I haven't yet, but I've got a 15% pulley, I'm out of warranty, and when I find someone with a tool for the JCW pulley, it's gettin' swapped. I'm basically looking to pick up another couple pounds of boost is all.

 

President of our club went from JCW pulley to 17%... but I don't know any JCW to 15%'ers.

 

The JCW reduction is based on taking the Eaton supercharger to it's maximum certified rpm. Anything smaller, ie 15, 17 and 19% turns the supercharger faster than it was designed and will likely reduce the overall life of the supercharger.

 

right!

 

wrong!

the JCW pulley, combined with a higher rev limit, still goes over the maximum speed for the charger.

ninjedit: but i am still highly skeptical of the safety of running high reduction pullies

 

Where is this "higher rev limit" coming from?? The JCW flash doesn't change the rev limit - it's still 6,950.

 

Maybe he was referring to the GP rev limiter . . . ?

 

sorry, I was referring to the GP.

 

Ah. Gotcha. Thanks for the clarification. I had forgotten that the GP had a higher limit.

 

well, i wasnt. i had incorrectly believed that the JCW ECU raised the rev limiter. with a 10% reduction and the stock rev limit, it is turning only 500rpms higher than the max shaft speed.

 

I’ve measured differences among standard and JCW size pulleys which has yielded varying percentages of reduction for the JCW size pulley. Whether it is from machining tolerances or measurement error I can’t say for certain. For the two shown here, 11.45% is the greatest reduction I’ve measured to date.



The JCW pulley is machined from solid billet, the standard is a welded two piece. The JCW appears more robust and might be able to tolerate conventional pullers better than the thin bolstered standard pulley; the standard’s lip almost looks stamped. I’ve seen a few badly bent standard pulleys which didn’t fair well with ducks foot/claw pullers. To remove this GP pulley, I started with a P&D style puller clamp and had it milled down on one side, this provided a very firm grasp on the smaller pulley.


About the performance gain when reducing to 15% from the JCW, I’ve read quotes from John Behe of Behe Performance (formally known as Lucky Dog Garage) in which he indicated the gains were in the 4-5 hp range.

 

it's the first time I see this kind of pictures showing real numbers. so 11.5% is the reduction for the JCW.
now I understand why forge sell the 11.5% pulley. and I'm sure the difference in hp/tq is not too much compared with the 15%

k-huevo about the SC, is it really the same? did you find any difference between the JCW and 05-06 mcs SC?

 

I’ve measured JCW reductions rounded off to 10.4, 10.6, 10.9, and now 11.45%; that’s the “varying percentages” point I was referring to. It is considered a 12% in the U.K., but from my measurements I think 11% is closer to the mark.

The only build difference a JCW SC has over a late model standard is the pulley.

 

So.... what does installing an 11% or 15% pulley do the air:fuel ratios?

The JCW upgrades includes larger injectors to deal with the increased air flow.

 

K-huevo -- I don't know . . . I really do need to see some more evidence that it's not 14.8%

based on what?

 

I did a few calculations on this and posted it up on MINI2, here's the post i made:

I've managed to source all the info for the stock pulley and for the Eaton M45 so here goes:

The stock crank pulley size is stated by MINI as 140mm
The stock supercharger pulley is stated by EATON as 65.5mm
This gives a ratio of 2.13 meaning at 6,750 rpm the supercharger is spinning at 14,427 RPM

Eaton state the maximum RPM for the M45 is 16,000rpm
So to stay within their tolerances when getting a smaller supercharger puller, the smallest you could get would be ~59mm
So that would equate to approximately a 10% pulley

For boost pressure i've had to base it on 4 degrees of overlap as i can't find any other info:
So with standard pulleys the supercharger produces 14.63 PSI
With a 15% pulley the supercharger produces 19.87 PSI
With a 17% pulley the supercharger produces 20.84 PSI
With a 19% pulley the supercharger produces 21.66 PSI

All of these are based on theory, they don't take into account thermal inefficienies due to going over the max RPM

 

the belt doesnt' keep constant length at the top of the pulley ridges, but slightly above that. has to do with the belt construction. So the ratio calculation is (R1+delta)/(R2+delta). Sorry to say, I don't remember what delta is. IT's about an 1/8th or a 1/16th of an inch or so. Also, for the boost curve, you can look at the temp delta vs pressure ratio for the eaton, and it starts to rise pretty fast as you approach red-line, this increases the boost more than your calc has done, but I have no clue how to do the math well. The pressure before the SC is what's important, and that's not ambient. It's also a function of RPM and pulley.

Matt

 

BTW, I have an '06 JCW and replaced my stock JCW pulley with a 15%, maybe 8 months ago. The difference was very noticeable. I am very pleased with the upgrade, and have gotten over the warranty issue. My stock JCW pulley measures at 58mm diameter, and I definitely believe it is in the 11 - 11.5% range and the 15% delivered 5hp, and at low rpm right where I needed them.
So, yes, even a JCW car can benefit from a 15% pulley. Recently though, I have considered a 17% as track days are rare, and a little higher reduction may be better suited for the street.

 

It is amazing that so many different numbers are flying around about the size of the JCW reduction. I always thought it was 14.- but I guess not... I was considering getting the JCW pulley once my warranty is up next year because I thought it was basically a 15%, but clearly thats not the case. I am leaning towards the 15% over the higher ones to preserve S/C life, even though I never hit the redline.

 

You thought wrong, but you have now guessed correctly!

 

I am sure you are doing this correctly... but i can't help but ask this question after reviewing your first picture. In the first picture it does not seem like you are measuring the pulley at its widest position. I am sure you wouldn't do that... its just the picture apears, at least to me, that you are measuring it slightly above the widest portion of the pulley and giving yourself in anaccurate diameter to compare against. Perhaps your calipers are really long and slender at the end and they are hidden by the grooves on the pulley?

 

The photos are for instrument reading illustration purposes; the measurements were taken at the top of the middle land at a sagitally devided line on each pulley. I could have supported the caliper and moved the JCW downward for the photo, but I would have lost a symetrical pulley comparison, which I considered a better way to frame them at the time.

 

These variations could be due to pulley temperature differences at time of measurement.

 

It is the JCM (John Cooper Motorsport) that has the 14.5 in their 225hp kit there are some pics of the JCM and stock JCW pulleys side by side on Robert Apons blog. You can visualy see the difference.

JCM on left and JCW on right.

 

Did a little digging and found out that JCM is now http://www.minimotorsportcentre.co.uk/ after John Cooper Works was purchased by BMW. Website has no info on its products whatsoever except the email of the chief engineer.