Supercharger pulley ratio's

markj113 · Post by **markj113** » Tue Jun 30, 2009 2:29 am

I know this was touched upon on Rob's thread but want to confirm it.

Will altering the outer pulley size of the double alternator pulley (the one driving the supercharger) have any effect?

In my mind it wont as the crank pulley size and inner alternator pulley (the one driven by the belt) are a constant and seen as the the supercharger is driven off the outer part of the custom double pulley varying its size wont have any effect as it will still be turning at a relative same speed due to the constant crank/original alternator pulley ratio.

I know altering the inner alternator pulley size would effect the speed as its the one driven by the belt but cant see altering the outer pulley on the double pulley having any effect on the supercharger speed.

If im correct ill have to fit a smaller supercharger pulley to create more boost as it cant be done by increasing the outer pulley on the double alternator pulley.

col2560 · Post by **col2560** » Tue Jun 30, 2009 10:05 am

its not the driven speed of the pulley, its the circumferential speed that changes. think of the difference when you fit non-standard size tyres to your car, engine speed stays the same but your speedo will be wrong. the outer diameter changes. there are mathematical equations that cover all this. ratios of one pulley to the other.

Post by **Bennoz** » Tue Jun 30, 2009 12:34 pm

These guy helped me out with that stuff in the past.

http://www.capa.com.au/eaton.htm

markj113 · Post by **markj113** » Wed Jul 01, 2009 12:48 am

Thanks for all the info, keep it coming in the future.

Good news for me that im wrong as it'll save me the cost and hassle of fitting a reduction pulley to the supercharger.

markj113 · Post by **markj113** » Fri Jul 03, 2009 2:28 am

Ok, so it looks like I will need a reduction pulley to make the charger more suitable for the FTO.

Problem I have is that the merc boys seem to fit a larger crank pulley rather than smaller supercharger pulley for some reason, must have something to do with pulley ratios?

So now im wondering if a mini cooper S reduction pulley will fit the mercedes charger as basically they are both Eaton M45's with different casings?

col2560 · Post by **col2560** » Fri Jul 03, 2009 8:47 am

the smaller the pulley, the tighter the belt must bend, and they wear out quicker.
i know 1 site i went to that said the pulley is a shrink-fit on the shaft and is almost impossible to remove without the right equipment.

markj113 · Post by **markj113** » Sat Jul 04, 2009 7:32 am

with help from members from the ftooc ive come up with this -

just redid my calcs and got them correct this time

65mm outer alt pulley =

127mm (crank) / 54mm (alt pulley) = 2.352
= alternator shaft turning at 2352 rpm at 1000 rpm engine speed.

65mm (alt outer pulley) / 76.2mm (supercharger pulley) = 0.853

0.853 X 2352 = 2006 supercharger rpm at 1000 engine rpm

2006 X 8 = 16048 supercharger rpm at FTO 8000 rpm

70mm outer alt pulley =

127mm (crank) / 54mm (alt pulley) = 2.352
= alternator shaft turning at 2352 rpm at 1000 rpm engine speed.

70mm (alt outer pulley) / 76.2mm (supercharger pulley) = 0.919

0.919 X 2352 = 2161 supercharger rpm at 1000 engine rpm

2161 X 8 = 17288 supercharger rpm at FTO 8000 rpm

Edit: another question as the charger is not driven directly off the crank but via a double alt pulley -

0.853 X 2352 = 2006 supercharger rpm at 1000 engine rpm

so does this give a drive ratio of 2.006

and

0.919 X 2352 = 2161 supercharger rpm at 1000 engine rpm

does this give a drive ratio of 2.161

if the above is correct then its pretty much spot on for the required drive ratio as calculated below -