Quote:
Originally Posted by mrg666
Superchargers, especially centrifugal types, increase the boost with rpm; that is its advantage over turbo. Why would you implement a boost-by-rpm with a supercharger where it already works that way? Useless idea. All that discussion for nothing, unrelated to the original question anyway.  All that is done by adjusting pulley size and valve spring. |
A supercharger is a disadvantage in ways, and it is an advantage in ways, especially for not breaking rods on stock blocks. The OP didn’t give specifics, so we don’t know if this is for him or a hypothetical. Regardless, it is worth being exhaustive.
Someone who was doing boost-by-gear would also likely want to do boost-by-rpm. Boost-by-gear is about limiting the boost ceiling in a particular gear, so 5 psi in 1st and 8 psi in 2nd and 17 psi in 3rd and 25 psi in 4th through 6th. Something like that. The turbo would still have the same torque profile, so if it could hit 25psi by 4k then it would still be hitting say 7 psi by say 1500 rpms, so someone may want to ramp up boost a little for first gear for traction.
On a supercharger, changing the pulley changes the max boost and the aggression/steepness of the torque profile, so it would make just as much sense to do the same. Yes, a centrifugal supercharger wouldn’t need as much adjustments by rpm, but here is a sample for an unspecified supercharger: 4-6th gear is 20 psi by 8k, so 10 psi at 4k and 5psi at 2k. Maybe 5 psi will be the max desirable boost in 1st gear, but it would still hit by 2k, so maybe that would be too much. Someone could limit the slope so 5 psi is hit at redline in order maximize traction at WOT launches. Considering most stock cars will break loose if input isn’t controlled, it would make sense that someone might want to do boost-by-rpm instead of just limiting boost to say 5 psi (boost-by-gear) and holding that from 2k to redline. Get it?
Again, this can be done, and it is done, but at a greater cost in heat generation, efficiency and supercharger wear.