Wouldn't a 2% crank and 15% sc pulley = more than 17%?
 

I was just thinking, the crank pulley is quite a bit larger than the sc pulley, so wouldn't a 2% increase actually be significantly more than 2% when it reaches the supercharger pulley? If that makes any sense..

Yup

On that note: if the pulley (15%) and and the crank (2%) are upgraded, not just the pulley; is it true that its safe to run for prolonged time? I know M7 sells the pulley & crank for 17% total and claims that it wont kill my supercharger. True or false? :popcorn:

I am currently on my 3rd R53... On my first one I ran a 15% pulley and never had a problem on around 80K hard miles, on my second MINI I had a 19% pulley, and ran that for close to 70K miles with no problems, and I currently have 6k miles on a 17% pulley and 3% crank pulley, I have never had any problems with any of my cars, so I would say your fine, but thats just my experience.

I am currently on my 3rd R53... On my first one I ran a 15% pulley and never had a problem on around 80K hard miles, on my second MINI I had a 19% pulley, and ran that for close to 70K miles with no problems, and I currently have 6k miles on a 17% pulley and 3% crank pulley, I have never had any problems with any of my cars, so I would say your fine, but thats just my experience.

The increases are multiplicative, so the total effect of a 15% supercharger pulley and a 2% crank pulley is more than 17%, but not very much more.

1.15 * 1.02 = 1.173

So the combined effect of the two pulleys on the supercharger is 17.3 %

I don't think I was clear in my original post.

What im saying is that if you multiply x% by the supercharger pulley size, you will end up with a much different 'decrease' than you would if you mulplied x% by the crank pulley size, given that the crank pulley is much larger than the sc pulley.

In other words, 1% x sc pulley size < 1% x crank pulley size.

Am I way off base?

The actual physical size of the pulleys doesn't matter - just the percentage of increase/decrease.

If you increase the circumference of the crank pulley by 2%, the supercharger will spin 2% faster. If you decrease the circumference of the supercharger pulley by 15%, the supercharger will spin 15% faster. That's all the math you need - the fact that the crank pulley is so much larger than the supercharger pulley doesn't come into it.

Makes sense, now i feel dumb

No need to feel dumb - it's tempting to over-think the problem and make it harder than it really is.

The smaller pulleys won't give you problems as people think they do. The point of not going to a 17% or a 19% like you have, is because they actually hurt the power due to all the heat being created in higher rpm's.:impatient: If you only drive your car on the streets and want good down low torque then that is fine. However, on the track and auto-x a 19% will hurt your hp numbers.

^^^ exactly. I have a 17% reduction, and it's great. but I drive on the hwy and country roads, the engine barely ever touches 5500rpm and when it does, I pretty much shift. If you're car is a track monkey though, that lives up at >6k rpm for prolonged periods of time (not just to get there and shift) you will be hurting your HP and possibly shortening the life of your supercharger.