I've taken my Evo up to the mountains a few times, and I was under the assumption that a Turbo would compensate for the loss of Oxygen at altitude, but for some reason I still feel a lack of power. Now that I've put my boost gauge in, I am getting a consistant 1.3-1.4 bar of boost, so it should be producing the same amount of power, but it isn't.

I read an article that suggested that if the turbo is too small, then it won't be able to compensate for the lower oxygen pressure, but since I'm getting full boost, then I would think that's not the case.

The only thing that I can think of is that the MAF sensor is seeing the lack of O2 in the air and the computer is making the A/F mixture richer. Is this why?

Any ideas?

-CJ

 

Cause the higher you go the less dense the air is and the harder it is for the turbo to make power. Less dense air mean that there is fewer air molecules entering your motor hence the lower psi of boost.

 

I am seeing 1.3-1.4 bar of boost though, so that compensates for the lack of O2 at atmosphere...

 

The same psi means the same air flow to the engine but not necesarily the same amout of oxigen then power does not have to be the same too.
When i travel to mountains (4800meter above sea level) i also feel the power decrease.

 

From http://auto.howstuffworks.com/turbo1.htm:

This still doesn't explain why the Turbo doesn't compensate fully for the thinner air. 1.3 bar should be the same at sea level vs. 7k feet, shouldn't it?

Oscar, are you saying that the amount of O2 vs. CO2 or Nitrogen, etc is different?

-CJ

 

I think the gauge is comparing the manifold pressure to your current atmospheric pressure. So yes, you are getting 1.3 bar greater than atmospheric but since you're at altitude atmospheric is already lower. Make sense? The boost gauge is not measuring absolute pressure. I guess if you had a boost controller you could set it really high and compensate.

 

The amount of O2 is definitly lower...air is less dense then that´s one reason why you get lower atmospheric pressures.
So image turbo is rotating at the same rpm (in fact turbo spins faster in high altitud when comparing with sea level) hence the same air flow should be going to the engine but it doesn´t mean the same O2 quantity is going to the engine and you need O2 to make more power.

The same happens with IC. You need to cool air in order to get it more dense (more O2 molecules) and generate more power.

 

That makes perfect sense. Is this is the reason, then the computer would be bleeding off boost based on what it thinks the boost is based on what is currently atmospheric pressure... Does the computer know what the current atmospheric pressure is, and can it pull more boost based on what the pressure is? Or is the Turbo at it's limits?

-CJ

 

I'm not an expert on the ecu stuff but I don't think the EVO's ecu actually knows the boost pressure. It's not a closed loop control so far as I know.

 

The lack of power has to do with air density, at higher altitudes the air is less dense. Air density is why we have intercoolers and also why we (or any vehicle) makes more power when it is cold out. You will still be moving the saim amount of air (boost pressure), but it is just less dense, so it actually has less molicules of everything. I hope this helps you understand.

 

Great question! You initial thoughts are correct, the turbo charger actually does help altitude performance, but there is a limit to the benefit. In other words, a turbo charged engine may deliver the same power at 3,000 msl as at sea level. But.... unable to compensate above 4,000 msl.

With a naturally aspirated engine, horsepower drops off 3 percent per 1000 ft (300m) because of the 3 percent decrease in air density per 1000 ft (300 m). If fuel delivery is not reduced, smoke level and fuel dilution will increase with altitude.

With a turbocharged engine, an increase in altitude also increases the pressure drop across the turbine. Inlet turbine pressure remains the same, but the outlet pressure decreases as the altitude increases. Turbine speed also increases as the pressure differential increases. The compressor wheel turns faster, providing approximately the same inlet manifold pressure as at sea level, even though the incoming air is less dense.

However, there are limitations to the actual amount of altitude compensation a turbocharged engine has. This is primarily determined by the amount of turbocharger boost and the turbocharger-to-engine match.


Speedlimit...

 

How about this for a simplified version, due to the thin air the turbo has to spin faster to produce the same pressure. This causes the turbos efficiency to drop so the air temps are higher comming out of the turbo. This is where a bigger turbo can help, of course this also why they make more power at any altittude. With the turbo working harder it also creates more back pressure in the exhaust manifold. Also the thinner atmosphere reduces the efficiency of the intercooler.
So even though you have the same pressure, the air temps at the intake manifold are higher (less dense) and you have more back pressure, not a good situation for making power.

The same pressure at the same temperature gives you the same air molecule density. At higher altittude your intake manifold air temps are going to be higher for a given boost pressure.

 

That's an excellent explaination! Thanks much!

Now, a second question... Would this mean that since it's not getting as much air, that the A/F mixture would be on the rich side, or does the computer know to fix the mixture?

-CJ

 

Great posts guys.....Dale K has it correct

However, the ecu most certainly does know the boost level you run, hence the prescence of fuel cut at a certain level.

Where are you measuring boost - at the turbo itself or at the manifold?

 

I'm measuring boost at the intake manifold... That should be the most "correct" place to measure the boost, right?

-CJ