136 | How Altitude Affects your Tune
Barometric pressure affects the way our engines operate and modern EFI offers the ability to help maintain a consistent tune as conditions change. In this webinar, we’ll discuss the effects of barometric pressure on the engine performance, see how the turbocharger performance is affected by altitude, and look at some options for dealing with air density changes in our tuning.
Guys, it's Andre from High Performance Academy. Thanks for joining us for today's webinar where we're going to be delving into some of the aspects surrounding how we can account for altitude changes in our tune. Now this is a topic that's become quite a hot-topic, I think, recently, and I guess we got a little bit more involved during our recent trip to Pikes Peak, where we were involved with tuning a vehicle that needed to contend with really large changes in altitude. I think other than that, probably through the majority of my career, altitude is something that I probably haven't really given a huge amount of consideration to, and I think that's probably something that could be also said for a majority of tuners out there in the industry. Unless you're really in a situation where the cars that you're tuning are going to be undergoing really dramatic changes in altitude, it's probably not something that's going to be crossing your mind too frequently, so I think it is often overlooked.
Now what we'll start with is why altitude has any effect on our tune at all. The key point here is that as we move from sea level where our standard air pressure is 101.3 kPa, as we move up in altitude what we find is that our air pressure decreases. What this means is that our air density is lower, essentially we have less oxygen available in the air, and it's that oxygen that's so important to the way our engine runs, so lower air density means that it provides less air mass to the engine. Now this is an important differentiation, I think even a few tuners that are fairly experienced still overlook is the difference between volume and mass. So when we're converting between a volume, so if we're talking volume of air, we might be talking in cubic feet per minute for example, that's a volume flow measurement, and really it's the mass of air that's the important part, and in order to convert from a volume flow of air entering our engine into mass flow, which is the important part the ECU needs to know and understand, we need to account for that air density and that's the part that's changing as our altitude changes.
Now while I am talking here predominantly about altitude changes, that's not to say that we aren't going to see air density affect our tune, even if we are sitting at a fixed altitude, let's say sea level for example. Have you ever watched the weather report, you'll notice that the weather is always talking about the atmospheric pressure, so even at sea level we are still seeing changes in our atmospheric pressure, albeit less than what we may expect to see as we drive from sea level to the top of Pikes Peak for example, we're at 4,100 meters, so in order to maintain a consistent air-fuel ratio, the ECU needs to be able to account for this changing air density. Now a little less obvious is that as air density decreases, we may also need to alter our ignition timing to remain optimal, to remain at MBT. So there's the two aspects there, these are our two key or fundamental aspects that we are controlling when we're tuning the engine, the fuel delivery and the ignition timing, so both of these aspects are affected by the altitude, or the barometric air pressure as our altitude changes. Now that's probably worth discussing, how the OE manufacturers deal with this.
Obviously OE manufacturers are in a situation where they really need to maintain very accurate control of their fuel delivery, very accurate control of the air-fuel ratio, if they can't do that then what happens is their emissions are likely to increase and that's really one of the biggest aspects that OE manufacturers need to watch is their emissions. If their engines aren't emissions-compliant, then their engines aren't going to go into production in the first place. So for this reason, along with a few others, for this reason OE manufacturers tend to prefer using a mass air flow sensor on their engines, a mass air flow sensor connected to their ECU. So the mass air flow sensor actually offers some quite large advantages here because it is directly measuring the mass of air. This means that the ambient pressure, the barometric pressure, the air density, as well as temperature is predominantly accounted for directly by the mass air flow sensor, and it's reporting to the ECU the mass of air directly entering the engine.
Remember, when we're talking about an air-fuel ratio and maintaining an air-fuel ratio, it's that mass of air that's the important part, it's a mass ratio between air and fuel, it's not a volume ratio so that's why I keep reiterating the point that mass air flow is so important. Likewise with the factory ECU, the OE ECU using a mass air flow sensor, when it comes to their ignition timing typically they are using a load axis of, perhaps grams per cylinder or grams per revolution, both of these, again, are really describing mass flow into the cylinder, so hence as the barometric air pressure decreases, our air density decreases and our mass air flow decreases. This simply puts us in a different part of the ignition table, and the ignition timing, the ignition advance can be tailored to that lower mass air flow. So in other words, if we drive from the bottom of a mountain to the top of a mountain, what we can expect to require is, as we get to the top of the mountain with our lower air density, our lower mass of air, we're going to quite likely need a little bit more ignition advance to achieve MBT timing under those conditions. Now in the aftermarket we traditionally don't use a mass air flow sensor, the most common way of controlling our ECUs is via the speed density principle where we are looking predominantly at the air pressure inside the inlet manifold.
Now this works on the ideal gas law where instead of directly measuring the mass of air entering the engine, the ECU is calculating mass air flow. Now this does work incredibly well, this is the basis for most of our aftermarket ECUs as I've just mentioned, however it does rely on a few compensations and calculations going on in the background in order to maintain a consistent state of tune as our ambient conditions change. Now there is an argument when it comes to tuning for high altitude, or for that matter changing altitude, that manifold absolute pressure is manifold absolute pressure, so all we need to consider is this factor, manifold absolute pressure in our ECU. However, there is a little bit more going on than that. While yes, the manifold pressure is a really key consideration, what's easy to overlook is that as our altitude changes, the pressure on the exhaust side of the engine is also being affected, so as our barometric pressure drops, the pressure being exhibited on the exhaust side of the engine is also dropping, so what we really need to consider here is that the volumetric efficiency of the engine is affected by the pressure ratio across the engine, it's not just the manifold absolute pressure that we need to consider, it's the ratio between the manifold absolute pressure and the barometric pressure, at least in a naturally aspirated engine that's going to be very close to what our exhaust pressure is.
So it's this ratio that we need to account for if we want to do a really accurate job of accounting for changing altitude, changing barometric pressure. We're going to talk a little bit more about that and I'll give you a quick demonstration of how that works using our Toyota 86 very, very shortly. Before we move into that, I've just mentioned naturally aspirated engines, that keeps things really, really nice and easy because we only have our inlet manifold pressure and our barometric air pressure to really consider in that equation. With turbocharged vehicles in particular, things get quite a lot more tricky because we need to consider how the turbocharger works. So let me just, let's go across to my laptop screen, okay.
Bear with me for a second while I just get that back, I don't know what's happened there. Okay, I apologize for that. So I've got here on my laptop screen a compressor map for a BorgWarner EFR 6758. Doesn't really matter what turbocharger we're looking at, I've simply chosen this because this is the turbocharger fitted to our Toyota 86 here. Now most people consider boost pressure on its own, they're looking at the pressure on the inlet manifold and really not considering anything else when they take that into account, but as far as the turbocharger's concerned, the pressure on the inlet manifold is only one aspect that's important to it.
The other aspect is the pressure at the inlet to the compressor wheel, so really what we're interested in when we're looking at turbocharger performance is the pressure ratio, and that is simply the outlet pressure from the compressor divided by the inlet pressure. Now the key point here is, we're dividing it by the inlet pressure, typically that's going to be at or very, very close to our barometric air pressure, so as you can see, as our barometric air pressure changes, so does the pressure ratio that the turbocharger is working at, assuming that we're maintaining a fixed, or consistent outlet pressure, so a fixed or consistent pressure in the inlet manifold. So just to demonstrate that, we're going to look at a few points, the pressure ratio is the number on this vertical axis on the left-hand side, so remember it is the outlet pressure divided by the inlet pressure, both expressed in absolute units. So if we look at a few points, let's say we're running 200 kPa of boost pressure, absolute, and our barometric air pressure is 100, so the calculation there is simply 200 divided by 100, that gives us a pressure ratio of 2.0, so we're going to be operating somewhere around about here. Let's just pick an arbitrary point and let's say we're operating right here, which we can see that we're almost in the peak of our efficiency.
We're going to be somewhere in the region of perhaps 110,000 RPM in terms of our turbocharger speed, we can see these lines through our compressor map represent the turbocharger speed lines as well. Okay, so that's our first point, let's consider now if we start increasing our altitude and we climb up a hill to the point where our barometric air pressure drops from 100 kPa down to 80 kPa. Now if we hold our manifold absolute pressure the same, if we hold that still at 200 kPa, we've still got 200 kPa of pressure in our inlet manifold, but now our pressure ratio is quite different. Now we're dividing 200 by 80 instead of our original example where it was 200 divided by 100, and this increases our pressure ratio now to 2.5, so it's going to increase our operating point, or change our operating point here up to a pressure ratio of 2.5. Now at the moment that's not necessarily a bad thing, we're still operating well inside the peak efficiency area of our compressor map, what we can see though is that we've jumped up quite significantly in our turbo speed.
We're driving the turbocharger harder in order to achieve that higher pressure ratio, so this would be exactly the same as if we were still at sea level, still with our original 100 kPa barometric air pressure, and instead of climbing up the hill, we had simply decided that we wanted to increase the boost pressure in the inlet manifold. That would give us exactly the same effect if we had increased our boost pressure from 200 kPa up to 250 kPa. Okay, let's look at one third point now, we'll imagine now that we're still at 200 kPa, but this time we have dropped our barometric air pressure down to 63 kPa, now that might sound dramatic, but that's actually the barometric air pressure that we saw at the finish line of Pikes Peak at 4,100 meters. So at this point our pressure ratio now is 200 divided by 63 which takes us up to 3.2, so in this case we'd actually be operating beyond the surge line. I've only obviously taken an arbitrary point here, what I wanted to really iterate here is, or show I should say, is that as our barometric air pressure changes, if we're maintaining a fixed boost pressure in the inlet manifold, then the pressure ratio that the turbocharger is operating at changes.
That affects the efficiency of the turbocharger, it's going to affect, as our efficiency drops we tend to increase the air temperature of the air. The other effect that's often overlooked is because our turbo speed is increasing, in order to drive that turbocharger harder, what we're going to also see is as the pressure ratio increases, we're going to see an increase in the exhaust manifold backpressure. We're physically needing to close our waste gate to drive the turbocharger harder in order to increase that pressure ratio, and what we see there is while we have maintained a consistent inlet manifold pressure at 200 kPa, the part that's easy to overlook is while that inlet manifold pressure has been remaining fixed, our exhaust manifold backpressure has been increasing. So this affects our pressure ratio across the engine, remember pressure ratio is the ratio between the pressure in our inlet manifold and our exhaust pressure, be that barometric in the terms of a naturally aspirated engine, or in this case we're actually looking at the backpressure pre-turbocharger in a turbocharged vehicle. Now that's not to say this is all bad though, one of the advantages of using a turbocharger in an application that's going to see large changes in barometric air pressure is that we can drive the turbo harder, like we've just looked at, in order to claw back some of that lost power that we're naturally going to see as our barometric air pressure drops.
Again, as our barometric air pressure drops, there's physically less oxygen available in the air, it's that oxygen that is the key element to producing power in our engine, and hence if we have got less oxygen in a given volume of air, our engine is not going to be able to make us much power. So in terms of our experience at Pikes Peak, I used this to my advantage with the vehicle that we were tuning by maintaining a relatively fixed boost pressure in the inlet manifold as the barometric air pressure drops. Let's just have a quick look at some data from the Pikes Peak International Hill Climb. This is a little bit messy because there is a lot of data here, this hill climb is around about 10 minutes to run, so the interesting points here are, if we look at our ambient pressure for a start we can see near the start of the hill climb we're sitting at around about 72, 73 kPa, so we're already incredibly low and we're only at the beginning of the hill climb. If we get to the end of the hill climb we can see that our ambient pressure dropped to only 61.9 kPa.
So two things there, first of all, dramatically different barometric pressures to what we saw here in Queenstown when we tuned it. Typically our barometric pressure here, we're at about 350 meters above sea level and we'll typically see somewhere between 95 and 97 kPa, depending on the day. So obviously that's one consideration, we're taking a car that was tuned at sea level, sending it in a container across the world, and even at the start line of the hill climb, we're already well below the pressures that we were seeing. Now in terms of coping with this, we've got our inlet manifold pressure here, so this is a result of a closed loop boost control strategy that I was employing, the MoTeC ECU that is actually fitted to that car uses quite a unique boost control strategy where the boost is actually controlled in terms of gauge pressure. Gauge pressure is the pressure relative to our barometric pressure, so what this means is that if we controlled the boost to a fixed gauge pressure, what we're actually going to see is the manifold absolute pressure, so the pressure relative to absolute, an absolute vacuum, is going to continually drop as our barometric pressure drops with altitude.
Now that's one way of controlling the boost and what that's going to do is mean that our pressure ratio that our turbocharger is operating at is going to remain fixed. Remember back to our compressor map? So we're not going to be driving the turbo harder as our altitude changes, the result of this is also going to be that our backpressure is going to remain relatively consistent pre-turbocharger because we're not trying to overspeed the turbo in order to maintain a fixed manifold absolute pressure. This is also, however, going to mean that as the altitude increases, we're going to start losing power, so obviously that's not really what we wanted to do. So in order to combat that, what we did was use the fourth dimension of the boost control table in the MoTeC M1 ECU which is for ambient pressure, or barometric pressure in other words. So what I did was I actually increased the boost target in terms of manifold gauge pressure so that as the barometric pressure dropped, I maintained a relatively consistent manifold absolute pressure.
Maybe that's confusing, you do need to understand the terms manifold gauge pressure and manifold absolute pressure. Manifold gauge pressure, remember, is the pressure relative to our current barometric air pressure, whereas manifold absolute pressure is the pressure in the inlet manifold relative to an absolute vacuum. So let's just have a quick look at a couple of points. So at this particular point, I'm probably a little way into the hill climb, once the driver has sort of settled down and so this is about the first section where he's actually using a little bit of throttle. We're seeing around about 173, 174 kPa of boost, remember this is absolute pressure here.
We do see a few spikes here, just to do with the way the padle shifted gearbox works, when the gearbox unloads, or does a gear change ignition cut we do tend to see a little bit of a spike there with the closed loop boost control because when the engine torque is reduced, the closed loop boost control strategy actually tries to bring the boost back up, so this results in a very slight overboost once the driver gets back on the power. Let's just have a quick look here. So this is another good section here to look at, and you can see that our boost pressure is sitting pretty consistent, around about that 173 kPa with the exception of those little spikes I've just mentioned. The other thing here, if we look at our lambdas, this has got dual lambda, so at the start of the hill climb here, this full-throttle section, we can see that we're right on our target of lambda 0.784 both banks and really consistent there. Again, we're seeing small fluctuations in our lambda as a result of the gear change ignition cut upsetting what's being seen in the exhaust.
I'll just zoom back out here. What I wanted to do is look at how the changes, so at the moment we're sitting at 69 kPa ambient pressure, I'll just move through to closer to the finish line, and we can see that near the finish line we've gone down to 63.5 kPa of ambient pressure, and I've actually got a slightly higher boost pressure in the inlet manifold, we've actually gone up to 180 kPa absolute. So this is the boost control strategy that I've employed there just to try and claw back some of that lost power that we're going to see, and again, you can see the two points we've looked at here, let's just zoom in, we can see that again, despite the significant difference in our altitude and our barometric pressure, we are still sitting within about 1% of our lambda target. I'll just mention here that I was employing a strategy here, you can see my lambda target now is slightly richer, 0.76, we were having trouble with this car overheating in terms of the engine's cooling system capability at altitude, so I purposely chose to target a slightly richer mixture once the engine coolant temperature exceeded a certain aim, and the aim there was to help use some additional fuel in order to help cool and control the combustion temperature. What I wanted to do here is just add one more group that we can look at, and what we'll look at here is turbo speed, we will not because I do not have that channel in here unfortunately.
I wanted to just show you the effect there of the driving that turbocharger harder, what we tended to see was the turbo speed at the beginning of the hill climb was significantly lower than the turbo speed that we saw near the finish line. So this is what we can do in terms of our tuning with altitude in order to try and maintain some consistent engine power, however, that turbo speed is really a key point. We need to understand how that's going to affect our tuning, so first of all, as we've already discussed, our exhaust manifold backpressure is going to increase as we're driving that turbocharger harder, so in essence what this is going to do is reduce the flow across the engine, so we need to be able to account for that. The other thing that's really important, probably mainly important at such a dramatic place such as Pikes Peak with such a large altitude change is we do need to be careful because if we're driving a turbo very, very close to its limits, we can end up overspeeding it and breaking the turbocharger. There is a point where a turbocharger simply can fail from being oversped, so that's why we see a lot of the turbocharged cars competing at Pikes Peak will use a turbo speed sensor.
Another element that's quite common with cars developed purposely for Pikes Peak is that they're fitted with a turbocharger that on face value may seem, possibly a little bit larger than what you would need for that given size of engine, and the reason for this is we are going to be operating that turbocharger, perhaps at a much higher pressure ratio than what we would conventionally, or typically need to do if we were running at lower altitude, perhaps at sea level. So there's just a little bit of insight into how the altitude, or barometric air pressure does affect our tuning in particular, also how we can use a turbocharged vehicle to help combat that. In general though, I wanted to also talk about how we can account for barometric air pressure in our actual tuning, and often what we can actually do is probably simply ignore it, and use manifold absolute pressure as our load input to our ECU. That doesn't sound particularly scientific, it doesn't seem like a very great explanation, and I'm going to go deeper into it, but this is the technique that 99% of tuners around the world are using, and I've used it myself, it does work, and again, it comes back to one of those first comments I made, if you are, unless you are dealing with a situation where you are experiencing very large barometric air pressure changes, the effect of minor barometric pressure changes that you're likely to see due to fluctuating weather at a fixed altitude, or the relatively minor altitude changes that most of us are going to see in our day-to-day driving, you're going to probably find that manifold absolute pressure is absolutely fine, and is going to do an adequate job of controlling your engine. You've gotta also understand that there's a variety of other aspects that are coming into play to affect the accuracy of our tune, temperature compensations just to name one, so the barometric pressure fluctuation is just going to be one other small factor that can add up to make a minor impact on your tuning.
The other way we can do this, if we are going to use manifold absolute pressure is we can account for it by using a compensation table, perhaps this is a reasonably conventional way of dealing with barometric air pressure. This obviously requires a barometric pressure sensor, so this wants to be fitted so that it can measure our current barometric air pressure. You want to be careful of where you fit that, somewhere in the engine bay or in the cabin of the car, somewhere where it's not going to be exposed to a ram effect, or something of that nature that's going to maybe affect its reading. Now this is generally applied using a three-dimensional, two-dimensional compensation map that's going to make a change by positive or negative to our fueling, our final fueling values, based on what the current barometric pressure is. It is a potentially messy situation to go and apply corrections to that, and in my own experience if you want to do that, really what we need to do is get some logging and see what's actually going on, and then apply changes to that barometric correction table, based on what our logging's told us.
Not particularly complex to do, and it can be a reasonably effective way of doing this, but what it does require is some logging in order to get that dialed in. We're not going to be able to do a perfect job of programming that table without having a look and seeing what our actual logging is going to need. Okay, the other way that's common in a few ECUs, in particular the Link G4 range of ECUs that were used in a number of our examples, our worked examples in our webinars, is to use manifold gauge pressure, or MGP for short as the load axis for the fuel table. Now this is a technique that I actually used myself for a number of years, in fact, I think probably our Nissan 350Z is still setup with MGP as the load axis for the fuel table, and on face value it seems to make sense, particularly if we read Link's description of why we should be using MGP. What it's doing is it's looking at the difference between our manifold absolute pressure and our manifold gauge pressure, so in other words if we are at 100 kPa inlet manifold pressure and our barometric pressure's 100 kPa, then we're in our zero kPa zone in our manifold gauge pressure axis.
However, if we are at wide-open throttle still in our naturally aspirated engine, and our barometric pressure drops from 100 kPa to 80 kPa, we find that we're now still in our zero kPa row of our volumetric efficiency table because we're still looking at the same point with, we're looking at the difference between our inlet manifold pressure, 80 kPa and our barometric air pressure at 80 kPa. So without getting too deep into the calculations here, this was actually brought up by Andy Wyatt from Adaptronic and it really made me think a little bit more in detail about this, and Andy was questioning whether he should be adding MGP as a load axis option to his Adaptronic range of ECUs, and when he actually ran through the calculations, the point he actually brought up was that manifold gauge pressure does work quite well at wide-open throttle, 100 kPa, or on boosted engines into positive boost, but as we drop into vacuum it actually creates more error than manifold absolute pressure. That's not to say that manifold absolute pressure, which is the conventional load input for our fuel table is perfect either, what we find is that there is still a significant error with that over what the engine truly wants, and the correct way of calculating this is, which is what Andy was pointing out, is to use that pressure ratio across the engine. We were looking at the inlet manifold pressure divided by our barometric air pressure, or in a turbocharged engine, our exhaust gas backpressure. So the ideal option there, which is something that we've been investigating ourselves, is to use pressure ratio, and I just want to go through a quick demonstration of that using our Toyota 86.
So I'll just get that up and running. After this demonstration we'll be moving to some questions and answers, so I see there's already a few questions in there, if you do have any more questions, please make sure that you ask those in the chat and I'll get through to those as soon as I've finished this demonstration. What I wanna do for a start, just while we're letting the engine warm up, is I'll just give you a quick look at what we've got going on inside the ECU here on our MoTeC M1. Now I will also mention that this particular setup that we've got in our MoTeC M1 ECU is not a factory offering from MoTeC, so this isn't a load axis option that we have by default in the M1 ECU, this is something that I've actually gone through and added in using a M1 build, so if you go and buy a MoTeC M1 ECU, currently this isn't going to be offered. So let's have a look at what we've got here, this is our volumetric efficiency table which we can see in front of us right now.
This looks pretty conventional, we've got engine RPM on our horizontal axis. The terminology that MoTeC use for the load axis is probably a little bit unusual if you're not aware of it, it's engine load normalized, and in this case this is our inlet manifold pressure divided by our exhaust manifold pressure, so this is our ratio here. Now the problem with using this as a load axis is it does give us a slightly unconventional path through the volumetric efficiency table. So if, for example, we were talking about a conventional manifold absolute pressure load axis and we've got a turbocharged engine, and we're running at a fixed boost pressure, what we're going to find is that our engine's going to run something like this through our table. This makes it really easy to figure out exactly where to make changes if we're running 200 kPa of pressure, then obviously the 200 kPa row through our efficiency table is where we're going to make these changes.
If we're using the engine pressure ratio as our load axis however, things change a little bit. What we'll find is that if we're running a fixed boost pressure, at low RPM our pressure ratio will be high. We've got a fixed boost pressure, but at low RPM our exhaust manifold backpressure is relatively low, so that when we divide inlet pressure divided by exhaust pressure, the number we're going to get is going to be a higher number. If we maintain that fixed boost pressure though, as we move through the RPM range, what we're going to find is that our exhaust backpressure naturally tends to increase. The turbocharger starts to become a little bit more restrictive, and we see that backpressure climb, so then if we have maintained that fixed manifold pressure in the inlet manifold, the ratio starts to drop away.
So what we conventionally see, and we'll see this in this example, is we're going to end up climbing up, we're going to, oh we're not actually going that high, but this will still show what happens. We'll reach peak boost at around about 3,000 RPM. I think for this example we're going to be up around about 1.3 or 1.4 as our pressure ratio, and then we're going to find that at higher RPM we start dropping back, and by the time we get up to about 7,000 RPM, the exhaust manifold backpressure has actually exceeded our inlet manifold pressure, so what this means is that our pressure ratio drops away, so we actually end up slightly below a pressure ratio of one. Let's get the engine running though, we're going to do just a single power run here. We'll jump across to the dyno screen and watch this power run, and then what we're going to do is come back and we'll have a better analysis of what exactly happened inside M1 Tune.
We'll just let the engine settle for a second and we'll get our run underway. Okay, so 217.5, or 291.7 horsepower at the wheels there, let's just pause our time graph there and we'll just fullscreen this so we can have a look at it. So first of all, excuse me, at the top of the screen we have our engine RPM, nothing particularly unusual there. We've got our ramp run from around about 2,000 RPM through to 7,500 RPM through that range. The next group that we've got here is a little bit more interesting, here we have our manifold pressure, we've got our boost aim, and then we've got, below that, our exhaust pressure.
Now I just wanna explain this, what we've got here is our boost aim, which you can see is one fixed value. It's not really relevant on this graph anyway, is in gauge pressure, remember that's the inlet manifold pressure, the difference between that inlet manifold pressure and our barometric pressure. So for this particular dyno run we were targeting 70 kPa gauge, 70 kPa above our barometric air pressure. We can see that we've got our inlet manifold pressure, and for the most part we're sitting at around about 165 kPa. That is actually our target because our barometric air pressure is below 100, if we're 65, 165 kPa here with 95 kPa of atmospheric pressure, that brings us to about our target, so that's why we've got that.
The next one down though is our exhaust pressure, we can see this is exactly what I was talking about. At the start of the run we're at our ambient or barometric pressure, about 96 kPa, you can see that as we first come up on boost pressure, about 154 PSI of boost, 118 kPa of exhaust pressure. As I move through the run though, we get to a point at around about 6,500 RPM where our exhaust manifold pressure just starts to overtake our boost pressure, and our exhaust manifold pressure peaks at 180 kPa. So that translates into our IMAP divided by EMAP calculation, remember it simply is that easy. We're looking at the inlet manifold pressure and dividing that by the exhaust manifold pressure to calculate a pressure ratio across the engine, and that's what we've got here, remember, it is uniquely named here as engine load normalized.
Let's not do that, let's just make a little bit more room so we can see exactly what that does. So you can see, when we're sitting at full throttle, 2,000 RPM and just settling the engine prior to the run, our engine load normalized where our pressure ratio's 1.15. We've got more inlet manifold pressure than we have exhaust manifold pressure. We get up to a peak here around about 1.3, 1.32 where we've got our exhaust manifold pressure has come up a little bit, but obviously our boost pressure is much higher, and then we can see what I was talking about, our pressure ratio starts to drop away, and by the time we get up to our rev limit, 7,500 RPM, our pressure ratio has dropped down to 0.91 because our exhaust manifold pressure is now above our inlet manifold pressure. Okay, so let's just have a look at what that means to our table here.
I'll just shut that down. So at 3,000 RPM this is the area that we're operating in, around about that 1.3, 1.32 pressure ratio, and if we move across to the right-hand side, by the time we get to 7,500 RPM we are down here. So the advantage, or reasoning behind using pressure ratio as a load axis is that it, to a degree, automatically compensates whereabouts, it's looking in that table as our pressure ratio changes. Now at a fixed altitude that's not particularly significant, but what we are going to find is that if we take the same engine and we now drive it up Pikes Peak, if we're maintaining a fixed inlet manifold pressure, then our pressure ratio is going to change by virtue of our exhaust manifold pressure increasing. So if we've correctly calibrated our IMAP over EMAP volumetric efficiency table, in that case we're going to theoretically end up with a much more stable tune as our altitude and our barometric pressure varies.
Now personally this isn't something that I've been able to prove just yet with our Toyota 86, so I'm presenting this as a well-researched and well-accepted technology. There's plenty of professional tuners around the world who are using this with great results. Personally at the moment, as I say, I haven't had the opportunity to test this over a large change in barometric air pressure. I just wanted to present to you the technique and how that works. There are a number of ECUs that provide this natively as a load axis option, including Adaptronic, including the MoTeC 100 series ECUs.
Autronic have been probably one of the first to offer this as a load axis option, and the other one escapes me for the moment so, oh Haltech is the other one that I wanted to mention there, so it is an option that you may have. Before we move into questions and answers, one more point I just want to make here is that if you are monitoring exhaust manifold pressure, which is obviously a requirement for this load input type, then you do need to give some thought into how you're doing that. What you'll find is that if you run a pressure sensor straight into your exhaust system, well it actually needs to be run through an extension of some brass pipe or something of that nature to remove the heat out of the exhaust, but if you do it like that what you are typically going to find is you're going to get quite a noisy signal, and that's not much use for a load source like this. Obviously it needs to be relatively smooth and consistent, and also react very quickly. So this is what we found with our Toyota 86, initially when we first started testing we ran a 150 PSI pressure sensor into the exhaust manifold through an extension of some brass, or copper tube I should say, and we found that the signal was very, very noisy, we couldn't use it for the load input.
By filtering it electronically inside the ECU this gave us a nice smooth signal, but by the time I'd actually applied enough filtering to really smooth the signal significantly enough, it also responded a little too slowly to be useful, so we ended up using a mechanical exhaust pressure dampener and this is a little module that Tony Palo from T1 Race Sales, just goes inline with the pressure sensor and it just dampens down those pressure oscillations, or pulsations. So again, if we just jump into my laptop screen, what I'll do is just add an exhaust pressure here, because on this ECU I do actually have it. Oh, we've already got it even, there we go. The bottom group here is our, no it's not, it's, sorry, exhaust pressure, we've already been looking at it. It's nice and smooth as we can see, this was our yellow line here as our exhaust pressure, so we've got a nice smooth line that's going to be able to be used as a load source through that calculation.
If you haven't got a dampener, you're going to end up with a really spiky, noisy signal that's going to make it all but useless for a load input. All right guys, we'll move into some questions and answers, now again, if you do have any more questions please ask them and I will do my best to answer them. Okay, our first question comes from F1 Jim who's, "Somebody tell Andre that we measure Pikes Peak "in feet, not meters." Fair enough, 14,100 feet I think. I remember taking a photo of the sign, and I remember distinctly how hard it was to breathe up there, but we're in the Southern Hemisphere, we're in New Zealand and we work in the metric system, so sorry about that. Barry's asked, "Given the data you just showed, does having "a forward-facing turbo affect the pressure ratio of "the turbo favoring increased performance?" Look, I did this actually on my drag car, I had a forward-facing turbocharger, that ran to 180 mile an hour.
I did put a pressure sensor in the inlet to that turbocharger at one point, and I never measured anything particularly worthwhile. That's not to say that it's useless, I think there's plenty of studies or experiments that have shown that a ram air effect can work. It may have just been the fact that I really didn't have any bell mouth or intake attached to the inlet of the turbocharger to really provide that, but if you have a positive pressure on the inlet to the turbocharger, then this will affect the pressure ratio that the turbo's working at. Remember it's, I've used barometric pressure as the example, there for the inlet pressure on the turbocharger, but of course that's not necessarily the case. If we've got a really effective ram feed to the turbocharger that proved to add positive pressure, then that would effectively drop our pressure ratio for a fixed inlet manifold pressure, and likewise what we can see, this is really common on a factory installation, factory intake application, we see that there is so much restriction through the intake plumbing, the air box, the air filter, et cetera, that we actually end up with a lower than atmospheric pressure at the compressor inlet, so this makes everything worse.
So my barometric pressure example there wasn't strictly accurate. Andy's asked, "Is it better to start lower on "the map so you're in the center of "the map at say, 10,000 feet?" Okay, I think the problem here is where is our line in the sand, the line in the sand is probably going to be at whatever altitude our dyno is, or whatever altitude that we've been able to perform our testing at, so at that point how I dealt with this in the Ford Focus that we took to Pikes Peak, the M1 ECU fitted to that was purely a manifold absolute pressure-based system. MoTeC with the M1 do offer the ability to have different volumetric efficiency maps at different barometric pressure points. Something I've never really been particularly comfortable with, feels like a really messy way of doing this, so what I ended up doing with the Ford Focus was simply gathering some data and seeing where we were compared to where I was at sea level, or 350 meters right here. What we actually found is, this is quite interesting, the air-fuel ratio was marginally leaner at altitude, probably not on face value what you may think.
So what I did was basically got us to a point where we were back on track at the lowest section of the mountain which was where we did our first day of testing and then the other way of combating this, once we were already back in the ballpark, and I think it only required a few percent of fuel, it was a relatively minor change. I should also mention we were running on a slightly different blend of E85 at Pikes Peak than what I tuned on, so I can't, there were too many variables that got changed for me to say that everything I needed to do was purely due to altitude. I must admit, I was quite surprised how close everything was when we were at the start line, which I think, off the top of my head, is still somewhere around about 10,000 feet. So then in conjunction with this I used closed loop fuel control in the M1 to account for any other variations. The closed loop control in the M1 I found exceptional, it's incredibly fast and very, very responsive so this made up for any small errors that may have crept in as we moved from 73 kPa, 75 kPa at the start line through to 62 kPa at the finish line.
Tyler's asked, "Can you setup a boost control system "to adjust for a pressure ratio target, as opposed to "a specific manifold pressure, given the required sensors?" That's a good question Tyler, it's really going to depend, I think, on the ECU that you are dealing with. Pressure ratio is not something I've seen as a load axis or target in any of the ECUs I think I've tuned. You can essentially do this in the M1 ECU that we've just used as an example, because they use manifold gauge pressure as their boost target, so we can vary that and essentially we're looking at a difference between inlet manifold pressure and barometric air pressure, which obviously, close to our pressure ratio anyway. Most of the aftermarket ECUs I deal with tend to use manifold absolute pressure as the boost target, it's possible you could then adapt a compensation table for that, or use a barometric air pressure as an axis for that boost target table. Those are the few things there that have just sprung to my mind, probably not the only way of doing it.
Andy's asked, "The drag racers look "at exhaust manifold to inlet pressure, any thoughts?" Yeah, absolutely, I did exactly that in my own era of running drag cars, it's a really useful tool, how to put it to, help guide us on our turbocharger sizing, and I mean it is quite a unique situation when we're drag racing, because boost response really almost doesn't matter at all. My own experience has primarily been in sport compact, so small capacity engines, and we've tuned a few that have run and held world records, so we were tuning a few at the pointy end. What I've found personally is that you get to a kind of a magic point when the inlet manifold pressure is higher than our exhaust manifold pressure, that almost never happens in a street car or anything where we need moderate kind of response. We tend to need to size the turbocharger to be restrictive enough to provide enough energy to the turbocharger so that we get good boost response, and this results in high exhaust pressure at high RPM and high boost. So this kind of kills performance, and as we've talked about, you can understand how that would affect our pressure ratio.
When we get to a situation in a drag engine where we're using a two-step to build boost on the lines, so hence boost response isn't that critical to us. It's still important, but it's not as critical. We can size the turbocharger much larger, and this means that the exhaust side can be much less restrictive, and we get to that point, as I've mentioned, where our exhaust pressure is lower than our inlet manifold pressure. Now through my own testing, and I would have to say it's not been extensive, I've never been in a situation to try five different turbochargers on one engine back-to-back with no other changes. So you've gotta understand that the changes I made were usually, sort of two, three, four, five different changes at a time.
But I sort of get to a point where the gains that you start seeing per PSI of boost pressure start to almost become exponential as we go across that threshold, and our exhaust pressure is lower than inlet manifold pressure. Hopefully that's helpful to you as an answer there. Godspeed's asked, "What fuel was Tony running on?" Sorry, we, the car runs on E85, we tuned it here on a, just a pump grade of E85. It was a true 85% ethanol, I say pump fuel, but it's not really, it is provided as a race fuel but it's not from what you may consider as like, a commercially available race fuel. When we got to the US to Pikes Peak the car was running on VP Racing C85, now that's again, a guaranteed consistency.
There are a few additives in the VP Racing C85 as well that includes MTBE, you need to be careful with that with some injectors, that additive doesn't work very well with some injectors, but back-to-back testing by a few sources that I have researched shows that C85 actually develops somewhere in the region of about four to 8% more power than a pump grade of E85. So as I said, that maybe part of the situation I saw at Pikes Peak. Motor Ray has asked, "I've heard of hill climb cars "with boost reference in strange locations "and dual MAP sensor setups, is this old-school methods "on old ECUs, or some other wizardry?" I really like to think that there is no wizardry in EFI tuning, that's, I think, one of our big sales points for High Performance Academy, however there are some really unique sort of ways of getting around certain problems. I think probably what you've just mentioned there might be quite accurate, in some ECUs we're forced to try and find workarounds for getting to the solution that we want, so really a lot of it is gonna come down to exactly how much control we have through the ECU, and whether that ECU's going to allow us to do exactly what we want, and again, like even with hill climbs in general, Pikes Peak is pretty unique, there's not too many hill climbs out there that have the kind of barometric air pressure changes that we do see from the start line to the finish line of Pikes Peak, so what we need for Pikes Peak is probably not something that you'd consider typical for every hill climb application. Andy's asked, "Does anyone use long-term fuel trims "to better hit, or hold the target air-fuel ratio?" Really I think that's dependent on the particular ECU that you're using there.
I've always been a little bit apprehensive of long-term fuel trims in some of my OE tuning, particularly in the GM LS V8 world, did a lot of tuning there, I think I'm probably not the norm there, but I quite often will disable the long-term fuel trims while still enabling the short-term fuel trims. This, my reasoning for this is the short-term fuel trims can still cope really well with day-to-day variations et cetera, and if our ECU is tuned correctly in the first place, they shouldn't be doing very much work, but the long-term fuel trims can affect our ability to maintain really accurate air-fuel ratios day-to-day if something goes haywire there. Now I wanna just use one example before I move on there of the Haltech Elite-series of ECUs, they have quite a nice function there of long-term fuel trims, they actually have some long-term learning capabilities with a few of their control parameters, and that particular system I do quite like. It runs closed loop control, or can run closed loop control of the fuel delivery, it uses short-term fuel trims to control to our target, and then it learns those fuel trims and applies those into a long-term fuel trim map so that next time you go through that same zone, the air-fuel ratio, or the short-term fuel trims should be doing almost nothing, and we should be right on our target. Now the reason I like that system over the OE system, I know it sounds like I might be doubling back on myself, the OE system, generally we have either one, either two or maybe three different zones for our long-term fuel trims, so you might have low, mid and high for example, and that means that when we're in the low range, the long-term fuel trim that's been learned for that range will be applied regardless if we're at the start of that zone, or the end of the zone.
So this means that if we have a zone that's rich and a zone that's lean, that all fits in that low area of the long-term fuel trims, that's going to actually make it harder for the short-term fuel trims to do their job. Complex there, I know, and I'm probably getting a little off-topic, so I'll carry on. CA18NA has said, "If MAP starts the same "but baro goes down, would a boosted car make more power "as there is less exhaust exit restriction?" Okay, what you need to understand there is if MAP stays the same, remember we are now running the turbocharger at a higher pressure ratio in order to maintain the MAP. Our barometric air pressure's gone down, so the pressure at the inlet to the compressor has gone down, but our outlet pressure has stayed the same. Pressure ratio's gone up, by its very nature that means that we have to drive the turbocharger harder in order to make that same manifold pressure, or that higher pressure ratio, so our actual exhaust backpressure pre-turbocharger will climb.
You're right in so much as the barometric pressure at the exhaust outlet that will have dropped, but that's much less of a significant effect than the backpressure pre-turbocharger. Tyler's asked, "Would engine load normalized load axis work "with ITB engines, boost or not?" Okay, so I think, probably might actually confused mate, the engine load normalized term is a MoTeC M1 lingo terminology, so engine load normalized is the way MoTeCs speak about the load axis for the volumetric efficiency table, regardless whether we're using manifold absolute pressure, in my case my modified IMAP divided by EMAP, whether we're using throttle position or whatever. So to answer your question though, we can't use this axis as a load axis for ITBs for the same reason we can't use manifold absolute pressure. Remember, the axis there is manifold absolute pressure divided by our barometric air pressure on a naturally aspirated engine. We're going to have that same scenario where, depending on the size of the throttle bodies, once we get past, perhaps 30 or 35% throttle opening, we're going to essentially be at atmospheric or barometric air pressure, so this gives us no resolution for the load for the VE table, so we would still need to use alpha N, this isn't going to fix that particular problem, alpha N or throttle position.
Barry's said, "I've noticed that when "the exhaust manifold pressure approaches "inlet manifold pressure, the engine seemed "to hit a sweet spot, the ignition timing doesn't need "to be tuned much, have you noticed this "on any of the motors I've tuned?" I think I've probably talked exactly about that already as an answer to Andy's question, but yes, absolutely, it's definitely something that is a very real scenario, and those of us tuning drag engines are likely to come across that. I'll also mention, I didn't add this to the answer to Andy's question, from my own experience, with that situation, because the engine's starting to act a little bit like a naturally aspirated engine, I know that's a very dumbed-down way of explaining it, but it's just a quick way of getting through an expression of how it works. What we find is because the exhaust manifold pressure is much lower, we can actually get away with using a larger cam profile with more overlap, more duration, which tends to work better in those applications at higher RPM as well, so basically everything just starts to work in harmony better, and allows us to mechanically modify the engine to take advantage of that as well. Godspeed has asked, "Exhaust pressure sensor mounted pre "and post-turbo, what's the difference?" Okay, so for the particular example we're using here on a turbocharged engine with the IMAP divided by EMAP, it's the pre-turbo backpressure that we're interested in. So this is what's going to be noticed at the exhaust valves when the exhaust valves are open, so this is caused by the restriction of the turbocharger.
With a turbocharged application, the exhaust manifold backpressure there pre-turbo will also be affected by whatever's happening in terms of backpressure and the rest of the exhaust system as well. With a naturally aspirated engine it's not probably typical to measure exhaust backpressure, obviously there we have no turbo in the system so exhaust pressure is the exhaust pressure, so the general input for the equation there that we're using would be the inlet manifold pressure divided by our barometric air pressure. Okay, right, we've got a question there from Alex, who's just asked about MAF scaling, probably not really relevant to this particular webinar there Alex, there is a webinar in the archive on MAF scaling that you can search for that explains MAF scaling in detail. All right guys, that's taken us to the end of our questions there, hopefully that's given you some more insight and some more knowledge on how altitude affects your tuning. Of course, as usual, if you do have any further questions, please ask those in the forum and I'll be happy to answer them there.
I look forward to seeing everyone next week, thanks guys.