Originally Posted by
Aaron
You're wrong, and here's the numbers. Now first, I'll say in general. There are electronically controlled, ambient pressure adjusting wastegates. They are extremely rare in automotive applications, used mainly for aircraft. In fact I've never heard of one being used in an automotive application. There are also ambient pressure supercharger bleed-off valves, that basically spin the supercharger faster than needed all the time, and bleed off excess boost. Again extremely rare, and never been in an OEM application as far as I'm aware. This is for 99% of automotive applications, including the Subarus, BMW's, and my Fiero.
Sea level = 14.69psi
Colorado Springs = 11.7 (That is the cells that my piggyback is at with ign on, engine off).
Anyways, we lose about 3psi ambient pressure due to altitude. For an N/A car, this is a 20.4% loss in air pressure, which would equate to a 20.4% loss in power (Assuming same ignition timing maps, which is likely).
Back in high school, I had a friend who moved here from Texas, on the coast, who had a modded GTP. Via his data-logging software, he saw dead on 15psi. Added to the 14.69, he had 29.69psia. However here in Colorado, due to how a supercharger works, he only saw 12psi. Combined with the lower atmospheric pressure, he only got 23.7psia. He saw a 20.2% drop in psia, equating to a rough 20.2% drop in power, near identical to the N/A numbers.
Boost is pressure above ambient. Superchargers will show less boost as altitude increases. This is because they are set to run at a certain speed, which means a certain airflow. So on the inlet side of the supercharger, they are down 3psi ambient. Because there is less air going into the blower (3psi less), there's less coming out (3 psi less). So you've got a 3psi loss in boost pressure, mixed with a 3psi loss in ambient pressure, about a 6psi loss in total air pressure, which gets us our roughly 20% loss in power.
However, things are different for a turbocharger, since boost (Pressure above ambient), is kept equal at your peak power. So for me, it was 14psi. Added to the 14.69, at sea level, I'd have 28.69psia. Up here, that drops to 11.7 + 14 = 25.7psia. A 10.4% loss in power, roughly half that of the N/A and SC cars, exactly what we'd expect.
Superchargers suffer just as much power loss at altitude as N/A cars, with turbo cars being about half of those.