The old set-up on my engines were 9:1 with 5 lbs of boost, no intercooler, and they lived but they were set up very fat, in the 11's. When we re-did them we brought the comp. ratio down to 8.3:1 and ran A/F ratio's in the low 12's.

I think 9:5 is high. It would be a good setup on low boost cause you could benefit in the low/mid range by having the extra compression and then the boost up high. The aluminum heads help a little also...

There's a reason Mercury goes with a very conservative 7.5:1 on their blower engines.

that would be my belief as well

Thanks for all the info, Not intercooled, when I cross refernced the pistons that were put in and seen they were 9.5.1, i knew that is where the problem was. Running MSD at 28 degrees timing NGK plugs. Anyone have the correct piston number I need or recomendations?

P.S. As I am not a mechanic, just the driver with a pissed off wife not being able to work on her tan, I really need the help everyone is providing, don't be concerned about hurting my feelings, just want to make sure it is right this time. Thanks, boatguy

find a engine builder that knows blower motors....

And lower the boost to 5psi and raise the richen the a/f to below 12.

The reason higher boost and lower compression is best is because you can pack more mix into the cylinder. But it is possible to make them live the other way around I just think you may have gone over the edge a bit with the 6.5psi and lean a/f. As long as your blower has the capacity, you should definately drop the static compression to match what the blower can do comfortably. In my case with 177s on top of 470cid engines, higher compression was necessary to get the bang out of the blowers. I would have to spin them too fast with lower compression and that would create heat and defeat the purpose.

besides finding the right tune up and lowering the compression a bit, Diamond Pistons make some very nice products and you can get special cotings on the as well.