I happen to think Dana is correct. This explanation will not try to prove any actual numbers, but more importantly talk about the fundamentals that are at hand when discussing the ability of a turbo to compensate for altitude. Mind you, you have to think like an engineer here and identify the core variables that make the difference in power. I believe the primary point is the turbo loses much efficiency and here is why. First an example, we always talk about larger turbos providing more efficient power because of lower charge temps, even at the same pressure. Wonder why that is? A few points. And this is simply a primer. And I am only discussing the system from inlet, through the turbo to the intake manifold.
1. Fact: High altitude air has a lower density. This is not a point in question.
2. Fact: Lower density air has a lower heat capacity primarily due to its inability to hold as much moisture(not to mention there are just less molecules of air there). Heat capacity defines how much energy it takes to raise the temperature of air a certain specified *F or *C. Therefore, it takes much less energy to heat up high altitude air than lower altitude air. Enter the turbo.
3. Fact: The turbo, at 5300ft is trying to take air which is already at a lower atmospheric pressure than sea level, and compress it to say 14psi. Therefore, the turbo is already working harder to compress it. More importantly, the energy that the turbo is putting into compressing the air is SIGNIFICANTLY raising the charge temps in comparison to air at sea level because it is applying the same(actually more) energy to compress it and the air has a lower heat capacity. Simply consult adiabatic charts in any thermodynamics book to see what I am talking about. This is no doubt having an impact on the efficiency of the turbo. I am not quantifying, just saying it is.
4. Fact: The partial pressure of oxygen at 5300 ft is the same as that at sea level. But there is less pressure at 5300ft, hence less oxygen. The partial pressures(for this purpose) remain the same. Compress it to the same 14psi at sea level and 5300ft, and no matter how you slice it you still have less oxygen. You simply don't create a higher oxygen ratio just because you compress it. That would violate every law of thermodynamics, and heat and mass transfer.
5. Fact: The intercooling system is cooled by atmospheric air. This outside air has a lower heat capacity, and is therefore unable to do as good a job of pulling heat from the charge air which is flowing through the intercooler. This explains why you blow humid air on yourself to cool off, not really dry air. Humid air has a high heat capacity. Dry air has a low one. Darn, now we have a less efficient intercooler.
Heat capacity, adiabatic charts, and partial pressure of oxygen= higher charge temps, less oxygen, and a less efficient intercooler. These are the facts and they are indisputable. And they all point to less power for a turbocharged engine. 6.7% less power? I really don't know, but could probably get fairly close if someone gave me a compressor map for the 15g. But jeez, don't Dana's numbers(actual) seem right on if it does???
Please Joe, dispute these facts, almighty one.
Edited by kenhoeve, 14 April 2004 - 05:38 AM.
