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They are worth a few horsepower and if the engine is apart the extra cost is only for the pulley and the pinning. I had one installed with the H/C pkg. at A&A. It was on for two years(4,000 miles) with no problems at all. Took it off recently to install Vortech.
I would recommend one if you have no future plans to add a blower.
If you do have supercharger plans, or just don't like the idea of turning some of the accessories slower, at least get an aftermarket balancer in the stock size. The stock pulley is crap.
I did one with my Cam swap. the extra power is great, my last batch of performance mods came with my stock pulley falling apart, definitely get an after market pulley if you need to swap it. even with the under drive my alternator produces over 14.4V. I to notice less power steering assistance when making a turn while the engine is at idle. I haven't had the chance to check the A/C yet, but I sure hope that isn't affected, because the summers are brutal around here.
So....I take it you can't use one with a Vortech s/c?
The UD pulley has less circumference than a stocker, so besides turning the supercharger slower, there is less area on the belt to drive the huffer. That's why I had mine removed when Andy did the Vortech install.