lower compression pistons

[bromak]

Can someone enlighten me as to what differences there are in marine pistons vs. a lower compression piston that is only a piston with the wrist pin raised .010 towards the crown?

[More Power]

Generally, pistons made for a marine diesel application are of a lower compression ratio than those normally available for an "on-road" application. Because they are most often available from a manufacturer of marine engines, the pistons are usually referred to as "marine" pistons.

Marine pistons are available with lower compression ratios because a lower CR improves durability in a constant high-load operating environment, like what a marine engine might see. 19:1 marine pistons are available for the 6.2L and 18:1 pistons are available for the 6.5L. Cummins produces a marine piston for their 5.9 that is a low 15:1 CR.

Current diesel pickup engines like the Duramax, the 6.0L PSD and the Dodge Cummins are 17.5:1 to 18:1 CR.

MP

[bromak]

Makes sense to me but what I guess what I'm trying to ask is, what ratio would I have with pistons that are .010 shorter. Is this enough height difference to reduce compression ratio a substantial amount?

[Kennedy]

When you are at 21:1 a little bit of volume makes a big difference, BUT keep in mind you will likely need to deck the block so a lot of that .010" is going to be used up...

The best way to figure it out is to CC everything and use a CR calculator program.

[Jim P]

When I was rebuiding my engine I was playing around cc'ing the heads and pistons.

If remember right, .010" shorter pistons cuts the compression about 1 point. So it would go from 21.5:1 down to 20.5:1.

I have measured two stock engines and the pistons stick up past the block .010 when the piston is all the way up.

I used the .010 shorter pistons and also bored my wrist pin bushings .010 off center to make the rods shorter. So with my new pistons in, mine stick down in the block .010 when the piston is all the way up. I also did some machining on the heads to further lower the compression. Mine is now real close to 18:1.

To get down to around 18:1 with only a piston change, I think they would have to be about .030 shorter.

Jim P.

[bromak]

Thanks for the info Jim. That was more in line with what I wanted to know.