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Discussion Starter · #1 ·
I have received good answers to my previous questions related to this build but here is another one born out of a discussion I had with another gearhead.

First some background on the motor:
1970 350 4 bolt (010/020) block bored .030", stock deck height
Forged crank (original to motor) - .010 on rod journals, .020 on main journals
Connecting rods are stock
Pistons are Sealed Power Flat tops #H345NCP .030
Rings are Sealed Power E-251K30
Cam currently to be Crane 100052 Energizer duration @ .050" is 216/216, Valve lift is .454/.454.
Heads - ???

I was planning to keep the motor at about a 9.1:1 compression which ruled out heads with 64cc chambers but my friend is saying this might not be the case.

His point is with a stock deck height and the head gasket from the Fel-Pro gasket set I have, there may be enough room there to accomdate 64cc chambered heads while maintaining a ratio in the range I am wanting.

The motor is going into either my 54 pickup to replace the SBC there or be used in a 58 pickup project I will be starting soon. Typical use will be daily driving, highway and general fun. No strip action.

My question is (Finally):
Can someone explain how I can measure and calculate what my compression ratio will be for this short block with the 64/65cc heads and also for 75/76cc heads.

A recent article I read regarding the AFR heads makes these an attractive option and come in both 65 and 75cc versions. I also have a pair of 487 castings (76cc) on the current motor which are fresh which could also be options since they are paid for.

If someone already knows the ratios for the recipes I noted please share although I am hoping I might learn something here.
Thanks for the help.

VW makes it, Chevy shakes it..
1,283 Posts
I've been hanging around this forum for awhile and have learned a thing or two. :D

You'll need to know these numbers.

Bore size. In your case it's 4.030"
Piston stroke. Which is 3.48" on a 350 SBC.
Head gasket bore.
Compressed gasket thickness.
Combustion chamber ccs. In your case it'll be 64 or 76, right?
Piston dome volume (dome, flat top, dish, etc. all have a +/- cc #). In your case it's -5cc (flat top with valve reliefs)
Piston to deck clearance. Which is the distance from the deck (head surface) to the top of the piston at TDC.

In order to figure out piston to deck clearance you add up your stack of parts. You have a 5.7" rod, which is what comes in a 350 sbc, you divide the crank stroke in half (3.48 / 2 = 1.74"), and you'll add the piston compression distance which is the distance from the center of the wrist pin to the top of the piston. In your case it's 1.56". Adding all those numbers up (5.7+1.74+1.56) it comes out to 9".

The stock deck height of a sbc, from the centerline of the crankshaft, is 9.025". Subtract your stack of parts above, which is 9", from 9.025" and you get .025". This might be a little off cause every block is slightly different from the factory.

Use one of the links the guys provided and plug in your numbers. You can play around with different gasket compressed thicknesses from gaskets sold as well as the gasket bore size. I would recommend staying within .035-.045 squish / quench. Quench / Squish is the distance from the top of your piston, at TDC, to the deck of your head. Add your deck clearance of .025" and the compressed gasket thickness to get your "squish".

I've attached a couple combinations of different sized heads with all your numbers. With a 64cc head you'll get a CR of 10.39:1 and with a 76cc head you'll have a CR of 9.13:1. The 76cc head puts you right about where you'd like to be.

I hope this has helped some. :thumbup:

Edit: Some spelling errors.


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