Originally Posted by swamp2
At some level the comparisons are splitting hairs though because all SCs operate off of the crankshaft. This is the key difference compared to turbo chargers which operate from exhaust flow and are fighting impeller inertia at lower engine rpms. This is why calling any SC as not working at low rpms is pretty much a fallacy.
Sure, the impeller spins at low engine rpm, but centrifugals produce no boost at low engine rpm. Same goes for a turbo impeller, which is also spinning at low exhaust flow levels, but producing no boost. The only supercharger that produces boost at low rpm is the twinscrew or older style positive displacement blower.
In comparison, a good turbo system will make full boost before the first half of the rpm curve. At that point, the centrifugal will make roughly the square root of its peak boost, and won't reach peak boost until redline (although most of the second half of the rpm curve is fun because boost starts building faster and faster). A good positive displacement blower system should make full boost from roughly the first quarter to one third of the rpm curve right through to redline.
The reason the positiuve displacement blowers have not been fitted to the S65 is partly due to the fact that instant boost at low rpm would increase the likelihood of knock on a 12.0:1 CR engine, cost would be greater because a twinscrew is more expensive than a centrifugal and harder to tune, and practicality because the S65 has those nice air intake trumpets under the intake manifold and they would have to be sacrificed to fit a twinscrew or the twinscrew would have to protrude well above the already substantial hood bulge. In years of tuners modding E46M3s, no positive displacement blower kits were ever developed for similar reasons, but they do exist for traditional throttle body BMWs.