Originally posted by rhombus
The pressure ratios for the turbo charged car are diffrent for the 2 diffrent elevations. So you could be loosing power just because you are going into a diffrent area of your flow map.
Pressure Ratio (ATM+Boost)/ATM
(14.7+10)/14.7 = 1.680
(13.25+10)/13.25 = 1.755
Higher pressure ratios will move your point higher up on the compresser map. Most of us will suffer from lower efficency, unless we are running very large turbos. We will probably even hit the surge line upto a higher RPM then we would at lower elevation.
Yeah, I agree with you here...but, like you say, just because the pressure ratio changes, it doesn't mean that your changing the compressor efficiency that significantly (if any). For instance, the difference between a ratio of 1.68 and 1.755 on my turbo at 10psi is about the same... It still says in the same 78% efficiency island on the compressor map.
Not only that, but your VE (volumetric efficency) is also lowered at the higher altitude, even on a turbo car. You dont have 100% efficency in getting all of the compressed air into the cylinders. The cylinders are starting out at a lower pressure then they would at lower altitude.
This is not a seperate point though... The efficiency of the turbo, boost pressure, and atm pressure deal directly with the volumetric efficiency of the engine.
Also - Boost (and efficiency) will vary at different altitudes dependent upon your boost controller. With a manual boost controller, your boost pressure is relative to the ATM pressure. However with an electronic boost controller, if you didn't change the boost setting between different altitudes, you would be boosting higher pressures (relative to ATM) at higher altitudes than lower ones.
Why? The electronic boost controller relies on an electronic pressure sensor to determine the boost pressure. Let's say you set the boost controller to 10psi, and a measurement of 10psi is 3.0 Volts on the electronic sensor... Well the electronic boost controller is totally ignorant to the current atmospheric pressure (because it assumes that the ATM is 14.7), and will strive to sustain a boost level of 10psi + 14.7psi absulute pressure.
Anyway...So if the atmospheric pressure drops 1.45psi (as in eric's example), then the boost (relative to the ATM of 13.25) will be 10psi + 1.45psi.
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