lowering compression on 502?
 

Hi everyone...

I've got a gen 6 volvo penta injection 502 block which I was going to supercharge, but I've found it's high compression (8.something:1) I'm told it's pretty pricey to lower this with pistons as it'll also need different rods.

Is there another option out there to lower the compression that's (hopefully a lot) cheaper? Are there thicker headgaskets etc? I know felpro do a thicker gasket, but I'm told I need to drop compression by quite a bit, down to the low 7's?

I guess my only other option is to stay N/A but it seems a shame to not use such a beautiful blower stack.

Cheers, James

To my knowledge 8.7:1 is fine for a blower.

not sure if its the same world but depending on how much boost you run, say 8.5.1 comp, you should be fine with boost around the 4-6 psi range, i relaying this from an automotive standpoint where 8 something to 1 is pretty low comp and to add a blower would not be a problem

8:1 is what I have on my 502 blocks. Running 7 lbs of boost. Nevertheless, if your internals are not high quality and designed to handle the blowers, you are asking for trouble. You are going to break your motors.

The stock 8.75 compression is fine to be supercharged as long as you keep boost around 5# and run 93 octane fuel.

You could have some port work done to the heads to enlarge the chambers and that would reduce compression by maybe .25. I would not go with a much thicker head gasket as it will increase the quench and actually increase the risk of detonation.

That compression in the 8's is just fine. Use the .060" Cometic head gaskets and then make sure the air fuel ratios are set and kept in the 11.0 to 1 to 11.8 to 1 range under loads and make sure the fuel system is capable of delivering the correct amount of fuel and at correct pressures.

Best Regards,
Ray @ Raylar

Thanks for the help guys, that's all very interesting! My engine builder was insistant that it was too high for the blower, though I did think it didn't seem all that high.

Internals are standard Volvo Penta forged 502 pistons with an Eagle crank, standard iron heads, roller rockers etc.

I'm only looking to run 3-5psi boost to just give it a little tickle anyway, would this be ok?

Cheers, James

It will run like a raped ape at 5psi. Just keep the a/f under 12:1 and back out the full timing to around 30*.

BTW, engine builders make money building engines. New pistons = engine build. No pistons = no engine build.

It will be fine as long as you run good fuel.

Here is a link to some good info and a chart that shows static vs effective compression ratios with boost.
http://www.holley.com/data/TechServi...ech%20Info.pdf

I would leave it alone as stated above. I would not however use a .060 head gasket. If the short block is stock then the piston is about .020 in the hole. With a standard .040 gasket, you have a quench of about .060. That is just fine. With a .060 gasket, you will raise the quench to .080, which is to high. It will be more susceptable to detonation, which is already a potential problem with the cast iron heads. Just run a good gasket (read: Cometic) and let it rip. You may also seriously consider a good set of exhaust valves.
I recently did a bone stock 502 mag with an M3 Procharger. At 5000 rpm and 3.4 psi boost, it made 625 hp and 681 ft lbs at 4000 rpm. It made for a really nice little piece. Good luck. give me a shout if I can helpin any way.
Eddie