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Old 04-02-2012, 11:11 AM
oldbogie oldbogie is offline
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Quote:
Originally Posted by Larry123
I have a 1976 Chevy 350, .030 over with low miles on a fairly stock rebuild. My plan was to install 64cc alum heads and a Lunati 60102 cam to warm it up and use in my 55 Chevy. I pulled the heads off and discovered flat-top pistons .043 in the hole. If I use standard .041 head gaskets the DCR comes out to 8.3:1 with this setup. Will this work ok for street use even with the poor quench?
X2 with "my87Z". If this is a real measurement than what you've got are rebuilder pistons used on decked blocks to restore the original .020-.025 inch piston crown to head deck clearance. This maintains stock compression ratios for the mass engine rebuilders that simply zero deck all blocks they run on a set up whether they need it or not. This is not like your neighbor-hood auto machine shop where they do not as a matter of course zero deck the block they are working on. That would only happen is the deck was warped or the customer asks for it.

If you don't yet have the aluminum heads the quick way out of this would be to go with a iron head with the Vortec style chamber and use a .015 thick shim gasket. This would give a squish/quench of .055 which is still pretty effective.

The problem this situation creates is that the parts industry makes two different pistons for a rebuild. For the Small Block Chevy these are standard compression height of 1.56 inch and rebuilder for stock replacement compression decked engines at 1.54 inch. Compression height is the name for the distance from the pin bore center to the crown of the piston. The crown being the top edge above the rings. This may be the same as the total surface as in a flat top, or it may be an edge above a dish, or an edge around a higher dome. But I compute an SCR of 9.7 to 1 with this set up which is a bit high for a cast iron head. You could go with an open chamber head at 74 ccs which would drop the SCR to 8.8 with no other changes which is a bit low.

You a bit between a rock and a hard place where you need tight squish/quench for the compression but can't get to the optimum .040 and the ratio is a bit high which needs all the squish/quench you can get. Going to a larger chamber drops the ratio under but pretty close to an optimum for an iron vortec style head. Unfortunately aluminum needs a thicker gasket to provide some sliding give to where the gasket doesn't erode the soft aluminum.

Bogie
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