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?

 

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. 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.