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Old 12-21-2012, 09:39 AM   #82
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Join Date: Dec 2007
Location: Overland Park, KS
Posts: 371
My Ride: '03 330i ZHP Sedan
Originally Posted by NFRs2000nyc View Post
A turbo is spooled by how much air is shoved through it. While a GT30 might spool at 3000rpm (fully) in an S2000, the same turbo in a mack truck will spool far faster due to the amount of air the truck motor pushes. So, if he is running a motorcycle turbo on an MR2, the airflow might be considerably greater. Having said that, I don't know too much detail in the setup, since like I said, I saw the car that one time, so I guess those that are more knowledgable in this spectrum can clarify a bit more.
Basic concept correct, details not quite accurate.

A turbo's "spool" is referred to the RPM point at which you've achieve full's not referring to how much air, but how much pressure. Volume and pressure are two different things, which is why there are compressor maps. Turbo size and PSI are related as based on the compressor, the volume of air pushed through the turbo may not match the demands of the engine, and thus the turbo is having to work harder to maintain the target Pressure...or in many cases with small turbos, cannot maintain that pressure and you see the PSI drop towards redline.

This is why, as I stated, a small turbo is good for autocross but sucks for everything else. Being so small, it most likely cannot maintain target pressure to redline and if it can, it's so out of its efficiency range all it's doing is blowing hot air and not making good power.
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