Thread: Elevation and the Turbocharger (very technical)

Elevation and the turbocharger. This is a very technical and long read. I hope it makes sense to you guys, I tried to make it as clear as possible so it is easy to read and understand. Feel free to share your experiences as well!

I have been told by many people that changes in elevation do not affect a turbocharged engine. I decided to look deeper into this because I have seen real-world data that says this is false. Looking simply at the variables in front of you, one can come to the assumption that the engine should be unaffected as long as total absolute pressure remains the same. I have heard wild calculations for “turbocharged engine” output at elevation, and I will go into this and hopefully we can all learn a lot from it.

I live at 4500ft. The ambient pressure here is 12.5psia average. Sea level is around 14.7psia. Psia stands for PSI absolute, meaning total pressure. Psig stands for PSI gauge, or pressure above ambient (boost). Psig is usually what is used to control boost levels, and is the pressure your gauge reads. In short, psig is boost or pressure above ambient. By looking at these two numbers you can see that there is an 18% difference in pressure between 4500ft and sea level. 14.7/12.5=1.18. SAE correction factors take this difference of air pressure into account when calculating HP at high altitude. At SAE temperature and 0% humidity you should have approx 18% more power at sea level than at 4500ft.

So the difference in air pressure is 2.2psia or 18%. Bare with me here because these numbers are going to throw you off a little unless you have a good understanding of turbochargers. Let’s take an engine that has boost, for this example we are going to use 6psig. Add the ambient pressures and find the difference in psia: 6+12.5=18.5psia and 6+14.7=20.7psia so 20.7/18.5=1.12. Notice the difference here is only 12%. Again, 14.7/12.5=1.18, an 18% difference in pressure; and with boost only 12% difference in pressure. You can see now that the difference in pressure goes down as boost pressure goes up. Let’s look at the difference in pressure when running 25psig boost: 30+12.5=42.5psia at high alt and 30+14.7=44.7psia. There is still a 2psia difference in total pressure, but the difference between the two here is only 5% at this point. Some have used this math to determine that the turbocharged engine is affected less by elevation the higher the boost goes. I have also heard by many people that the SAE correction factor shouldn’t be applied to turbocharged engines at high altitude for this reason.

This makes sense until you dig deeper into the understanding here. By comparing the difference in total pressure, you are comparing the engines power output directly to total pressure. Let’s do the reverse calculation for an engine at 20K feet. Ambient pressure is only 7psi at this point. 14.7/7=2.1 or 210%. Wow, 210% correction? So at sea level you would be making 2.1 times the power than at 20K feet right? This relation does not make any sense in the real world. This is comparing the difference in air pressure directly to the power the engine will make. 7psia is half the pressure of ambient at sea level (14.7), so you must have half the power? 1 bar of boost must double your NA power then?

I have seen this example thrown around on the internet for years, but it simply does not take into consideration the vast variables the turbo throws in here. The above equations relate the total pressure directly to power output, which you cannot do in the real world. We have all seen the engines pushing the same boost pressure making wildly different power numbers. You cannot simply relate power to psig. The difference in pressure between sea level and 4500ft is only 2.2psia, so a turbo engine should make the same power at high alt as long as boost is raised by 2.2psi right? Wrong. You cannot simply add the 2.2psi and expect the power output to be the same as sea level. Let’s look at an SRT4 neon for a second: 14psig of boost, so 14+14.7=28.7psia total pressure on the valve seats, and at high alt: 14+12.5=26.5psia. Raise the boost 2.2psi to the high alt engine and you get 16.2psig boost and the same 28.7psia total pressure. Same total pressure, but will the power output remain the same? Total pressure is the same right? Let’s look into the turbocharger a little more with some science and physics mixed in…

An engine is governed by the volume of air it can swallow, so this becomes the main limitation for how much power can be made. An engine is only efficient at certain rpm ranges, and therefore it cannot fully consume its full displacement. For example, a 2.0L engine consuming 2.0L of air would have 100% volumetric efficiency. This is not the norm, as air has a limited time to get into and out of then engine. Most engines are going to operate at 80-95% peak efficiency. When you throw a turbo into the mix it is possible to get over 100% VE.

Air density is a major factor in power being made. Density = Mass/Volume. We can just assume for a minute that the mass of air stays constant. Density can be increased by lowering the volume. You lower volume by compressing the air into a smaller space. Make sense? So adding pressure can increase density by decreasing volume. If you have 2L of air at ambient pressure, then compress it to take up only 1L of space, then you can fit another 1L of compressed air into the engine. After everything is compressed, you now have 4L of air at ambient pressure. You just doubled the displacement of engine in a manner of speaking. The engine can only displace 2.0L, but you have compressed the air so you can fit 2 times more air in the same amount of space (volume). The less volume the air takes up, the more that can fit in the cylinders. Air taking up less volume means extra space in the engine to fit even more air.

The turbocharger compresses air. Compressing air heats it up. How? Well the ideal gas law states that changes in volume are inverse to pressure, and linear to temperature. PV=nRT. Volume goes down as pressure goes up (and temp goes up with it). The less volume, the more dense it is, and the more air going into the engine. The more air and the more fuel, the more power can be made. It is important to note that temperature is directly related to volume. As temp goes up, volume goes up with it. Eventually you get to a point where compressing the air will no longer decrease the volume, but rather, will increase it!

It is sometimes easier to think of the turbo moving a certain Volume of air. Since the compressor efficiency measures out in LB/min of air flow, this is a direct look at what a turbo does. You will see on a compressor map later on that at a higher pressure ratio (and the extra heat that comes with it) the turbo will be unable to move the same amount of air because of the higher temperatures. Higher temps mean more volume, and this is why a turbo can only move so much air. I find it weird that turbo manufacturers map their turbo flow charts in LB/min of air flow instead of Mass air flow. LB is a weight measurement (the effect gravity has on an object) and air weighs less as it is heated… blah blah blah. You catch my drift?

Compressors have rated adiabatic efficiencies that can be mapped based on the amount of boost run, the volumetric efficiency (VE) of the engine, and how high the engine revs. A compressor that is 70% efficient will heat the air 10% more than a compressor that’s 80% efficient. The compressor that is 80% efficient will heat the air 20% more than the ideal gas law states. Now we get to play with the turbocharger numbers!

The turbo compressor maps look directly at the pressure ratio and the air flow to map out efficiencies of the compressor. Let’s look at the pressure ratio (PR) first: PR is calculated by taking total boost pressure and comparing it to atmospheric pressure. Looking at the compressor map for the GT35R, we can map out our pressure ratio real quick. Don’t get lost in the numbers.

25psi of boost. High alt PR compared to sea level PR. (26psig+12.5psia)/11.5psi ambient. I guess you are wondering why boost pressure is 1 psi too high, and ambient is 1psi too low? Well the extra boost takes into account the loss of pressure across the average intercooler. The lower ambient pressure takes into account the depression caused by the air filter, which gets more pronounced as boost goes up (at 30psi the drop could be very substantial). These numbers are just for calculations and estimates, and could be different for your car.

(26psig+12.5psia) / 11.5psia = 3.35 PR at high altitude.
(26psig+14.7psia) / 13.7psia = 2.97 PR at sea level.



Let’s graph these on the GT35R compressor map for around 40lb of airflow (find map at Welcome to TurboByGarrett.com) and you can see the difference this makes! At 40lb of air flow the sea level engine borders the surge line, and at high altitude the compressor won’t even operate! Even at 50lb/min airflow the PR is maxed out of efficiency at high altitude. The compressor will operate “ok” at sea level, but at high altitude the compressor will choke and over-speed. The instability caused by the higher PR will likely cause damage to the turbo. You can see that the speed of the compressor at high altitude will exceed 117,000rpm! The difference in compressor efficiency will create a ton more heat to deal with, and the kicker of the whole thing is: We haven’t even added the extra 2.2psig to make up for the loss of ambient pressure! If we take that into consideration:

(28.2psig+12.5psia) / 11.5psia = 3.54 PR !!!

I highly doubt that the compressor is going to like running at that, and if it DOES make the boost, it will be so inefficient that the volume of air will go up to the point that corrected mass air flow will suffer. This is why turbo size is so important. A bigger compressor can move more air, more efficiently, and that’s why they can make more power. Otherwise (if the ideal gas law didn’t exist) the small t25 turbo could just spin faster and faster and make more power. There is always a limit. Also, just like a water pump can spin so fast before it cavitates, the turbo can only move so fast before air speed reaches mach levels and it cannot move any more air. As efficiency drops, the temp goes up, volume goes up, which means the air is less dense and power is affected.

Even if you have an intercooler that is very efficient to bring the temps back down, you still have the loss of airflow caused by the drop in efficiency. You can see on the compressor map that the compressor can only flow 52lb/min of airflow at a PR of 3.3. Boost will have to fall if the turbo is to flow the 600hp it is rated. This is where efficiency takes its toll. A drop in efficiency of only 2% can mean 30HP on some engines. A drop of 6% seems small, but is huge. You can play around with the compressor maps and PR to see how the turbo will spool as well. At sea level the turbo will spool much faster before surge is present.

Eventually you hit a wall where pressure increases will no longer decrease the volume because the temperature of the air is heated to high levels (upwards of 350*F)!

Let’s go back to compressor speed. The compressor is directly related to the turbine, and speed is affected by boost pressure. At the 2.97 PR at sea level, the compressor would spin around 110,000rpm at 50lb/min of airflow. At high altitude the compressor is over 117,000rpm. How much power does it take to spin that turbo an extra 7000rpm? How about the extra backpressure to provide that spin? The other truth is the fact that the higher the turbine speed, the more backpressure. This is something that seems to be overlooked. Higher backpressure means a lower VE, this is a fact. Some engines may suffer from more reversion as the backpressure increases by a few psi. It will be harder to get into crossover at high altitude because of this.


Now let’s go into the other workings of the engine. Higher boost is not the cure-all for high altitude engines. More boost will lead to more engine blow-by. I have seen in person 50whp increase from removing the oil cap from a boosted mustang. The drop in crankcase pressure reduces pumping and windage losses and leads to more power. Adding boost increases this problem.

Changes in compressor efficiency will affect how much power is used to drive the turbine. A better compressor efficiency will make it easier for the turbine to power the compressor. In short; the more efficient the compressor, the less power it consumes from the engine.

Ultimately you can come to conclude that the turbo engine is more affected by temperature change than altitude, but temp change is directly related to altitude. The higher the altitude, the higher the temperature! Having a higher PR means the air is being compressed more, regardless of boost pressure. Even if boost is 2.2psi lower at high alt than sea level, the PR will still be higher. The more the air is compressed, the more heat is produced. The more heat, the more volume, the less dense the air is.

Oxygen concentration changes based on elevation also. At 5000ft there is a .5% drop in oxygen concentration. This is a small number, and stays around 1% at 10,000ft but it is a small change that needs to be compensated for. A boosted engine will only exaggerate this small number. Once boosted this number will take on more significance. With all the other variables to consider, this one is worth overlooking for now, since no one can calculate the difference it makes accurately.

Many in the racing world refuse to take into account the vast variables that need to be taken into consideration when changing altitude. NHRA correction is less for turbo cars at high altitude, but it only makes sense to do so. A turbo engine can raise the boost to limit power loss, but it can also be equipped with a larger exhaust housing rather easily to help improve VE. An NA motor will require compression ratio changes to maintain the same power output at high altitude. Higher compression ratio will raise the BMEP back to sea-level “levels” and similar performance can be maintained. It only makes sense to handicap the turbo engines for altitude in racing because small changes can be made to help compensate for the higher elevation without going into the engine.

You cannot simply relate power directly to pressure. More boost doesn’t mean more power. I lost power when raising boost over 24psi on my car. The higher boost doesn’t mean more airflow, or more power. By changes in compressor efficiency, PR, blow-by, and VE, you can see that some engines can be affected MORE by elevation than an NA engine. There are few cases where the compressor efficiency can actually be improved by higher PR provided by higher elevation, but only on the most detuned and low-boost setups. Bottom line is, don’t expect to make 600whp uncorrected on your GT35R turbo at high altitude. You likely won’t get more than 550whp. That’s nearly a 10% difference right there.

When it comes to SAE correction factors on dynos, it does not take into account the turbocharged engine. It is merely an estimate of power at sea level, and it has a 3% room for error. Instead of arguing that SAE correction shouldn’t be used on a turbocharged engine, one should ask how it works for an NA engine. The SAE correction factor for dynamometers is NOT a law of physics; it is just a calculated, estimated guess to what the engine will actually make on a perfect day. There is no such thing as a perfect day. Everything I have seen with Corrected Dynojet numbers at high altitude suggests that the SAE calculation gets very very close to corrected power at sea level, regardless of NA or FI engines. In either case the numbers should be used as a guideline.

The reason why I believe so strongly that the SAE number should be used for FI engines, is because the turbocharger air temps are affected drastically by ambient air temp and pressure. Using the SAE correction is the only way to get an idea of how much power you have made with each adjustment to the engine. For example: You dyno 420whp uncorrected when it is 100* in the shop, remove the exhaust and dynoing again you dyno at 400whp. Temp in the shop is now 108*, so even though you removed the exhaust, you are making less power now. How does that make sense? The SAE equation gives you some sort of comparability between numbers, regardless of weather changes. I have run my own car on the dynojet 248c and seen 390whp uncorrected on 20psi of boost, but only 375whp uncorrected on 22psi. Ambient pressure remained the same, but temperature and humidity changed. The corrected numbers showed 20whp increase in power, but the uncorrected numbers showed 15whp decrease in power! The only way you can get numbers that you can compare with each other is if you USE THE SAE correction.

Bottom line is that I have direct experience with cars that have dynod at sea level and high altitude and the numbers are amazingly close, within 1% of the “corrected” numbers. I have even seen people lose MORE than the SAE corrected number shows when going to high altitude within the same week. But ultimately, the SAE number gets very very close, regardless of engine configuration. One of my best friends bought a dyno this last year and It has been a good learning experience. I have learned so much about the dynojet and how it works, and it has confirmed what I was told in Automotive physics and engineering in college. The SAE numbers are to be taken with a grain of salt, NA or Turbo. But the end result will always be that the numbers are amazingly accurate for such a basic equation. Thanks for reading, now post up your thoughts.

No comments? I have fixed a few typos and tried to clarify things a lil more. I guess this thread doesn't really apply to most of the guys living on the coasts etc.

They prob still won't get it lol.

Wow, just plain wow. Great job on the explanations

I was able to keep up for the most part, some parts just blew my mind haha .

It's nice to keep in mind this information, even though I'm on the east coast, my city is at 1,325 ft (404 m) elevation.

Last time I could get my engine to redline (well 7k rpm) without peeling the tires off I had 236bhp, which seemed kinda low for 9psi ball bearing turbo (227 bhp stock), with 2.5" DP, 2.37" resback, UR Pulley 2 set, JWT Pop charger.

Thanks guys. There is some revising I would like to make but overall I think this is really informative. I hope it really helps those guys out who move from FL to CO and wanna explain why their corrected numbers don't match. I have seen guys lose more power than the sae correction says they should based on altitude change alone.

You guys should move east or little further west

But mountain driving is pretty awesome if the car can handle the turns