James B.

iTrader: (1)

I used to run 10psi with an intercooled Whipple on a 9.4:1 383 SBC with 18 degrees of timing. That was a 7.5" crank pulley and 2.5" S/C pulley and only 6-rib belt. The missing power is not a matter of the cam, the boost, or the compression in my opinion, it's timing. You don't have enough advance. Dialing in your timing will help with the fuel economy too. I think 8.5:1 compression is perfect, you've got room there 15psi boost on 91 octane.
Keep a close eye on AFR when you go into power enrichment mode. You need wideband logging capabilities. A lot of power can be lost from being too rich.
There's another thing here that's slightly confusing that I think should be mentioned -
Roots and twin screw superchargers require an amount of power to turn that is a factor of the volume of air they are moving and the pressure of the discharge plenum. The efficiency map also plays a smaller roll here since each compressor has different VE at different speeds and loads. Whipples are more efficient at making high-psi boost because they are truly compressors, Eatons (Roots) are not. Because of differences in VE, a large compressor (3.3L) spinning slower to make the same amount of boost for an engine that a smaller compressor (2.3L for example) would be spinning much faster to make, would take slightly less power to turn because less of that power would become heat due to air friction with the faster-moving rotors. Also, even though a larger displacement compressor has larger and heavier rotors, they're spinning so much slower that less inertial energy has to be overcome during RPM changes. An Eaton positive displacement blower and a Whipple twin screw compressor have different characteristics in rotational inertia for two reasons - for each CID Whipple rotors weigh more. Also, the second rotor in a Whipple is overdriven 1.66x faster than the input shaft. There is more rotational inertia in a Whipple than in a an Eaton. You'd notice this free-reving the engine (which is hell on belts and tensioners powering superchargers.)
You mentioned that the 3.375" pulley is effectively the smallest pulley you can run without slippage, and you think that's because of the supercharger size. Something's wrong here. The size of the supercharger is less relevant than the amount of work it's doing, and pushing only 10psi into a 427CID engine is barely enough to make that unit break a sweat. If it's slipping with pulleys smaller there's either not enough tension on the belt or not enough wrap around the S/C pulley. You shouldn't be getting slipping problems with a 10-rib belt on this setup, so I think something else isn't right. Running a 2.875" pulley making 15psi with the 10-rib drive shouldn't be causing you problems.