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Compression Ratio too high?

Posted By Butch Lawson Last Month
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Butch Lawson
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Fresh 312, zero deck, .040 , G heads.  Compression ratio calculator says static is 9.4 and dynamic is 8.1.  Is this too much for a street engine?  Thanks.
Butch

Butch Lawson

Manchester,  TN

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No, Ford wanted .010 in the hole with a .021 head gasket (steel shim) that is a .031 deck height, however most engines I see are .030 in the hole with the .021 head gasket (original engines), don't use book values to calculate the ratio it won't be close, always measure.
Dobie
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I think you should be OK on pump premium with timing all in at 36* BTDC but hopefully Ted will see this and give us his $.02. I have the same question about my Merc 292 which will soon have a set of freshly built G heads.

How are you doing? Haven't seen you on any forums for awhile.

Dobie Gillis from FordBarn/Tom in Tullahoma.
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I used gofastmath to calculate static and dynamic compression ratios. According to gofastmath, my dynamic compression ratio would be 7.99. However, Ted says you can't use IVC @ .050 to calculate dynamic compression ratio. He calculated that my dynamic would be 7.56. If you haven't zero-decked block, milled G heads, and used domed pistons, and small cam, I don't see how your compression ration ratio will high enough to be concerned about running premium pump gas. 

My dynamic is based on 3.81 bore, zero deck, flat-tops/w no valve reliefs, G heads milled to 67cc, Best gaskets, and cam/w 214 duration @ .050. A bigger cam will lower the dynamic. 

Hope this helps.
Butch Lawson
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Hey Dobie
Wife and I have both had a few health problems.  Hopefully we are about to get back to normal.


Butch Lawson

Manchester,  TN

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I had a 40 over 292 with G heads that I ran a McCullouch supercharger on. Never a problem.

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire
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Butch Lawson (2/15/2021)
Fresh 312, zero deck, .040 , G heads.  Compression ratio calculator says static is 9.4 and dynamic is 8.1.  Is this too much for a street engine?  Thanks.
Butch

Assuming the DCR number is correct, this engine combination will require a regular diet of 91-93 octane fuel.  While a DCR of 8.0:1 has a bit of a safety margin built into it, the 8.2:1 value should be considered the max for pump gasoline but that same 8.2:1 value does not give much latitude for accidents such bad gas, running hot, etc.  As Jeffrey brings up, the DCR calculation does rely on the actual valve closing event and not the 0.050” number.  IVC does means ‘intake valve closing’ and that occurs much closer to the advertised duration number rather than the 0.050” number.


Lorena, Texas (South of Waco)


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Have you measured the cranking pressure?   I seem to remember Ted quoting 180lbs max for pump gas.
Still learning.
stuey


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