Bigger runners on the heads slow spool up. Spool up is about velocity......smaller runners spool faster , but choke top end power. Same goes for larger cams. I had 2530's on my car with the same heads and the graphs are lower throughout even though the pump pressure on the trans was lower then. The auto makes the graphs completely apples and oranges. The fact is if you only changed turbos on your car I can guarantee the whole power band would be better. The difference in design of the compressor housing is like the difference in a 4 cyl to a V8!
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