Compression Ratio Guestimate
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From: St. Joseph Mo
Compression Ratio Guestimate
I have almost no idea what pistons are in 'Old Blue' nor do I have cam specs. All I do know for sure is he is a .040" overbore 427 with #3933148 heads that, according to Mortec is open chamber 109 cc.
The cranking compression is a nearly flat 180 psig all around. 175 psig if the lowest and 187 psig is the highest. Given that the spec in the Chassis Service Manual is 160 psig for an L36 must I assume that the C.R. is above 10.25 : 1?
How much does the inlet valve timing (closing point) have on the cranking compression? If the inlet valve closed earlier on the unknown cam than on the L36 cam wouldn't that increase the cranking compression by itself?
D&%#@ I can't find the folder I put all those box labels in back in '93.
The cranking compression is a nearly flat 180 psig all around. 175 psig if the lowest and 187 psig is the highest. Given that the spec in the Chassis Service Manual is 160 psig for an L36 must I assume that the C.R. is above 10.25 : 1?
How much does the inlet valve timing (closing point) have on the cranking compression? If the inlet valve closed earlier on the unknown cam than on the L36 cam wouldn't that increase the cranking compression by itself?
D&%#@ I can't find the folder I put all those box labels in back in '93.
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Usually the 10.25 motors actually measure out around 9.8 or so in real life. The .040 over part depends on what pistons were used. Often 'rebuilder" style pistons are used and they actually have a dome that is even smaller than normal and they are made shorter than usual to allow for decking of the block. Usually you end up with less than stock compression.
Most heads will CC a good bit larger than what the books say too. After a few valve jobs and the fact that was a nominal number anyway changes things too. I had a set of BB closed chamebr heads that were supposed to be 100 cc. They were 115!! We had to angle cut and flat mill them to get them down to 95cc.
Yes, a different cam will drastically change cranking compression. If it has a tighter LSA, is a smaller lobe, is a very aggressive lobe or it is advanced a lot, they will all show higher cranking compression.
The good news is you have good compression for pump gas use. You should be able to run fine right there on good gas.
JIM
Most heads will CC a good bit larger than what the books say too. After a few valve jobs and the fact that was a nominal number anyway changes things too. I had a set of BB closed chamebr heads that were supposed to be 100 cc. They were 115!! We had to angle cut and flat mill them to get them down to 95cc.
Yes, a different cam will drastically change cranking compression. If it has a tighter LSA, is a smaller lobe, is a very aggressive lobe or it is advanced a lot, they will all show higher cranking compression.
The good news is you have good compression for pump gas use. You should be able to run fine right there on good gas.
JIM
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Safety Car
Joined: Feb 2003
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From: St. Joseph Mo
Rebuilder type minimal dome pistons are what we used back then being the intent was to end up at 8 1/2 or 9 to 1 in order to run 89 or lower octane fuel. It does, I might add.
The other week when changing plugs I ran a compression check and much to my suprise it topped the stock figure by 20 psig. I was expecting about 150 psig. This leads me to believe the cam has an earlier inlet valve closing than your ordinary L36 cam. Must be advanced some.
The other week when changing plugs I ran a compression check and much to my suprise it topped the stock figure by 20 psig. I was expecting about 150 psig. This leads me to believe the cam has an earlier inlet valve closing than your ordinary L36 cam. Must be advanced some.
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my 11:1 (advertized) L-79 measured out to be 9.4:1 in the back cylinders and 10.1 in the front.
yes, both decks ran uphill front to back !
that has since been corrected
yes, both decks ran uphill front to back !
that has since been corrected
