why remove underdrive pulleys when supercharging?

The problem with it is, that as sgarlic stated, the smaller (under-driven) crank pulley is going to slow down all of your accessory pulleys, including your blower pulley. And yes you’re right….it may be as little as 1-2psi, or it could be more in the line of 3-5psi depending on the type of blower you’ve got and how high strung you’ve got it running. If you’re making in the 8-15hp range for every pound of boost, does it really make things better freeing up 6-8hp running under drive pulleys, just to lose 8-50hp because now cut the blower speed dramatically?

Now, the problem with just making up the boost with a smaller blower pulley, is that as it is with most systems running within the 8psi+ range, the 6-rib pulley design provides very little grip on the belt to start with. Going with a smaller blower pulley, to increase blower speed also decreases the surface area in which the blower pulley has contact with the accessory belt. This will lead to some serious belt slippage at even 8psi….and it only gets worse the harder you spin the (even with a true grip pulley) when the blower is under load.

To increase belt contact again, you could convert to the truck 8-rib pulley set up, but this tends to be an expensive venture for most street set-ups. Especially when it costs you nothing to just revert back to the stock 6.85" crank pulley.

Does that clear things up for you guys?