I've poked around these forums and didn't find a sure-fire answer on this, so here goes:

If you have for example a 2% crank pulley and a 15% supercharger reduction pulley, do they add up to an overall 17% increase in the supercharger speed as it would seem?

 

Good question and I'd like to see an answer too.

But if the two pulleys are just cummulative then why not just go with a 17% sc pulley? It would be cheaper!

 

So you get a lighter crank pulley?

 

From my understanding (I'm not an expert), they don't add up to quite 17%. I think it's something like 16.XX%. Even though they don't add up to be the same %, from what I've read, you're going to gain more power having the 2% CP with the 15%. You lose mass on the crankshaft which in turn would help it rev a little easier, and there are also other benefits as well.

Randy Webb has an article on this, on his web page. I plan on going the 15% + 2% eventually, or doing the M62 Supercharger upgrade (if it becomes a kit).

Here's the link to Webb's page http://store.webbmotorsports.com/ind...roducts_id=132

 

The two do not share the same values relative to one another. 15% SC pulley represents the reduction value relative to the SC, the crank pulley to the circumference.
If you know the measurements of the 2 ( OE SC pulley - OE crank pulley V aftermarket ) pulleys you can figure the ratio.

 

... a guy from Germany did some calculations in an excel spreadsheet and posted it to a german MINI forum.
Have look:
(to open, save on your hardrive, remove .txt extension, open with WinZIP and extract file, open with M$ Excel)

 

besides the shortcomings of our math education!

The percentages quoted aren't right, because the effective radius of the belt/pulley system isn't the diamter of the pulley.

To do compound percentages.... Multiply! (or devide, depending....)

So a 2% crank pulley and a 15% reduction SC pulley would give (ignoring the belt issue above)...

effective reduction = (1.02 / 0.85) - 1
About 20%......

Matt

 

Thanks, very nice spreadsheet and info.

I have a 15% pulley, so I guess I an comparing the numbers used for the GTT 15% right? I am wondering for other people with greater S/C pulley reductions... Maybe the others listed on the file represent other reductions?

EDIT: Well, maybe with the Doc's observations above the numbers are not quite accurate on the Excel file...

 

Yes that's just about right. You would measure from the outermost edge of the pulley groove

 

Andy had posted what the difference was, and I just don't remember....

Matt

 

I haven't seen anywhere what the JCW reduction pulley comes out to compared to stock.
I have a JCW and have added a 2% M7 pulley, and I'd love to know what I have now.

BTW - the crank pulley does add a whole new revving ability - much more free than before.

 

Top to top groove, SC to crank, C to C & that is the ratio.

 

Turns out that the part of the belt that doesn't either stretch or compress is closer to the outside of the belt than the top of the groves. This makes the effective diameter of a pulley = actual diameter + a little bit. I think this little bit is on the order of a couple of mm, but it does make a difference to the calculation of actual SC RPMs.

What you wrote would be correct if the part of the belt that remained constant length rode exactly at the top of the grooves of the pulley.

Matt

 

C to C is constant. No argument here about the belt moving up in the groove but it is not a constant. There would be a lot of factors involved in the calculation, type of belt, belt temp, centrifigul force..... All C to C will give you is the actual ratio of the pulleys

 

then it will all be clear...

https://www.northamericanmotoring.co...1&postcount=18

What it means is that the effective diameter is about 1/8" larger than the measured top of groove to top of groove diameter. That's all that's needed (along with the relavent pulley diameters) to figure out actuall drive ratios....

Matt

 

Stock crank pulley dia. is 5.46 inch.
Stock SC pulley is 2.58 inch.
Correction for belt is 0.126 inch. (thanks Andy)
Effective stock crank dia. is 5.586 inch.
Effective stock SC pulley is 2.706 inch.

Crank _______SC___________% SC
Pulley________Pulley _______Overdrive
% over _____% under
0 ___________0 _____________0
0 ___________15 ___________16.69
0 ___________16 ___________18
0 ___________17 ___________19.34
0 ___________19 ___________22.12

2 ___________15 ___________18.97
2 ___________16 ___________20.31
2 ___________17 ___________21.68
2 ___________19 ___________24.51

3 ___________15 ___________20.11
3 ___________16 ___________21.46
3 ___________17 ___________22.84
3 ___________19 ___________25.70

4 ___________15 ___________21.25
4 ___________16 ___________22.61
4 ___________17 ___________24.01
4 ___________19 ___________26.90



That is, a 17% undersize SC pulley spins the SC 19.34% faster than stock.
A 15% undersize SC pulley + 2% oversize crank pulley spins the SC 18.97% faster than stock.

 

I have 15% pully and 2%Alta crank pully, and love it

 

Ahh! Now the mud is gone.

 

I have a JCW that I am interested in bumping up to an effective 16% or so. If I understand correctly what I've read here, then I can add a 0% crank pulley and achieve (roughly) what I'm looking for then? I also would like the the motor to more freely rev from the lighter crank pulley.

 

I believe that a JCW pulley is a bit smaller than 15%.
A 15% SC pulley turns the SC 16.69% faster than a stock pulley.
A 16% SC pulley turns the SC 18% faster than stock.
A 15% SC pulley + 2% crank pulley turns the SC 18.97% faster than stock.
A JCW SC pulley + 2% crank pulley would turn the SC a bit less than
18.97% faster than stock.

A JCW pulley + 2% crank pulley would be close to an effective 16%.