13:1 Compression Pistons in 340 Small Block

yellow rose said

I don't think anyone told the OP to use them as 13:1 Pistons. I'm saying use them as 11:1 Pistons. Done correctly, he can do that on pump gas, get .040 quench and have a very low dome. Then the piston will be lighter, he won't be through the roof on compression and he can use what he has.

BTW, to get 13:1 out of those Pistons isn't easy. They still have to be out of the deck .040-.050 to do it.

Can't tell you how many engines came through the shop that were some claimed compression ratio and when it came apart, it wasn't even close.

Especially the MoPar stuff with open chamber heads and a zero deck. Even with the piston out .017 like some books call for, you are nowhere near the claimed CR.

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I agree with yellow rose. I have a set of those Pistons from years ago. I had mine cut for a little extra valve clearance, but as assembled on a typical 68-70 cc head they only actually came out to 10.9:1 compression. I believe those Pistons were advertised as 13:1
at a chamber volume that is much smaller than the typical x or j or w2 head. One of the things I learned with my new stroker build is that because of the addition swept volume of a stroker crank, it is much easier to get a really high compression ratio without a heavy domed piston. I think the op should assemble one cylinder assembly and do some ccing and see what he actually has for compression.
Bet you it is not a true 13:1. My new 410 build with a 69cc w2 head has 12.7:1
Compression with a .005 out of the hole .100 thou thick flat piston that is a least 200 grams lighter than that trw piston.