moosestang

Find a more generous dyno.

danzcool

Positive displacement blowers like the M90 have a linear output based on how many times the screws/rotors turn, the M90 moves 90 cubic inches of air with every revolution of the supercharger pulley. Let's just assume the supercharger pulley turns twice for every revolution of the crank pulley. So at 5k RPM, your supercharger is turning 10k RPM, 10,000 * 90 is 900,000 cubic inches of air per minute. 10% overdrive means the accessories spins 10% faster, so instead of 10K rpms your supercharger spins 11K rpms and gives you 90,000 more cubic inches of air, which is of course 1/10th more than you had before. so, if you were getting 5PSI, you would get 1/10th more for 5.5PSI, so 1/2 LB of boost, the 12% would be at 5.6PSI., so definitely not 2-4LBS of boost more on a positive displacement blower.

going from a 2.875 pulley to a 2.49 pulley gives you 13.3%, so there's your comparison, the 12% overdrive won't quite get you as much boost as the supercharger pulley swap would have.

On a centrifugal blower, they have an upward curve at higher RPMs, they get more efficient as they spin faster. And that's why I said that it might give 2-4LBS on a centrifugal supercharger.

That's one way, ha!

09mustangt

Contacted a BBR rep and he told me that a 10% would put me around 7 lbs on my m90. Depending if 12% gives me over 8.5 lbs I'm going to go with the 7 lb pulley. Thanks for your help guys!

danzcool

Be sure to let us know what the actual boost and numbers end up being.

S197GT07

OP, don't get your hopes up with the above #'s. For reference my buddy is running 7psi on his M90, JLT intake, with O/R exhaust and a mild comp cam. He put down 405 to the tire.

You can't just say each PSI = xhp. The M90 becomes extremely inefficient above low boost levels, so your return in HP for each psi becomes less and less.

I can't recall a M90'ed car being above 450-470 to the tire with full boltons.

09mustangt

I'll probably go with the 10% crank with 7lbs, and live with the results!