I recently bought a Mini S with 15% Supercharger Pulley. What is the stock pulley? How does the % effect boost? I do not know much about the performance of my engine. Any coaching will be appreciated.

 

The pulley you have is 15 percent smaller than the stock one, so it spins 15 percent faster, generating 15 percent more boost giving you 15 percent more power (oversimplified).

 

It's, IMHO, the "best" sized pulley, especially if you ever hit the track.

You won't get any more power with a further reduction--i.e. 17%, but you'll max out the SC earlier, meaning you'll get your power earlier at a sacrifice of heat (you'll be spinning the SC above where it's making power as you go up high in to the RPM band, not creating any more torque/HP, but generating quite a bit more heat.

Although 17% is fine for a daily driver. It's just that going from 15% to 17% isn't earth shattering; 15% still maxes out the SC a little early (The JCW pulley, which is about 12%, was designed to max out the SC near redline, as a comparison), but, like I said, from a wear and tear/performance perspective it's my favorite.

 

The pulley gives you your boost ....the power you feel...and the whine...at a lower....and for most folks...more usable driving rpm. Think it is as moving the peak tq/HP to a more street friendly range ....making the car more fun. You DO gain some peak hp/tq on a dyno....but driving feel in town ....the extra zip is the biggest plus.

 

We tested the 15% vs 17%, same car, same day on our Mustang MD 250 Dynamometer. The 17% produced 6whp more than the 15%. The area under the torque curve was better with a slightly greater drop-off near red line. IATs were higher at red line by 5-7 degrees.

cct1: Your advice here is probably good, but there is a prevailing "truth" on the internet about the 17 and 19% pulleys 'not making power' in the high rev range. This is what I call an internet truth: logically it makes sense, but in practice it's not correct. Actually, the 17 and 19% pulleys make more power at stock red line.

That having been said, there's more to it and you need to examine how you are using the car before choosing a pulley. I won't go into it now, but there's a very good story about how the various sizes of pulleys were established...

 

Good to know--I was taking for granted my car, where the rev limit is set significantly higher than stock. I know you've dyno'd a bazillion MINI's--have you dyno'd the different sized pulleys say up to 7500 RPM's? Be interested to know what happens between 6500-7000, and 7000-7500, both with HP/torque and IAT's (water/meth is in my future).

I have the "Grand-Am" cam, and BVH, and spend more time up high than down low. 15%, from what I've read/been advised made the most sense for what I was doing, and the consensus from what I've read is it pretty much leaves all your options open. I've always been convinced the 15% was the "sweet spot" with regards to practicality, but that's the first I've seen anyone write about there being slightly more power with further reduction (and coming from you, I don't question it, you're one of the few on the boards who is unconditionally trustworthy.).

I've also read that wear and tear on the supercharger increases after 16%; that below 16% it's pretty much like stock wear and tear, but above that, it's noticeable on SC wear--any truth to that?

 

Above 7000 RPM, the 17 and 19% really fall on their face. The 15 percent pulley on car with BVH and track cam makes more power than a car with a JCW pulley at 7500rpm, so for sprint use, that would be the best solution. For endurance you might consider getting a JCW pulley.


That sounds like marketing talk. To my knowledge there have been no controlled tests comparing wear of various pulleys. It would be a difficult and expensive test.

 

Not to jack this thread.....but Helix, how much more boost would a 15% or 17% pulley make over my stock JCW pulley on my 03' JCW MCS? I think the JCW pulley is around 14% right? Would it even be worth the trouble to change the pulley on a daily driver?

I'm making around 12-13psi boost with the stock jcw pulley depending on the ambient temp.

 

A JCW pulley is about a 11% reduction. With good vanes in the charger, and 15% should make about 15psi, and a 17% should make another pound over that.