I have always heard that the altitude you race at makes a difference with naturally aspirated cars but not with supercharged or turbocharged cars because the engine is pressurized. Does this sound right?

 

Altitude does have an effect on a turbo/sc car because you're looking at a pressure differential.

As an example, you can make 15psi at sea level, or 5k feet, but it will work harder to make it at 5k feet because the pressure on the inlet side is much lower, which will also give you a hotter, less dense intake charge.

 

I live in Colorado, altitude is 5280 ft, give or take. typical DAs are 7000ft on a good day. typical dyno correction factors for NA cars are aproximately 1.23-1.25, so that gives you an idea of the power we lose... here's how it works.. a supercharged car will suffer the same losses as a NA car, because the blower is still spinning at the same speed, but it has only 12.1 or so ambient air pressure to work with, so it makes that much less boost. and an NA engine has only 12.1 or so psi to work with as well. However, a supercharged car is still more beneficial than an NA car, because all they have to do to get back the power they loose from altitude is spin the blower up faster so it will again make the boost it made at sea level, or more. turbo cars will not see the losses of an NA car.. they typically see 5-9% losses from my experience. It will depend on how close the turbos are to being maxed out at sea level. If at sea level a turbo car's turbo(s) are close to being maxed out, then when they come to altitude they will see power loss, becasue the turbos cant work any harder.. and a turbo will have to work harder at altitude to make the same boost because it is working with thinner air. Spool times are also increased because of this. A car with turbos that are not maxed out, will not see much power loss, but still some (the reason for this I am not 100% sure), but possibly because the turbo is working harder and generating more heat in the process. hope this helps. This is why I want to turbo my LT1