Well, there are tons of threads about this but the way I see it is the following:
15%:
The most conservative one.
Give less boost at low RPM than 19% (obviously!)
Still a great upgrade over stock!
Safe for autoX and trackdays
No belt problem at all
17%:
A good alternative to 19%. for low-end torque
Still safer than a 19% for heat concerns...
Have to change belt but never heard of problem with it...
19%:
Great low-end torque as boost is created sooner.
A few people had problems with belt wear but I never saw a blown s/c or water pump...
Not recommanded for AutoX or long high-RPM driving... (creates a lot of heat!)
There are not a lot of power gain with an ECU upgrade for a 19% setup yet... (don't ask me why)
I chose a 15% because I didn't want to have any problem and I want to keep my MCS for a long time! Probably a 17% would have been as reliable as a 15% but I decided to follow Randy's experience and go with a "safer" 15%...
Make sure you order a pulley that is NOT presss-fitted... It is a pain to install and to align... If you are not sure, a good way to tell which pulley is good is to look for bolts and 2 or 3 pieces... Alta, MINI Madness, Webb motorsports (Alta reseller), and many others carry those!
