Thread: 396 compression, too much!

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Hi all,
My brother has a 396 in a 69 Chevelle that I need to know what is the current compression ratio. He could never get it to stop pinging on 93 pump gas. I have torn the engine down to the short block and here are the head and piston numbers: Heads- 3872702, pistons TRW L2242F30, gasket appears to be a felpro blue. From what I can find, this head has a 98cc chamber. From the Summit catalog, these pistons have 10.58 compression with a 109cc head. Could someone help me figure the ratio with this 98cc head? Would it be possible to get the compression down to 9.5 or 10 with just a head change? If so, what head? Thanks in advance for any help!
Steve

According to the piston chart in Summit, these pistons have a 38.3 CC dome, they were originally for a closed chamber head. I have a set of 990 rectangular open chamber 119cc heads lying on my shelf. Any reason why you think that it would still rattle with this cc head? Thanks for your help!
Steve

That's a bummer as he is tight on funds and does not want to pull the engine, change pistons, etc. At this point, I guess that it would be best to use a head with a big chamber and run a thick gasket. Mortec.com says that an 049 head has a 122cc chamber, maybe I could round up a set of these on Ebay or find a cheap set locally. We will probably try this and a different cam, could you recommend a cam to bleed some of the pressure?
Thanks,
Steve

I'll have to check with him and see if he has any specs on the cam. All I know at this point is that it has a Lunati solid cam and he wants to change it back to a milder hydraulic. I'll check the engine tomorrow and see if I can figure how deep the piston is in the hole. I don't believe that the block has ever been decked so it should be at factory height.
Thanks,
Steve

Thats really too bad with that situation. Most of the proper fixes cost more money, but things you can do is try to get a cam with more overlap which will bleed off pressure as techi said. Heads with more bigger chambers could help. I run about 10.7 or so on my 355 and I just use 91-93 and never detonate but that's also with the properly set up quench or 'squish'. You could deck the block for better quench but then still have to use bigger chambered heads to bring the CR down. This depends on how far down the pistons are just as Richard said. Cheapest fix is a bigger head and a cam with more overlap, but keep in mind the downsides of more overlap too.