In Reply to: You mean like a 'roots' style blower?? posted by PM3 on October 06, 2000 at 14:25:10:
A 'roots' type is one type of positive displacement supercharger. There are others such as Vanes and Lysolm screws but the 'roots' is by far the most common. All the factory supercharged cars use a 'roots' type made by Eaton.
"...but I've driven both a Dinan
SC (stage II) 3.2L, and an RMS Vortech stage whatever, with a 3.5L stroker (forged internals) kit making 400 hp at the wheels. Granted this was in CO, at 5.5k feet of altitude, but no where near what I experienced in a 450 bhp (now it's 480, actually) Stage II+/Gen II AA Turbo...."
Of that I have no doubt. Both SCers you mention are centrifugals. They don't have full boost until redline due to the way a centrifugal works. Their airflow is not linear with RPM... it is a square of their driven speed. Because of this centrifugals make for a peaky sort of power delivery.
The turbo would have had much more boost at lower RPMs so it would be putting out far more power then a centrifugal at a comparable RPM.
BUT, a positive displacement blowers airflow is linear with RPM. Hence the term 'positive displacement' (IOW it flows the same amount of air every revolution no matter if it is turning at 1000rpm or 15,000 rpm) Because of this they will give full boost through almost the entire rev-range and will have dramatically more power then a centrifugal down low. On my Eaton supercharged Z3 (D/A kit) I get 2psi of boost at 1000 rpm.
If/when you drive a positive displacement supercharged M3 you are simply going to think it has a MUCH larger engine in it. If you are ever out on the East Coast (Ma.) you are welcome to take my Z3 for a spin.