My 1957 312 has the ECZ G heads (69cc) and according to the spec's on John Mummert's site the compression is 9.7 to 1.  Does anyone know what the compression ratio is now since its been bored .060 over and the heads milled .010 ?

buddy

See if this helps.  Here’s the static compression ratio for a stock 312 and assuming a zero deck.  This is using fibre or composition head gaskets and not the original steel shim head gaskets.  If the pistons are sitting 0.028” in the hole, then reduce the compression ratio by another half a point.

6.252	Rod Length
3.800	Bore
3.440	Stroke
69.0	Head cc's
0.000	Deck Clearance (in.)
10.00	Head gasket cc's
312.11	Cubic inches
9.09	Static compression ratio

 

 

And here’s the static compression ratio for a 312 bored +060 over (same zero deck) with the heads milled 0.010”.  This is assuming the heads were 69cc to begin with

6.252	Rod Length
3.860	Bore
3.440	Stroke
67.4	Head cc's
0.000	Deck Clearance (in.)
10.00	Head gasket cc's
322.04	Cubic inches
9.52	Static compression ratio

Lorena, Texas (South of Waco)

Ted,

Thanks a lot for the great detailed information.  This probably means I'll need to run premium gas. 

buddy

Buddy; Just for your information; The 9.7 to 1 compression figure you mention, was as advertised. John Mummert has indicated in the past that these figures were theoretical. In practise, allowing for deck clearance & head gasket volume, they were about 9.3 to 1 actual...

Paul

Thanks for the info Paul.  Back in the day (1963) I used to run my 312 timing two to four degrees more advance than what specs called four.  With the lower octane rating today I doubt I'll be able to do that with this engine without getting a lot of spark knock.

buddy

You might get away with it if you calculate the dynamic compression.



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Lawrenceville, GA