Hey every! I’m trying to figure out what pistons I have in my Hemi. Sounds stupid as you’d think I would know after having the engine built. So they were suppose to be CP 12.5:1 pistons but I think the builder put the wrong ones in. I pulled the oil pan off to see if I could see a part number and all I found was the CP logo and 426H-C, I can’t find any info on these pistons. The motors a 4.28 bore and 4.15 stroke with a 6.86 rod. The motor made 827@8000 and didn’t really perform like it should have that’s why I pulled the pan, and I have a buddy with the same short block but with Ross 13.5:1 piston which is completely different. Any help would be appreciated
426 street hemiroid motors are hard to figure the correct compression ratio without knowing the exact deck height on the pistons above the deck, the dome volume, combustion chamber volume, head gasket volume and the bore and stroke.
I would check the actual deck height at TDC first with one head off and then move the piston down to either zero deck height or maybe .500 below the deck surface and then seal the ring lands up to the cylinder walls with chassis grease and then find out what that actual volume is and then do the math on a flat top piston that far down and then due the math to get your dome volume and use that information to get the true compression ratio
Don't be surprised if it is not exactly what the piston maker said they would be
