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-   -   Can you guess my compression ratio? (http://www.njfboa.org/forums/showthread.php?t=28785)

JL8Jeff 09-12-2007 09:35 PM

Can you guess my compression ratio?
 
I don't really have enough good information to even hope for a good guess, but here's what little I know. 1970 396 engine with 1971 oval port heads casting 3993820 which are listed as 113 cc's. Assuming the heads had a basic shave when they were rebuilt, what might the compression ratio be based on these dry cylinder readings:

Driver Passenger
200 --- 184
196 --- 184
199 --- 186
195 --- 185

Looks like one head was shaved more than the other or the head gaskets weren't the same thickness. Either way, both sides are pretty tight. I know the motor has a hydraulic cam and not the stock L78 solid lifter cam but I have no idea what pistons or overbore might be done to the engine. Engine seems to run fine, I'm just a little curious about the overall combination.

johnjzjz 09-13-2007 07:37 AM

a 70 - 396 is a 402 = 030 396 - and iron headed motors were 9 1/2 for the most part - if the cam has short duration or is advanced more than spec then the compression #s ( static ) will be high - also if you started on the drivers side the starter was cold and the battery was at top by the time you switched sides the #s always fall off - most crank the motor till the # is at its highest - race cars use the 3 pump method - at 3 hits on the Gage you stop and go on to the next one - overall it looks as if the cylinder pressure is real good -- had you gone back and re done # 1 my guess its the start point it would not have been 200 -- its the difference between them not the top # that matters -- jz


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