Dan and I have had this discussion many times.
If your going after torque, then I would say go for the lower pulley. You will not gain as much hp. Just a load of tq. But this comes at a cost. Primarily heat.
A quick reason why there is more tq achieved than hp. If you take a car that is running a 2.75 upper only. The car reaches roughly 14lbs of boost and lets just say at 6,500 rpms, the blower is spinning between 13,000 and 14000 rpms. With this combo average #'s are 460hp @ 6200RPM's & 484lbs tq @ 3200 RPM's.
Add a lower pulley and now you are making 460 - 470hp and 525 - 540lbs tq. But why is there more tq achieved than hp??
The reason for more tq is because at 3200 engine rpm's instead of spinning the blower at 6400rpm's you are now spinning it closer to 9,000rpms. Forcing more air at a lower engine RPM.
However, when you get to 6200 engine rpms the blower is now spinning between 18,000 and 20,000 RPM's. The rotors are losing their ability to grab and compress air.
Too much air is slipping past the rotors, kinda like a cavitation effect of a propeller in water. (Too much hp and you just spin the prop because their isn't enough surface area to grab the water. This does two things, one your prop spins and creates an air pocket, and two it super heats the prop.) Hence for a more aggressive pitch or redesigned prop (adding another blade), than can process water more efficiently.
At 14,000 RPM's the blower’s efficiency starts dropping off fast. It is taking more power to turn and producing more heat, eating away at the actual output. So the blower becomes better at producing more tq, but in the process creates a TON more heat. Leading to heat soak faster.
One reason the Kenne bell or screw type compressor is more efficient, is because of the rotor design. It can draw or pull air in without losing much air at the tip of the rotor. This increases volume and reduces heat.
A couple downsides:
1.) You will be heating up your air temp on the intake side, even at idle. You have to remember that when the motor is running, the blower is still trying to compress air. Even though you have a bypass, that doesn't prevent air from being brought through the supercharger and being compressed by the rotors. This is why; at idle, without taking the car for a ride your blower is usually hot. This may lead to problems with consistency at the track, especially on repetitive runs. Longer cool down may be required.
2.) The bearings will have a higher demand on them. Faster RPM's, and more heat will be generated in the bearings.
Well in no way, am I an "expert" but I have been reading and learning allot more about roots, screw, and centrifugal blowers.
Suggested reading: Supercharged! Design, Testing, and Installation of Supercharger systems. Book is written by Corky Bell.
Hank, if you want to talk more about this pm me or call my cell or work #.
I also have some ideas about the porting job's. But I'm still researching those theories.
:alcoholic
