| Higher elevation has less dense air and lower ambient pressure. When your boost gauge says "15psi" a mile up into the atmosphere, you are producing a relative pressure difference of 15psi from the ambient. The mass of air in a 15psig "charged air" volume a mile up is less than a 15psig charged air volume at sea level. Since the relative average pressure at that altitude is 12.3psi, when your boost gauge says you are making 15psi, you are actually only producing an absolute pressure of 12.3+15, which is 27.3psia. At sea level, the atmospheric pressure is typically around 14.5psi, so when the boost gauge reads 15psi, you are producing an absolute pressure of 14.5+15, which is 29.5psia. The difference in absolute pressure would mean that in order to flow approoximately the same mass of air, neglecting compressor efficiency, you would need to run the boost gauge up to 17.2psi (29.5-12.3). This is going to change where in the compressor's efficiency you fall and dependent on what turbocharger you are using you may help efficiency or you may hurt efficiency. With stock turbos, this would definately be moving into the less efficienct regions of the compressor's operating efficiency and I wouldn't recommend turning the boost up to 17.2psi. As for the SAE correction factor and how it plays into the dynocharts, I believe you are confusing some things. most of what you said is true except the inflation of hp on the SAE scale compared to sea level, it is possible that our turbos will find a better efficiency range at lower elevations and make more power at similar boost levels. At sea level, 30% relative humidity at 60 degrees, the SAE correction factor will be Zero. The numbers the dyno reflects are what the car actually put to the wheels. Now, if you go to high elevation, the SAE correction factor is going to give you a few extra percent because it sees the relative atmospheric pressure being considerably lower, however, a turbocharger with an intercooler can make up for the losses due to the atmospheric conditions considerably better than a naturally aspirated engine will, hence, the SAE correction factor DOES NOT WORK FOR FI/INTERCOOLED vehicles like it should. It works for comparing NA vehicles in any environmental condition, but the nature of a FI/Intercooled setup makes up for a lot of the losses than an NA powerplant will suffer as you start thinning out the air. The only time the SAE correction will be correct is when it isn't applying any correction at all. This will only occur when the atmospheric conditions are as what I detailed above. If what I am saying is incorrect, then I am very much open to hearing your explanation for why these cars, a mile high, can put out such bloated numbers for their setup. FWIW, I had a customer at stage3 in denver put out 385RWHP SAE at 15psi according to the dynojet (that's intake, exhaust, ECU). Most seasoned Z owners will know that this is a pure impossibility.
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