Belt size when changing pulley size?
 

Hi all,

This is a technical question but not Porsche-specific...

I am installing a smaller pulley on my supercharger (in a 911...) and want to calculate the new belt size instead of ordering a bunch of sizes...

Here's the diagram below...

http://forums.pelicanparts.com/uploa...1265170135.jpg


Pls check my logic/math:

If the pulley is 9mm smaller in diameter, then the outer diameter the belt needs to wrap around the pulley is 14mm smaller...

old pulley diameter = 73mm
Pi = 3.14
Pulley wraps around 1/2 of pulley
Pi*73/2 = 114.6

new pulley diameter = 64mm
Pi*64/2 = 100.6

Difference = 114.6 - 100.6 = 14mm

Now, since the outer edge of the pulley is also 4.5mm closer to the crank does that also mean each "leg" of the belt must also be 4.5mm shorter?

For an effective shortening of 14mm + 4.5mm + 4.5mm = 23mm?

The belt on there now is a PK1300, which I believe means a 1300mm length.

Thanks!

14mm shorter is correct. Forget the 4.5mmx2.

Thanks!