heatwave3

I reduced my M1-SC blower pulley (502 efi in a Baja272) from 4.25" to 4.0". Here's my results:

4.25" pulley:
WOT RPMs= 4900
fuel pressure = 52#s
engine temp = 150
top speed = 74
boost = 4#s
prop= 26" Bravo 1

4.0" pulley:
WOT RPMs= 5100
fuel pressure = 58#s (low of 49#s)
engine temp = 150
top speed = 76
boost = 6#s
prop 26" Bravo 1

4.0" pulley:
WOT RPMs = 4750
Fuel pressure = 58#s (low of 48 #s)
engine temp = 150
Top speed =78
boost =5#s
prop = 28" Bravo 1

I was hoping to generate more boost with the 28" prop and the 4" pulley. The engine was run on a laptop and there was absolutely no knock at any time, even during hard accleration and WOT. Plug check from WOT was black on the threads with a slight toast brown on the tip. In fact we dialed the fuel pressure back to 55#s as there was some soot build up on the back of the boat that I didn't have when the pressure was at 52#s.

Here's the question: Does anyone see an issue with reducing down to 3.75" pulley which would probably deliver 6#s of boost with the 28" prop and probably 5000 rpms. Maybe reaching 80. Is this a reasonable step up in boost or am I pressing the capabilities of the M1 and the engine?

It seems reasonable only because of the laptop readings show no knock under a relatively long run at WOT and several hard pulls out of the hole. Thoughts on my plans for a smaller pulley?

heatwave3

Belt was very tight and there was no slippage. After a hard run, checked to see if the belt was hot (slippage) and it was just warm to the touch. Additionally there was no belt dust around the pulley, therefore I'm confident there was no slippage of the belt.

mcollinstn

Yeah, if you pulled 6 pounds at 5100, then there's no reason you can't pulley for 6 pounds again. With equiv boost, I'd say you'll see 4900, doubt you'll see 5000.

As far a slippage, etc, you can stop trying to figure out where your boost "went". Centrif blowers make boost with rpm. You lower the rpm, you lower the boost. You raise the rpm, you raise the boost.

Turbojack

Everyone talks about belt dust but everytime my belt has slipped I have not been able to see any dust. The only way I know it has slipped is my son can hear it or in looking at log of run you can see where the belt starts to let go.

As Mcollinstn says the boost goes up with RPM. You will need to go to prochargers web site & see what the max rpm for the M1 blower is. Take you bottom pulley size, top pulley size, gear ratio & caculate the max blower speed. As long as you are under that you are OK. At least with the procharger.

It also looks like you motor likes less rpm, When yo go up in boost you may need to go even bigger on prop. The problem with going a bigger prop is the procharger does not give you much boost at low rpm so you takeoff & low end gets weaker as you go bigger in prop.

Back4More

Go for it...should hit 6psi i would think.

Scarab28

I'm runing 3.75 pulley, 7 lbs boost, 5400 rpm, 28p bravo. (carb'd)
Go for it.

heatwave3

Scarab,

Is your 7#s with an M3SC or an M1SC? Are you able to safely run on 89 octane gas?

Scarab28

I'm runing an M2SC. I always run 93 octane just to be safe. Second year and no problems yet.