copression ratio loss using 3.2 pistons on 3.0 rods?

[peon77]

I'm thinking of using 3.2 pistons on a 3.0 rebuild I'm working on. I will have the piston pin bushings offset. Ive read threads for the last five days. I like the idea of the better rod ratio, but I'm wondering if there is a loss in compression ratio. I think it should remain the same but I'd like to hear from someone thats done this.

Thanks
Pete

[Eagledriver]

I'm sure the compression ratio will be less because you have a shorter stroke. The volume of the cylinder will be less by about 6 percent so the compression will be less by about the same amount. 9.5 pistons will drop to about 8.9 and 10.3 pistons will drop to about 9.7. Of course Mahle pistons don't achieve these numbers at normal deck heights anyway.

-Andy

[fred cook]

Don't forget that the 3.2 pistons use a 24mm wrist pin while the 3.0 rods use a 22mm wrist pin.

[brads911sc]

Why do you want to do this?

Call Tom Butler. If yours are in good shape.. perhaps he will swap them out...

Originally Posted by peon77

I'm thinking of using 3.2 pistons on a 3.0 rebuild I'm working on. I will have the piston pin bushings offset. Ive read threads for the last five days. I like the idea of the better rod ratio, but I'm wondering if there is a loss in compression ratio. I think it should remain the same but I'd like to hear from someone thats done this.

Thanks
Pete

[lindy 911]

Using a 3.2 piston on a 3.0 crank and rod will not result in a shorter stroke if the rod is modified (lengthened by re-bushing) to put the piston at the same place in the stroke as a 3.2 crank. At TDC, there would be no CR loss. The reason to do this is a better rod ratio and piston dwell at TDC. The rod ratio change will relieve side load on the piston skirt and the dwell at TDC will create better cylinder pressure. CP=HP. This is common practice in other motor builds and is known as a "long rod" motor.

[brads911sc]

Thanks for the education...

Originally Posted by lindy 911

Using a 3.2 piston on a 3.0 crank and rod will not result in a shorter stroke if the rod is modified (lengthened by re-bushing) to put the piston at the same place in the stroke as a 3.2 crank. At TDC, there would be no CR loss. The reason to do this is a better rod ratio and piston dwell at TDC. The rod ratio change will relieve side load on the piston skirt and the dwell at TDC will create better cylinder pressure. CP=HP. This is common practice in other motor builds and is known as a "long rod" motor.

[peon77]

Originally Posted by Eagledriver

I'm sure the compression ratio will be less because you have a shorter stroke. The volume of the cylinder will be less by about 6 percent so the compression will be less by about the same amount. 9.5 pistons will drop to about 8.9 and 10.3 pistons will drop to about 9.7. Of course Mahle pistons don't achieve these numbers at normal deck heights anyway.

-Andy

The advertised 3.2 C/R is figured with a 74.4 stroke. The new 3.0 C/R needs to be refigured with the shorter 70.4 stroke and and thats a difference of 5.53%. But what is the difference in the volume of the 3.2 heads combustion chamber vs the 3.0? Right this second I cant find the published numbers for them.

[peon77]

I found out head cc is the same. Using the quick and dirty method 10.5 drops to 9.92:1, doing the full C/R formula 10.5 drops to 10.0:1

Thanks everyone

[lindy 911]

Misunderstood original question; I didn't realize you were talking about using original Porsche pistons. Of course identical dome shapes on different stoke lengths will result in differing CRs.
A set of custom JE's would keep the CRs in check.

[peon77]

All the advertised C/Rs assume a stock set up. Before I buy pistons I wanted to make certain I accounted for any C/R change as a result of the change in rod length, so I can end up where I want to be.