, do we need a blow off valve because now we don't need that blow off valve strictly for supporting the turbocharger or reliability, making sure the turbocharger thrust bearing isn't going to wear out.
There are also some instances where I'll point out that blow off valves have not been used. Competition cars, even back from the era where journal bearing turbos were the norm, it was quite common for blow off valves to be not used in competition applications and there's some specific types of motorsport such as rally where the anti lag system essentially for the anti lag to be effective really dictates that we can't have a blow off valve or at least it can't be functional when the anti lag strategy is active. So there's a bit of food for thought, basically no firm conclusions but a bit of information around the idea of blow off valves. So what we'll do now is we'll get our RX7 up and running and I'll just show you how I gathered some data here in our Adaptronic software so I'll just allow everything to sort of warm up for a little bit before we get into this test. And what I'm going to do is I'm going to come, if we jump across to my laptop screen we're going to come to about 4000 RPM, so what I wanted to do was get to a point where we were above the boost threshold so in this particular instance with our 8474 EFR turbocharger, on a ramp run we're all in by about 34, 3500 RPM.
So I wanted to be somewhere past there where we've got sufficient exhaust gas energy to spool the turbocharger up. At the moment we are running on the wastegate spring pressure so that's going to get us to around about 200 kPa so we're going to be running down around here in our fuel table. So information that we've got on this particular car, we've got our turbo speed, so actually if we come over here, I'll give it a little rev, the turbo speed sensorsdon't generally tend to pick anything up at idle but at the moment we can see, holding up at 2000 RPM we've got about 8000 RPM of turbo speed so the poor little thing's just barely ticking over. So what we did here is, I'll just quickly show you, I've got some logging functionality in the Adaptronic software. We've got the ability to log straight to the laptop which is great for tuning purposes because we don't need super high frequency logging.
I don't have the specifics but just looking at my log file, looks like the logging rate there for the PC Logging may be somewhere in the region of 15 to 20 hertz. Probably nothing strictly wrong with that but what I wanted to do was get a slightly higher rate of logging so we can also bring in internal logging. So we'll have a quick look at that setup here. And for this particular log file I chose to log at five millisecond intervals which is 200 hertz so that's pretty quick and the key parameters we're looking at here, manifold pressure, so this is after the throttle body, so the actual inlet manifold pressure. We're looking at our turbocharger speed so we know exactly what's happening to the turbocharger.
Engine RPM, just so we can get a bit of a picture on what's happening with the turbo speed, sorry the, yeah the whole picture there. And what we're going to do is basically replicate as accurately as we can, a situation where we've got a gearshift or a throttle lift. So I'm relegated to the dyno so I'm doing what I can here and I will talk a little bit about some of the limitations of this test as we go through this and how this could have influenced our results but what we're going to do is go to 4000 RPM. We'll go to full throttle, we'll allow the boost to settle up to the wastegate spring pressure and I'm just going to go through a series of basically in and out of the throttle as quickly as I can. This logging as well I'll just point out, if we go back in here, it will log basically any time we're above 3000 RPM and it will continue logging for a period of two seconds.
I don't need to worry about this 'cause I've already got the log files that we're going to have a look at in a second but I just want to show you how I'm doing that. So without further ado, let's get ourselves up and running on the dyno here. I'll get our RPM stabilised here at 4000. And what we're going to do is we'll go to full throttle, allow our boost to stabilise. OK so that's the sort of testing I was doing there.
Now there's a few problems with this, it's really difficult to do really consistent back to back testing here. Because there's a few variables that will come in. First of all, the longer we're doing this, the longer we're at 4000 RPM and we're going in and out of the throttle, the higher our exhaust gas temperature will be. So this affects the amount of exhaust gas energy that's available to the turbocharger so it's very difficult to get an absolute repeatable result here. We need to factor that in.
The other aspect here is I'm not a robot so it's really difficult for me to consistently get on and off the throttle, of off and on the throttle for exactly the same duration. And of course if we've got one section where I'm off the throttle for let's say 400 milliseconds and one where I'm off the throttle for 200 milliseconds, we're not comparing apples with apples so we need to factor all of that in and don't worry, I will talk a little bit more about how that works as we look at our data. But that test that I've just done there, that is with the Turbosmart recirc valve fitted, so I'll just mention how we got rid of that out of the system. So this is another little product that Turbosmart offer for the likes of those Borgwarner turbochargers. Usually they're offering this if you want to fit an external blow off valve from their range and you don't want to use the factory recirc valve on the compressor cover.
So all it is is a little CNC machined plug, it comes with a couple of o rings that seat against the relevant surfaces in the turbocharger. So for us this is nice because it literally turns the job of removing the blow off valve and blanking it into about a two second operation or a couple of minutes at the best so that's how we did that. So anyway we can log all of this data in Eugene, the Adaptronic software. I'm personally not a massive fan of the log viewer, it makes it difficult to do the sort of analysis that I wanted to do here. So they do also offer a log converter, it logs natively in Adaptronic's .ALG file which is native to, bespoke to the Adaptronic software but you can convert it to a .CSV which then opens up your options to explore it in the likes of MegaLogViewer HD which is exactly what we're going to do so we'll head across to MegaLogViewer HD.
I am zoomed a fair way in now so let's just have a look back out here, we'll sort of have a look at our two pieces of data that are overlaid. So in this case the solid line is with the blow off valve in and our lighter lines are with the blow off valve out. So again really difficult to get a good comparison in our data here but I kind of got the best piece of data that I could here so the red line up the top is our RPM. No big surprises there, for some reason my presentation pointer is not working which is ideal, seems to be a bit hit and miss on the MegaLogViewer HD software. What we've got here is the yellow trace is my throttle position.
So you can see as it drops from 100%, I'm not quite off the throttle and back on it for exactly the same amount of time and yes that's going to affect things. Also how do we align the data is important. So we'll have a look at that in a second. The finally our green line down the bottom, this is our turbocharger speed. So at the moment I'm sort of focused on the area where we get back on the throttle.
For the purposes of this, let's just start by, I'm trying to align this data, it's really fussy here, trying to align the data where I actually jump off the throttle 'cause this is important. Does the turbocharger stop, does it slow down? And here what we can see is that if I align the data so that the position where I get off the throttle is pretty much consistent, we see first of all our green trace here for inlet manifold pressure, almost identical, they basically follow exactly the same path so no big surprises here because nothing post throttle body really should be affected by whether or not we've got a blow off valve in the system so fair enough, didn't really expect a massive change here. If we look at our turbospeed though, the interesting thing is we really don't have any significant effect on the turbospeed from our blow off valve. Interestingly the blow off valve in which is the solid line actually slows down slightly more but I've put that down to a test discrepancy more than anything. The point I've got my cursor on here we're 59,000 RPM versus 57,400 RPM so it's 1000, 1500 RPM, I wouldn't draw too many solid conclusions from that.
Now what I'll do is I'll just try and get back, as I said obviously I wasn't off the throttle for exactly the same period of time so I'll just try and get back to the point where, yeah it's probably closest right there, where we sort of open the throttle at the same point so at this point here, before I open the throttle, our manifold pressure is basically identical, interestingly now I align the data this way, our turbocharger speed is identical within about 100 RPM so we're really at exactly the same situation here. Open the throttle, looks like my rate of ramp in in the throttle marginally different but again you're going to have to bear with me here, I'm not a robot so I'm doing the test as well as I can. But what we want to look at here which is really interesting is immediately I go to full throttle, the manifold pressure jumps up to atmospheric, no big surprises exactly what we would expect. So as soon as that's out of the system, we don't have any vacuum so we're straight up to atmospheric and this is where the boost starts to ramp up. But if you look at the boost and the rate that the boost ramps up, I mean I can't split hairs here, it's basically identical.
As we get further through the test, we actually find that the lighter line with the blow off valve blocked actually climbs above and so does our boost pressure. Kind of have to disregard that, I'm going to put that down to the fact that the tests were of different lengths and this comes down to what I was mentioning about exhaust gas temperature and adding a little bit more energy to the turbocharger. So really what we want to focus on is that initial drive to get the turbocharger back up on speed. And for this particular test, I personally can't pick a significant difference from the blow off valve in or the blow off valve out. Now the other argument that the blow off valve's essential to stop the turbocharger from slowing down significantly, obviously we can see here even at our minimum speed, in both tests we're still at 56,000 RPM so no, the turbocharger definitely doesn't stop regardless whether we have a blow off valve or not so that much is really clear there, the turbocharger's definitely not stopping and it's absolutely certainly not spinning backwards.
The other argument though is we need the blow off valve there to prevent the pressure spiking so yeah fair enough and unfortunately for this particular test I do not have pre throttle body boost pressure so I can't see the difference and show you the difference between the blow off valve in and out for that particular channel on our RX7. However we do have a little bit of information around this from our Toyota 86 racecar so we'll head across to our MoTeC i2 Pro and I just want to get rid of an overlay lap here and what we want to do is come down to our engine data and our boost. So there's a lot of data on here and really what we'll do is we'll just zoom in on one small segment which is actually relevant to what we're interested in here. So let's do exactly that. So we've got our throttle position here, we're going through the gears which is our RPM up here.
Sequential gearbox, no need to lift the throttle so we see the boost is dropping down a little bit here on each of our gearshifts. So this is boost pressure and gauge pressure. So we're seeing around about 90 kPa gauge pressure so around about 13 psi, thereabouts. So pretty consistent there as we ramp through. Another piece of data that is important here, the next channel down is our blow off valve position, so we've got that blow off valve position sensor that we already looked at, we can see the movement of the blow off valve.
What I want to show you here or concentrate on here is this piece of data here where I step off the throttle at the end of the straight and get on the brakes. So we see, that's the point I've got highlighted right there, the point that I jump off the throttle. So two pieces of information to take in here, first of all we can almost immediately see the blow off valve begin to open. And remembering the blow off valve on that particular application, it is fitted right by the throttle body. So the argument we often hear about getting it as close to the throttle body as possible, we can't get it any closer than that.
So the blow off valve is opening, we're seeing it's doing its job here, it's about three mil off the seat. Interestingly that isn't scaled but we can see that it is, the relative movement, it is open. However, what we can see is that there is this spike in our boost pressure. So even having what is quite a large blow off valve there on our SR86, our peak pressure there, looking at it, 111 kPa gauge. So we've spiked about 20, 22 kPa above the boost pressure that we're running as we close the throttle body, despite the fact that that blow off valve at this point is just about wide open.
One consideration here, something I do need to address, if we actually look at our boost pressure channel, this is only being logged at 10 hertz so probably not fast enough to show the true impact but actually if we had it logged at a higher rate we'd probably actually see a worse situation. If we look at the samples, if we look at the actual samples or every single sample, we've got the highest point is show there and the data basically draws a straight line between those or a line of best fit I should say between those. So there's a good chance that our actual boost pressure pre throttle body could have spiked higher. So that just shows that the blow off valve, the purpose of that is just to show that the blow off valve is not basically a complete solution, we're still going to see some pressure spiking. Now of course I don't have this data for no blow off valve and by all accounts I would expect yes, it's going to be higher pre throttle body with no blow off valve in the vehicle at all.
Alright we're going to move into some questions and answers in a second so this is a perfect time to let you know, if you do have questions, please start asking them. I just want to kind of go over a few of the caveats with the testing procedures I've gone through and what we can really take away from this. So the first things that I've already talked about, very hard to get a completely accurate back to back test with and without the blow off valve so we've got a few things that creep in there. Despite that, looking at the results there, what we've got is not particularly compelling one way or another, we're not really seeing any significant advantage from the blow off valve in terms of boost response which is what I wanted to get into because that is the argument we so often hear. However, that being said, this test is still being done at a relatively modest boost pressure, we're at one bar of boost so would we see the same results if we were running three bar of boost, 43.5 psi? Possibly but I'd actually suspect that the situation would be significantly different.
We've got a lot more energy in terms of inertia in the compressor, the turbocharger, the turbine wheel etc at that higher boost pressure which infers a higher turbine speed. We would need the blow off valve to vent a lot more air so we're probably going to see a significant difference in the boost pressure build up pre throttle body when we close the throttle, both with and without the blow off valve and that could significantly affect our results. So I can't sit here and say this is a all encompassing set of data that shows everything you need to know about the blow off valve, this is one data point on one vehicle. We need to keep that in mind and understand the data for what it's trying to show us. Ultimately, do I encourage you to get rid of the blow off valve? Absolutely not and particularly if you are running a journal bearing turbocharger then I would be fitting a blow off valve every single time.
As I mentioned, one of the core aspects there is the ability of the blow off valve to protect the turbocharger from thrust loading. Right let's jump into our questions now, if you've got any more questions, please keep those coming. OK our first question comes from Brendan J who's asked, do three port blow off valve bypass valves help modulate manifold charge pressure? I'm wondering this as my daily XR6 turbo ute has a stock bypass but tuned to over 16 psi and the stock bypass valve would be for seven psi. Like for bleeding pressure out of the manifold charge. Not 100% sure I've got the angle of your question here but let me just go through this.
So this is a problem we see with a lot of factory blow off valves where the blow off valve will be fine for a factory application but as soon as we start increasing the boost pressure, the blow off valve may leak. Now that's not always the case. What we need to understand is that as you increase the boost pressure, yeah the pressure trying to open the blow off valve is going to increase obviously. Using my little prop we've got here, we're going to have more boost pressure in the intercooler pipe trying to lift the blow off valve off its seat. But we've got spring pressure closing that blow off valve and we've also got boost pressure going to the top of the blow off valve trying to close it so not just because we're increasing the boost will necessarily mean that the blow off valve, a factory blow off valve is going to leak, they don't all do it but definitely some do.
The early Mitsubishi Evo blow off valves were a classic example for that. So three port blow off valves, I'm thinking you're talking here about a blow off valve that vents both to atmosphere as well as plumbs back. There are a few considerations around this. And this is actually something that's quite good to talk about as well. Blow off valves still serve a different purpose on factory vehicles which use a mass airflow sensor.
If we don't have a blow off valve, we are going to get compressor surge when we back off the throttle and what this is going to result in is the potential for air to basically back up and come back out through the compressor and go back out through the airflow meter and this can cause havoc with some airflow meters, depending on the style of airflow meter and proximity to the turbocharger, causes all sorts of problems around stalling, when we come up to a set of lights and take our foot off the clutch, or put out foot on the clutch. So a recirculating blow off valve in that instance can be really beneficial to remove that surge and basically keep everything operating nicely. That gets to our other question about the type of blow off valves that both vent to atmosphere and recirc, this is really more around noise. Problem with a vent to atmosphere blow off valve for an airflow meter car is when the blow off valve opens and vents to atmosphere we are venting metered air that's gone through the mass airflow sensor and been measured and reported to the ECU and they're now venting that air out into the atmosphere, it's no longer going to the engine so what happens is that the ECU doesn't know that that air is not going into the engine, it's scheduling fuel and less importantly ignition timing for that air so we run horribly rich when that blow off valve is vented to atmosphere. So considerations there.
DTB has asked, how much does blow off valve placement matter for performance response and turbo longevity? My motor's the VG30DETT. Well I did cover this during the webinar, there's arguments on both sides of the fence. My personal opinion is that you're probably best to consider the logistics of your engine bay layout and where you can physically mount the blow off valve. Where it's easiest to mount it, that's probably actually, in most instances, going to make more difference, make your life easier rather than the difference we actually might see in the performance and response there. Barry's asked, don't you think the airflow limit of the build in EFR blow off valve is low for a high horsepower car? That's a good question Barry, I don't know the answer to that.
I would like to think that Borgwarner, given that they know that the EFR, in this case the 8474 is designed to produce, 700 to 800 horsepower, I would expect that they probably engineer the blow off valve to suit but you're absolutely right. Particularly in comparison to the likes of the RacePort, the blow off valve diameter is significantly smaller so ultimately that will affect the flow. Now this again comes down to how would this pan out at higher boost levels? Probably at the boost level and airflow that we're at at the moment, the size of the blow off valve's probably a moot point. When we go higher in the boost it may become significantly more important. And this is why we probably want to re run this test at higher boost levels, probably use our SR86 for that part of our testing a little bit further on so yeah it is a consideration but I don't have the answers for you there Barry.
Wheezer has asked, what are the effects on a data log with a blow off valve open too little or too much on a MAP based centrifugal supercharger setup at idle? OK effects on the data log, well so first of all what we need to understand with a centrifugal supercharger we're in a different situation because the supercharger or compresor speed is fixed, it's driven by the crankshaft speed so whether or not we have a blow off valve, it's going to have zero effect on the compressor speed. We do still have thrust loading to consider with a centrifugal supercharger. Really the compressor section is absolutely no different to a turbocharger in that side. So probably the bigger issue with blow off valves, my experience with centrifugal superchargers is you are going to need a blow off valve that vents back into the intake system pre supercharger. Otherwise, it's going to drive you crazy with the amount of noise that it makes because obviously it's going to be venting a lot, particularly at high RPM and closed throttle, you also have it venting at idle.
But the effects on a data log, yeah I'm not really sure exactly on your angle there because that really wouldn't be a good comparison for our test here. The purposes of a blow off valve on a centrifugal supercharger, possibly could consider to be a little bit different than on a turbo application. Brendan has asked, would a Bypass valve help with turbo response, say venting pressure back into the turbo inlet on a long-path stock turbo setup, than say, aftermarket, venting back to a 4" short path turbo inlet? Without doing a specific test like that it's really impossible for me to say. There are arguments that venting the air back to the turbocharger inlet and directing the air so it's basically driving towards the compressor can be beneficial. My gut feel is probably if there was any difference there it'd be hard to measure but I haven't done that test so I can't confirm.
I would probably not be too worried about going to too much trouble trying to achieve this, I'd probably do what you can easily fit and what's going to be easiest to plumb in your engine bay because sometimes the tight confines of an engine bay, that can actually drive your decision more than trying to get the ultimate perfect solution. Ken has asked, so from what i can gather is that if you're running a journal bearing turbo, a blow off valve is probably a good idea, but if its ball bearing turbo probably not so much? Yeah so I need to be a little bit careful here Ken, yes 100% journal bearing turbochargers I would highly recommend a blow off valve for protecting the thrust loading on the turbocharger. But again our data point at the moment is one data point at a relatively low boost pressure so I can't really say what's going to happen at higher boost. It is pretty unlikely though that there's going to be any damage from thrust loading on a ball bearing turbocharger just because of the added strength. Bananaman has asked, can you please explain the terms recirculating valve vs blow off valve.
Look they're really two terms for exactly the same thing. We see in a lot of OE applications, the blow off valve is referred to as a recirculating valve, because it plumbs back it recirculates that air back to the turbocharger inlet. A blow off valve, as far as the aftermarket lingo goes, normally what's referred to as one that would vent to atmosphere but these days we have options that both recirculate or vent to atmosphere or sometimes do both. Red21's asked, what type of blow off valve is preferred, piston type or diaphragm and what are the pro and cons? Good question there, that's actually something I probably should have touched on. So the problem with some of the piston style blow off valves is that we are relying in the fitment between, or the clearance between the outside of the piston and the inside of the CNC machined alloy housing for a seal.
Now that's not all of them, some of them such as the Turbosmart blow off valves do include some seals in there so that's not an issue but on cheaper blow off valves this can be a problem. It'll be fine when the blow off valve's new but as it wears it can actually introduce a source of a vacuum leak and sometimes a boost leak as well, depending how the piston seals to the base of the blow off valve. So my personal preference is a diaphragm style, that insures that we have no chance of a vacuum leak in the top of the blow off valve. Manitou Black's asked, while blow off valve near the throttle body is best, have you any experience with the internal blow off valves on the Borgwarner EFR turbochargers? I'm going to guess that you haven't been in this webinar very long Manitou Black because that's exactly what's in the engine bay in front of us. I've talked about this aspect as well and honestly I don't know if there's a solid argument that near the, the blow off valve near to the throttle body is best but again, I'm not going to regurgitate the information I've already given out earlier in the webinar.
Green Addiction Garage has asked, do you expect higher boost pressures would give you a different result for turbo speeds or do you think the results would remain fairly consistent regardless of boost pressure? Look right now my hypothesis on this is that yes I think we would see a greater difference at higher boost pressures. But I want to test that for my own purposes. I've been, I have gone into this with my own ideas on what I was going to see which have been largely borne out but I have been trying very carefully here to be as objective about it as I can and use a scientific method. As I've mentioned, difficult for me to do accurately and I can't say that this test data is conclusive and yes, finally I would expect the results to potentially be different at higher boost. Thomas asked, would a journal bearing turbo be more susceptible to damage as opposed to a ball bearing turbo from compressor surge? Yes, for exactly the same reasons the thrust strength is not there.
We do need to understand that compressor surge under load as opposed to getting off the throttle can do damage to the entire turbocharger assembly so it's not just the thrust bearing, it's actually a very damaging situation. Now that more gets down to turbo sizing for your engine application though so slightly different type of surge. Richard has asked, thoughts on anti surge compressor housings? Yeah we've seen these are quite common now and I do believe there is an advantage in the anti surge compressor housings. Talked with a colleague of mine as well about whether or not the anti surge compressor cover on the EFR could have affected the results and again yes, absolutely it could have so we see these are so common these days on performance turbochargers and yeah I do think there is an advantage with those. Caleb has asked, have you had any testing experience with testing bypass valves and blow off valves on similar systems? Why do OEM manufacturers prefer to use a diverter valve? Will changing to an atmosphere valve hurt? OK so the primary drive here from an OE manufacturer's perspective is twofold.
The key one is emissions. Very very difficult, if not impossible on a mass airflow sensed system to use a vent to atmosphere style valve. Because of all the problems I previously discussed. So yeah changing to an atmospheric venting valve is going to be a real problem for that sort of system. The other aspect though for OE manufacturers is they have to consider noise, vibration and harshness in all of their testing and while we in the aftermarket might love the sound of an atmospheric venting blow off valve, or for that matter, compressor surge, that chuffing noise you hear, so common the WRC rally cars with their anti lag strategies when they're getting on and off the throttle.
That's not what most people want in a passenger car so the OEs go out of their way to engineer the system to be as quiet as they possibly can. Mathew has asked, as far as pressure sensors pre and post throttle plate is that beneficial for diagnosis and data logging? Also do you have any information as far as sizing wastegates? Can you have too big of wastegate? OK the wastegate question is probably a little bit beyond the scope of our webinar, we try and keep these webinars really focused on the topic that we are discussing. Undoubtedly a lot of information out there from the likes of Turbosmart if you want to research that for your own purposes. Pre and post throttle body pressure sensors, no strict requirement to have these. It's a strategy that some ECUs employ for their boost control.
So what happens here is that let's say our boost target is 200 kPa, so if we don't have a pre throttle body boost pressure sensor then the ECU is trying to manage 200 kPa of boost pressure in the inlet manifold. That's going to be very difficult to achieve at very low throttle openings. However if we look at what's happening pre throttle body, when we've got the throttle body closed maybe 50% or more, we're actually going to find that the boost pressure pre throttle body is significantly higher than what we've got post throttle body so it's just a different way of controlling our boost pressure, getting more stable boost pressure and better control of our boost. Not essential, just a different way of going about it. d T b has asked, if a stock motor uses a recirc valve and it dumps behind the MAF sensor shouldn't this lead to air escaping through the air filter and cause a rich scenario? OK so if it dumps behind the mass airflow sensor, in other words past where the airflow has been metered, this shouldn't be a problem but there are some other consideration, we need to think about how that air is being reintroduced.
So if we are bringing the air back in at a 90° angle, it's likely that as that comes in, particularly if there's a high volume of flow, we could still end up with some air going back towards the mass airflow sensor. This is going to be driven largely by proximity to the MAF sensor and we will still get that rich scenario. So this is why if I'm in this scenario, I will try and angle the recirc valve back towards the compressor, away from the mass airflow sensor just to give us the best chance possible of success there and getting rid of those rich on lift situations. AudiA4 has asked, how often do you suggest cleaning or lubricating a blow off valve? That's a good question there, probably something that is often overlooked. It's going to depend to a big extent on the condition of your engine and how well the air through the engine is being filtered.
Particularly if you've got an engine that is maybe getting a little bit on in its years and you've got a rocker cover, sorry a PCV valve that is venting back into the inlet track, you're going to end up with oil in the system and that's going to vent out of the blow off valve. I mean it's not essential that that means it's going to need to be cleaned but that can be a source of getting dust and dirt then stuck to the blow off valve which then can create wear. I can't really give you a one size fits all answer on what sort of timeframe you should be looking at for blow off valve maintenance though, be very dependent on your specific application. RPSS has asked, the position of the valve influences this data, say that the valve should be at a 45º instead of 90º does this influence? I can't tell you specifically because again I haven't done that exact test. My gut feel on this situation though is going to be a no from me.
The reality for this is again it comes down to understanding that the pressure in a system is always going to be equal so we're going to have pressure against the back of that blow off valve seat when it's closed, when it opens it's going to allow an escape path regardless whether this is at 90° or 45° to the flow. Alright thanks for watching there, that has brought us to the end of our questions and for our members, if you do have any questions after this webinar has aired, then please ask those in the forum and I'll be happy to answer them there. Thanks for joining us and hopefully we'll see you again next week.