I figured I would check combustion chamber volume to verify my compression ratio before I bolted the heads on. I'm using stock cast iron heads with new guides, springs, etc. One is dated 1972, the other is 1973, both are the same casting # 993's. So I c.c.'d the heads, the '72 has chambers of 74 c.c's, the '73 head has chambers of 76 c.c.'s. Back to the machine shop to mill the '73 head to match the '72, should only need to take .010 - .012 off to reduce it by 2 c.c.'s. So with 74 c.c. heads, flat top pistons with 5 c.c valve reliefs, composite head gasket, I'll be running 9.5 to 1 compression ratio - BINGO, right where I was shootin'.

 

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Excellent! How many horses are you planning on?

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Be sure you check your pushrod length on both sides. Also won't you have to have the intake machined to compensate for the different head heights?

Now that I think about it, I'll bet the '72 head was already milled as I seem to remember that the small block went to 76cc when Chevy dropped the CR for unleaded gas.

 

DV8, I'm not really aiming for a specific h.p. figure, I know it's gonna' be allot stronger motor than the factory L-48. And as you are no doubt aware of, it's the torque from the longer stroke that we feel. 73shark, thanks for the input on the pushrods, that was my next step as I was gettin' ready to bolt on the heads and decided to check the chamber size. From the different books I've read, Chevy made some very small changes in chamber volume in the early seventies and some heads actually had 74 c.c. chambers, I'm thinkin' the change occurred from the 72 to the 73 heads as they were fine tuning the the compression ratio for emissions. However, you may be correct, the 72 head could have been milled at some point, I don't know the history of the heads. The motor machinist said I shouldn't have any issue with the intake, he stated that you have to take .025 inch or more off before you run into trouble with things lining up. So I'm puttin' my trust in him, he hasn't done me wrong in the past.

 

I've been trying to find out whether my '81 has flat top, dished, or ft w/ valve reliefs or what so I can calculate my compression. I want 10:1 but if there is any dish (last time I saw stock pistons they had a 10 cc dish) it will put me at 9.3-ish:1 with the 67cc Summit Heads, anyone have any ideas?

If not,I will be pullin the heads off maybe Thursday or Friday anway, so I will let you know. The Fel-pro gaskets I am using are about .039", but I even went to GM to price the factory .018" head shims just incase I REALLYWANTED to get up to 10:1, we will see...

 

Just pull out one of the plugs and with a pen light look inside the cylinder. It will probably be a PITA but it is doable. Just be careful not to bust yourskull while you're trying to get a look, LOL! Another idea is if you know some one with a bore scope you can do the same thing with out endangering your skull.
Cheers

 

Drycreamer, do you know if your motor is still stock, is it the original motor for your car, and can you read the engine code stamped on the block by the front of the right head? L-48 should be 8.5 to 1 and the L-82 should be 9.0 to 1 compression ratio. These are factory ratio's with 76 c.c. heads. The 8.5 to 1 pistons are dished with 4 valve reliefs. The pistons that I'm using in my 383 are flat tops with 2 valve reliefs, the reliefs have a 5 c.c. volume and the piston comes up short of the deck by .025 in. With a Fel-Pro composition gasket that measures .038 -.039 compressed and 76 c.c. heads this combo would give me 9.3 to 1 C.R.

 

Drycreamer, you can use the LT-1 11:1 pistons w/ your 76cc heads and get a 10:1 CR according to TRW. That's what I did when I discovered that had to go .030 over.