Originally Posted by M&M
A super provides "absolute boost" just the same as a turbo does. A CF blower and turbo are essentially the same except one runs off exhaust gasses and has a wastegate and the other runs off the crank. I don't follow the second part of the sentence. Due to speed of the compressor being related to the rpm? The rpm range is the same at sea-level or at 5000ft. The car will rev from idle to 8500rpm at both elevations. So every revolution of the crank will produce the same amount of boost and spin the same amount of air into the engine at sea-level and at altitude. Incorrect
The difference being that the air is less dense up here. 17% less dense. So you are increasing the power from a much lower base. And the extra air you are spinning in is also deprived of oxygen compared to sea-level due to the air being thinner and less oxygen rich.
Air has less available Oxygen as the ambient pressure is less. The minute the air is compressed to the same pressure, the oxygen levels are the same. Air is 20% Oxygen / 80 nitrogen.
So I think Roman will run a smaller pulley due to the engine being nowhere near the limit of power one gets to at sea-level. You are starting with a 50hp deficit so one can run more boost safely up at altitude.
Engine power is not the issue we are chasing here. Boost pressure is. Once we achieve 5,5psi at high altitude, the engine will be receiving the same volume of oxygen is was receiving at sea level when the boost pressure was 5,5psi
engine RPM is directly related to blower RPM as its belt driven.
At sea level the air is denser and therefore the blower spinning at a particular RPM produces X amount of boost pressure.
At higher altitude, the air is less dense, and therefore the blower spinning at the same rpm as it was at sea level, now produces less boost pressure.
To compensate for the loss boost pressure, one must run a smaller pulley on the blower, thereby spinning the blower at a higher RPM, to achieve the same boost as it was previously making at sea level.