Installation of Steeda underdrive pulley affect supercharger boost?

[jrallen]

I recently installed a steeda underdrive pulley system on my 06' GT, how will the smaller diameter pulley afftect my supercharger boost? I have a Saleen supercharger with a 3.2 inch pulley on the supercharger, bumping the output of the supercharger to 8 lbs. What are the calculations that the underdrive pulley will do to the boost?

 

[o0Dan0o]

It will lower your boost, but the calculation is not so simple. Do you know the size of the UDP and the size of the stock pulley?

EDIT: Basically, divide the boost number by the stock pulley diameter and then multiply it by the UDP diameter, that will get you close.

 

[Romeo46LV8]

I had always thought you were not supposed to run UD pullies when running a SCer. Wouldn't it give you the reverse effect of that of UD pullies on a N/A car (ie. lowering boost, thus lower HP output by a greater number than what the pullies make up for)?

 

[bigcat]

why waste your money on underdrives when the boost more than makes up for it?

 

[gpp]

i wouldnt want to take mine off either if it only meant losing say 1 lb of boost.anyone have any idea at all on how much you lose?

 

[Romeo46LV8]

i wouldnt want to take mine off either if it only meant losing say 1 lb of boost.anyone have any idea at all on how much you lose?

I think what's trying to be said here is if the UD pullies give you 10HP, but the 1lbs of boost gives you 20HP - if you put the UD pullies on with a SCer and lose 1lbs of boost, you'll be making overall less HP in the end.

I'm by no means an expert so these numbers are only for example. The loss of HP could be more or less & heck maybe someone can prove UD pullies w/certain types of FI actually produce more HP.

What about upping the boost? What about torque? What about the fact that some of the accessories are being driven slower so then their life maybe extended? All questions I'd have...

 

[Gearbanger 101]

You could always swap out to a smaller blower pulley to bring boost back up to previous levels. You'd have to check to make sure you're not running too small a pulley though, otherwise you'll suffer from a loss of traction on the belt because of the smaller contact surface area of the pulley.

If the stock 3V crank pulley is the same diameter as the 2V, then it measures about 6.85" (which it may, or may not be. You’ll have to measure for yourself to know for certain)

D = Diameter of the Crank pulley
BP = Diameter of the blower pulley
R = Over all pulley ratio

6.85 (D) / 3.2 (BP) = 2.14:1 (R) pulley ratio.

What you want to do is measure the diameter of the under drive pulley, then swap out to a smaller supercharger pulley that still gives you the same ratio.

ex...if the under drive pulley is roughly 5.0" in diameter (which is just an example), then you'll need to do a little reverse math to calculate the size of the new blower pulley to give you that same ratio. 5.0 (D) / 2.14 (R) = a 2.336 (BP)

My guess though, is that you're going to see some belt slip with that small a pulley, unless you go with an 8-rib conversion....which isn't going to be cheap.

 

['01 Steed]

Brian is absolutely right. But even if you bring the ratio back into line, all you've really done is thrown a couple hundred bucks out the window.

You've lost a great deal more power than you thought you would have gained by installing those pulleys.
U/D's on the modulars is good for 7 rwhp, maybe, on an n/a car. You've just lost a good 20-25 hp worth of boost, if not more, and that's not accounting for the probable belt slip that you'll experience.
My advice to you, and anyone else in this situation, is to remove them, and go back to the stock damper.