For those of you with a greater physics bend, what is the relative difference between these two principles? Down sides? Upsides?


Michael

 

All things being equal (which, comparing one mod soley against the other, they arent; a 2% crank pulley does not change the boost to equal a 15% reduction pulley) making the crank pulley larger has some benefits compared to making the 'charger pulley smaller:

Pros:
No reduction in belt gripping area (there is an increase actually) that can be a problem with making the 'charger pulley smaller. There is less chance of belt slippage and a corresponding loss of boost,
Potential to use a lighter crank pulley,
Generally easier to change the crank pulley rather than the supercharger pulley,
A good way to increase JCW kit HP without mucking with the 'charger pulley and potentially reducing the kit/vehicle value,
Less overall tension on the belt.

Cons:
All the accessories (alternator, a/c compressor) are now being overdriven creating additional drag (and possible wear),
Longer belt needed in some cases,
Still some untested reliability issues regarding long term use of lightweight crank pulleys

Best usage:
Install a crank pulley in conjunction with a lesser (15-17%) supercharger pulley to gain the benefits of the higher boost pulley without running the risk of not having enough belt area on the 'charger pulley and causing slip and heat buildup. A 2% crank and 15% supercharger pulley gives the same results boost-wise as a 17%, but you dont need the shorter belt in most cases.

Wildest usage:
2% (or more) crank pulley in conjunction with 19% (or more) for insane amounts of boost most effectively. Screw reiability concerns, just gimme all ya got!

Some non-MINI applications (Ford Lightning trucks, for example) have found that a bigger crank pulley was the only way to effectively make any more boost since the smaller blower pulley was causing slip and wear or belt breakage. MINIs are seeing this with 19s and extended track usage in some cases.

 

Thanks Greatgro!