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Old 04-20-2012, 08:56 AM
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When I tore apart my engine, I found the last rebuilder used Silvolite pistons with a 1.540" compression height, undecked block, and a .043" head gasket. This gave a whopping quench of .088". No wonder the previous owner had to use 93 octane even though the engine was about 8.5:1 compression! So YES engine builders more often that not ignore this dimension.

Also keep in mind Vortecs are advertised as 64cc, but in reality they are closer to 66-68. You will need to cc your own to find out. I did mine the other day after the shop milled .010" and they came out to 66cc. Also the cc dish will likely be greater than advertised, so be sure to measure your own. After you get your measurements, buy your head gasket LAST to achieve your quench and compression ratio. You are dealing with several variables: piston dish volume, head volume, and machine shop decking (or factory variance if undecked), and possibly core shift/misaligned main saddles. Measure all these, then pick the gasket.

For comparison, here is how L31 engines left the confines of the General's compound with 87 octane friendly specs:

L31 engines came in two different styles light duty (cars and trucks) and heavy duty (for HD trucks).

Bore: 4.000"
Stroke: 3.48"
Combustion Chamber Volume: 64cc (probably closer to 66cc-68cc)
Compressed Head Gasket Thickness: .015" steel shim
Deck Clearance: .025"
Squish: .040"
Piston volume: 12cc dish (for light duty)
Piston volume: 18cc (for HD)
Compression Ratio: 8.93 / 9.5 : 1 (HD / LD)
Compression Ratio: 8.68 / 9.22 : 1 (HD / LD)

I listed a range of compression ratios here based on variable head chamber size, not even counting piston dish variance, etc.
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