baronbob

7.4 MPI with a 5 psi pulley and blow-by. Tearing it down now and I am planning forged pistons. What ratio would you reccomend and where can I get them.? I also plan larger rod bolts into the stock forged rods. What should I do with the valves? Thanks for your response

PatriYacht

A little more info would be helpful. Depending on the gas you can get on the water and whether it's intercooled, you could go as high as 9.5 to 1. If all you have is 87 octane and no intercooler, stick to 8.5 to 1. Less compression and more boost will make more hp. If you have any idea that you might want to change pulleys in the future, stay with the lower compression. If you have the 3/8 bolt rods, don't machine them for larger bolts. That will just weaken the rod cap. Replace them with the best 3/8 bolts you can get. ARP Wavelock or equivelent. Valves in a supercharged engine should alway be top quality. Manley or Ferria stainless intakes and inconel exhaust. Get a good valve job from a person who knows what they're doing like Jim Valako aka Jim V here on the board. Use 2.25 intakes and 1.88 exhausts.

baronbob

Thanks for your respose. I can only plan on the lowest grade of gas. I plan to stay with the 5 PSI pulley. The stock comp ratio is 9+/1. I have knock sensors. I do not believe I have had any pre-ignition; however I also have weak cylinders.Where can I go for piston selection. Thank you Bob

PatriYacht

www.flatlanderracing.com has a good selection and good prices. Try the SRP's. Stock forgings that Merc used to use gave 8.8 to 1. That should work ok. I think I'd run 89 octane though.

PatriYacht

If you use the small dome pistons .020 or .030 oversize, without decking the block and use the .042 thick Fel-Pro teflon head gasket, you'll end up at 8.8 to 1. The flat tops will be below 8 to 1 and thats not good for what you want to do.

boatman22

This is good info. keep it coming. I am upgrading my 454 mags in a few weeks. Going with PC after freshining the motors.

Pat McPherson

Change the heads don't spend the $$ to modify them.
The L-29 heads are no good for superchager engines, probably why your engine has failed. The point of the heart shaped cumbustion chamber gets red hot and melts holes in the pistons.
I would suggest a set of Edelbrock or Worldproducts large ovals when you rebuild.
Good Luck!

Pat McPherson

The 7.4MPI has 100cc heads, 9:1 with flat tops. Dome pistons will hit the point of the heart shaped chamber.
Like I said in my earlier post, I would suggest different heads.
The MPI intake should bolt up to any large oval head.

PatriYacht

My bad, Didn't realize that engine used vortech style heads. My piston advice is only good for open chamber heads. Thanks for the correction.

baronbob

I would like to keep my heads but I will look into the heads you suggested. If I keep my heads I guess what you are saying is that new valves, perhaps larger will be wasted compaired to the cost/benefit of the larger port heads and you also state my plennum will still work.