I am considering putting new aluminum heads on my 350 sbc. I am looking at some AFR's. Looking at the 68cc chamber. Question is, I don't want too high a static comp ratio. It is currently 10.25:1 with I believe 76 cc heads (current heads are the 3932441 casting). I do not think the pistons are domed. Used an inspection camera and the pistons are stamped "L2256 0.30". I know they are TRW. Any thoughts on what I might be looking at for a comp ratio? I know the head gasket thickness can play a role too, so any input folks have is much appreciated! Thanks!
Pretty sure you are no where close to 10.25:1 with those pistons if your heads are 76 cc combustion chamber. You might be closer with the 68 cc heads, but I still don’t believe it will be that high. I believe those pistons are just flat tops like I have run in several 350’s over the years.
Playing with the summit calculator with BS numbers (for what it’s worth) you would need roughly a 10cc dome to have 10.25:1 with 76cc heads. I shot from the hip at .020 in the hole and .040 gasket thickness and it spit out a 10:1 compression ratio with flat pistons and 68cc. Someone better at this could do more accurate spitballing but it gives you an idea
Betting your compression ratio is closer to 9:1, or maybe even slightly under that. With the 68cc chambers you should be up around 10:1 or close to what you think you have now.
Ok, thank you. This is helpful. So sounds like the original owner of the engine had the wrong comp ratio. I wondered how it could be that high a ratio in a 76cc chamber with flat top pistons. If I go the 64cc route and get that ratio up to around 10:1, sounds like that might be a nice pairing with my 280 dur cam then?
Not sure but know one is adding the negative for the 4 eyebrows of the standard flat top pistons. I’ve heard it’s 7cc’s but that always seemed high to me. I use the calculator on the Keith Black piston sight which takes in everything including cam over lap if you want that over “static” CR if you want that too.
What you need to do is actually CC your piston, not guess. It doesn't cost that much for a little graduated cylinder and some flat plastic to at least get very close on the actual volume. If you guess at it and get it wrong it will cost you a lot more in the long run. Besides, you actually learn something in the process. Probably find what you need used or cheap on Facebook. Remember, you also don't know if the block has already been decked during a previous rebuild or the thickness of the head gasket. I'd be sure before buying an expensive set of heads. Amazon has a graduated cylinder for about $10
I cc everything. Heads and do molds of pistons tops. At onetime I used plaster of Paris but now casting resin. A plastic burette 100cc works just fine and like EKIMNEIRBO says…inexpensive!
Your not even close to 10.25 -1 with those heads and flat top pistons even if it was zero decked and had the heads milled and a thin gasket . "" Here’s an example of a 350 with a 10.1:1 compression ratio: 4.030-inch bore and 3.48-inch stroke Heads with a 64cc combustion chamber Flat top pistons with 6cc of valve relief that sits 0.005-inch below the deck 0.041-inch thick head gasket" That is a near zero decked block ( .005 down vs likely .020 or more if a block has not been zero decked honestly with an aluminum head that allows almost a point of compression over a cast iron Id go with a 64 CC head but the 68 cc along with the much much better flow will give you a very substantial increase over what you have now
Again, you are better off checking what you have . Lots of speculation on what you will end up with, but remember you could also end up with a lower than desired comp ratio. There is a squish chamber between the head and piston that needs to also be correct. Its especially important in a larger combustion chamber. I have a shortblock sitting in my shed right now that a seemingly knowledgeable Chevy guy built for his own driver. He told me that he ended up with too low a compression ratio and an ineffective squish chamber, and regretted it. I believe he said he had to buy a can of octane to add to his gas every time he filled up. He was moving and now wanted to get rid of it rather than correct the problem. Met a guy at the grocery last month. He parked next to me in his old Chevy truck. You could tell it had a cam and sounded kinda nasty. We talked and he said he had a 12.5:1 comp ratio and had to run aviation fuel in it. Guess thats what he intended to do.........but its sure inconvenient when you get the compression ratio too high or too low. You don't really know what has been done to the block before, and apparently the seller didn't either.
The piston you have spec's out at +6.10 cc, the Summit Racing compression calculator shows them at a 10.25 ratio with a 0-deck block, .041 head gasket and 64 cc heads. Remember aluminum heads forgive about 1 point of compression, you will be fine with 64 cc aluminum heads I'm running the same piston with 64 cc cast steel vortec heads and it's on the edge of detonation with 91 octane, I mix in race gas or octane boost when racing, would not have that problem with aluminum.
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