what does a 2% crank pulley add a 16% reduction pulley? whats the 2% pulleyt for?

 

Whats your question?

 

the combo will provide the equivalent of 18% reduction pulley. The advantage is that the belt has more surface contact with the 16% pulley than if you just got an 18% reduction pulley. . . plus the 2% crank allows you to go from 16 to 18% reduction without changing your 16%, if that's what you already have.
Additionally, manufacturers (like ALTA) claim that the lighter crank pulleys are equivalent to reducing the car's weight by 100lbs.

 

Regardless of what a pulley reseller may claim a 16% sc +2% crank is not the same as 18%. It increases the supercharger to crank speed by 21.43%. An understanding of math is critical for tuning!

 

dmh - thanks for the clarification.

I was basing my response on comments like #814 and #823 on:
https://www.northamericanmotoring.co...=41766&page=33

So, is it your experience that adding a 2% crank to even a 15% reduction pulley will result in increasing supercharger speed by more than 19%?

 

All of this is my experience and it is simply math. You can also get it all out of the Bosch Handbook.
15% w/2%=20%
19% w/2%=25.93%

There is more to pulleys than simply installing them. MINI had a good idea of when they made the JCW: it increases the supercharger to crank speed by 16.69%.

 

I see by the (15% w2%=20% supercharger overdrive compared to stock) and (19% w2%=25.93%) that you are modifying the pulley size based on the effect of belt thickness.
If you consider belt effect, (15% pulley = 17.65% SC overdrive)
(17% pulley = 20.48% SC overdrive)
So a 17% pulley turns th SC slightly faster that a 15% undersized SC pulley
with 2% oversized crank pulley.

I am not positive but I believe that the JCW pulley is 14.8% smaller than
stock. A 14.8% pulley = 17.37% SC overdrive.

 

I measured the JCW as 14.3% smaller than stock. Translation; 16.69%.

 

Maybe I'm missing something, but it seems to me that this statment is still true. I realize that 18% pulley increases crank speed by more than 18% -- but there's only a negligible difference from running a 16%+2% instead of an 18% (or a 15%+2% instead of a 17%).

 

I flunked geometry class . Thanks for the clarification.

 

An 18% pulley (if there were one on the market) would increase the supercharger speed by 21.95%, which is almost identical to what you get from the 16% + 2% combo.

So, while the advertised percent reduction in size of the pulley is NOT the same as the percent increase in supercharger speed, it *is* true that a 15% + 2% combo will result in almost-identical increase in supercharger speed as a 17% pulley alone. (20.00% versus 20.48%)

Likewise, a 16% + 3% combo will overdrive the supercharger by almost exactly the same amount as a 19% pulley used alone (22.62% versus 23.46%).

 

I was told that 2% crank pulleys were not an actual 2% larger, but larger in an amount to be equivalent to a 2% reduction on the original diameter supercharger pulley. Obviously this would still need to be adjusted depending on the actual sc reduction, but not as simple as 2% larger diameter crank pulley. Was this wrong information?

 

WOW... they say you learn something new every day! Thanks guys... very good to know!

Doing my install next weekend on the 16% + 2% M7 Kit