In regards to pulleys, It seems to be common knowledge that they are not the #s as advertised, a few different measurement points and you can come up with some different actual sizes and ratios, but there seems to be an across the board inconsistancy about size of the pulley in relation to the blower speed which changes at very different rate than the pulley changing size, so I am looking for a direct explination for this.

Here are the #s that have been found so far.

 

but the marketing labels are the ones that persist.

Matt

 

Note: I'm not talking boost levels or air temps but speed vs size of a mechanocal linkage.

 

I'm not diagreeing with his #, though I don't trust the marketing as much. I am confused as to how speed changes at a different rate than size as the friction levels should remain the same and should be unaffected by air temp and pressure unless you reach detination levels and the ECU adjusts your timing.

 

but it's still true that the pulley diameter isn't the only thing that goes into the math. It's still true that the drive ratios are calculated with the diameter of the crank pulley, the SC pulley and the effects of the belt. And it's still true that Andy's math is the only one that recreates the drive ratio of the SC that is listed in the Mini Cooper S technical literature.

Matt

 

Right so what other factors am I missing that makes these ratios of speed vs size not match up? The engine turns at the same speed and that turns the crank at the same speed which turns the A/C, water pump, SC Alternator and so on, so changing pulley size shold yeild a porptional change in the speed at which the bloower turns in Direct porption (%) to the size of the pulley change. Unless the rubber compund is significantly different causing different amounts of streching amongst the properly sized belt.

 

Its all about the belt. The accessory pulleys are all driven by the belt. There is a woven core in the belt that prevents it from stretching while its being pulling around. This woven core is the only place you can measure belt speed.

What determines the overdrive or underdrive of accessories is the ratio of the diameters created by wrapping the belt around the pullies and measuring the distance from the center of the pulley to the center of this core, which is ever so slightly greater than the pulley diameter.

You're correct in that its only determine by the ratio of the diameters, its just not the pulley diameters. Hope that helps.

 

the reduction SC spin ratios compared to stock are the same as the ratios of reduction SC pulley dia to stock pulley dia, measured the same way. measured at the bottom. top or middle of groove...it doesn't matter.

(SCP1,2,4...\CP)\(SCstock\CP)=SCP1,2,3...\SCstock

 

Theoretically, a 5" diameter pulley and a 15" diameter pulley.

15/5=3

If the belt centerline was .5" further away,

15.5/5.5=2.81

What am I missing!?

 

your method seems right to me

 

Yep, just like jlm said, doesn't matter where you measure, as long as you stay consistent with where you make your measurements.

 

there really is an appropriate amount to add over the diameter of the pulleys, and that is determined by the belt construction. Only this amount will give you correct drive ratios, and ratio changes with different pulley diameters.

Matt

 

Well at least we're learning, I understand the belt plays factor in this but I don't see how a different size belt is yeilding not linear, changes that are different from the rastio the pulley was changed, cause in my mideval way of thinking if the pulley goes down a certain x%, the belt needs to be x% shorter which would mean the same x% increase in blower speed. I have the fisrt % and the last % but I'm trying to figure out why somewhere in the middle things are changing.

Please forgive me if this is nieave or uneduacted afterall thats why I'm asking!

 

the real effective radius is the pully diameter plus a bit. If you only take the 15% off the pully diameter and not the bit, you end up with a new effective radius that is not 15% different in diameter!

Matt

 

And I calculate effective radius through??

 

try this:
[(SCP1+K)\(CP+K)]\[(SCstock+K)\(CP+K)]=(SCP1+K)\(SCstock+K).

[(SC1+K)\(SCStock+K)]not=[SC1\SCstock]