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Discussion Starter · #1 ·
I recently picked up a 350 Chevy short block that has a 4 bolt main, domed pistons and supposedly a cam with a lift of approx. .525.

I do not want to tear it apart because it's a recent rebuild, but according to good/reputable friend that the engine ran in the 12s with no problems.

I have a Weiand Tunnel Ram with 2 450 Holleys which I plan on using with Steel heads that are 2.02/1.60, with 64 cc heads. They have been slightly ported.

The ignition I am planning to use is a Unilite distributor; haven't decided to going with an "MSD" box or not.

Also, would like to use my Crane Roller Rockers which are 1.6s.

My question is whether or not it is possible to bring the compression down to a usable level to run on pump gas using a thicker but durable head gasket? Or, any other thoughts? The engine will be going into my 69 Chevelle.

Please add anything else that you think I may need to know.

Thank you!
 

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Not knowing your actual numbers, I'll give you an answwer based on some simple assumptions.

A 350 Chevy with 64 cc chambers and a .019" thick steel shim type gasket with a 20cc piston dome produces an 11.635:1 C.R.

A .040" thick gasket like a Cometic copper gasket produces a C.R. or 11.00:1 or a reduction of about .6 in C.R.

The key to better accuracy is knowing the head gasket thickness you now have, the cc's of the piston dome and how far down the cylinder the piston (not the dome) is at TDC, but this should give you a flavor for the change.
 

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Discussion Starter · #3 ·
Thank you for the info! Today I found that the casting number for the heads I was thinking of using are: 376450. They appear to be used on vehicles ranging from 262s - 350s, but have only a 60cc head. Which means I should expect quite a bit of torque, with a loss of power at about 5000 RPM ? Since the head has been opened up to 2.02/1.60s would I still expect the same?

From what was mentioned based on a "64cc" head using a .040 head gasket my engine would have a CR of approx. 11:1. I was unable to find something that was truly "stupid" proof when it came to calculating the compression ratio, and was wondering if you could assist with calculations based on a 60cc head using a .040 head gasket vs. a .060 head gasket.

Based on the final estimate could I expect to be able to run on pump gas ?

Thanks again!

Brett
 

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Old Rotor Flap said:
Not knowing your actual numbers, I'll give you an answwer based on some simple assumptions.

A 350 Chevy with 64 cc chambers and a .019" thick steel shim type gasket with a 20cc piston dome produces an 11.635:1 C.R.

A .040" thick gasket like a Cometic copper gasket produces a C.R. or 11.00:1 or a reduction of about .6 in C.R.

The key to better accuracy is knowing the head gasket thickness you now have, the cc's of the piston dome and how far down the cylinder the piston (not the dome) is at TDC, but this should give you a flavor for the change.
I don't know what calculator you used but, that Cr. isn't even close. with those #'s it would be 14.95 Cr and with the .040" gasket 13.87Cr. This is with .020" Deck height.
 

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srnoncom said:
Thank you for the info! Today I found that the casting number for the heads I was thinking of using are: 376450. They appear to be used on vehicles ranging from 262s - 350s, but have only a 60cc head. Which means I should expect quite a bit of torque, with a loss of power at about 5000 RPM ? Since the head has been opened up to 2.02/1.60s would I still expect the same?

From what was mentioned based on a "64cc" head using a .040 head gasket my engine would have a CR of approx. 11:1. I was unable to find something that was truly "stupid" proof when it came to calculating the compression ratio, and was wondering if you could assist with calculations based on a 60cc head using a .040 head gasket vs. a .060 head gasket.

Based on the final estimate could I expect to be able to run on pump gas ?

Thanks again!

Brett
Stock these heads are 69cc to 74cc, http://www.chevytech.com/3c376450.html with the Larger valves & milling they may be 60cc chambers.
Many use the old LT1 style Dome pistons, these have a .100" high Dome that is 2.4cc. Easy to check if the heads are off the engine.
A few examples>>
4.030" Bore X 3.48" Stroke, 60cc Chambers, .020" Deck Height, 2.4cc Piston Dome, .028" Gasket. .048" Quench = 11.76 Cr.
With .040" Gasket & .060" Quench = 11.37 Cr.
With 64cc Chamber,.028" Gasket = 11.16 Cr.
With 64cc Chamber,.040" Gasket = 10.81 Cr.
 

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srnoncom said:
Thank you for the info! Today I found that the casting number for the heads I was thinking of using are: 376450. They appear to be used on vehicles ranging from 262s - 350s, but have only a 60cc head. Which means I should expect quite a bit of torque, with a loss of power at about 5000 RPM ? Since the head has been opened up to 2.02/1.60s would I still expect the same?

From what was mentioned based on a "64cc" head using a .040 head gasket my engine would have a CR of approx. 11:1. I was unable to find something that was truly "stupid" proof when it came to calculating the compression ratio, and was wondering if you could assist with calculations based on a 60cc head using a .040 head gasket vs. a .060 head gasket.

Based on the final estimate could I expect to be able to run on pump gas ?

Thanks again!

Brett
The 376450 head does not appear on 350s, it's a small chambered, small valve and port head found on 305 and smaller engines. While it was popular back in the 1980s to put these things on engines to get the compression up, the compromise in port/valve flow and poor crowded chamber characteristics required a lot of extensive and expensive modification. The lack of a GM performance heads for 350 and larger engines is what spawned the aftermarket head industry as modified 305 heads just couldn't support the required power levels with anything close to the required durability.

Using a high lift cam and sticking 1.6 rockers on these heads is not the solution to inadequate flow. When the port/valve becomes maxed, adding lift at the valve is of no influence on flow, only adding duration to get a low flowing port to do so for a longer period of time will provide a solution, but one that brings its own unique problems.

Like everything about an engine, it's how each of its characteristics work with each other that makes a great engine. Pushing one feature to its maximum effect does so at the reduction or elimination of effectiveness in other characteristics. Pushing the compression above 10 on a street driven engine causes so many compromises to timing, mixture, and operating temps as to be useless as a tool to regain bottom end power lost to the big cam. But trying to force your combination of parts to a 10 to 1 compression with a thick head gasket also creates problems in that as the distance between the block deck and the heads' squish/quench deck increases; the combustion chambers tolerance to detonation decreases, which forces further compromises in reduced ignition and richer mixtures. Reduced ignition timing is a problem for your head selection as these are essentially large chambered smog heads with wings cast in next to the valves to reduce the chamber volume. The problem these heads then bring is the plug is off on the far side of the head and with a 350 bore diameter results in a long burn just like 1970-1980 era 350 smog heads, which therefore wants, but now can't tolerate, more spark lead for best power.

I'd recommend springing your VISA card for a modern set of heads instead of screwing around with these boat anchors.

Bogie
 

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srnoncom said:
Thank you for all the comments! I think that I'm going to spring for some nice aluminum heads and use these for something more tame in the future!

Brett
Wise decision. Be aware, if your pistons have the old style domes the new heart/kidney shaped combustion chambers on most new aftermarket heads wont work. The Domes will interfere/hit the chambers. Check to be sure before you buy the heads.
 
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