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      04-12-2010, 02:36 PM   #9
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Originally Posted by Bob MG View Post
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
Technically speaking, that is not true. When density changes, the volume remains the same.

But you are also compressing air that is less dense. As air gets thinner the molecules are further apart. So there is less oxygen per square inch at 5000ft than there is at oxygen rich sea-level. Ever wonder why climbers use oxygen masks at the top of Mount Everest? Each breath you take, you ingest the same amount of air you ingest at sea-level. But the air is pretty much useless as the 02 molecules are so sparse it doesn't feed you.

Originally Posted by Bob MG View Post

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.
Ok on that point I agree with you. When density changes, pressure varies by:

P2 = P1 (D2/D1)

So a 5.5psi charger at 5000ft will produce = 5.5psi(850mbar/1000mbar)
= 4.675psi or 15% less boost.

So you would need a 15% smaller pulley to make the same boost.

But even if you get the same boost, the quality of air is not the same. So actually you will need an EVEN smaller pulley to compensate quantity for quality.

But hey, that's just my opinion. It's a great discussion with lots of grey areas si I am open for correction.