Unknow compression ?

[STREETREBEL]

I have a built up big block, but don't know what's inside.
Can I do compression test to tell what octane I should use?

 

[Garys1969RR]

I have a 13 to 1 C/R 451 that needs at least 97 octane at 5200 feet. It gets cranking compression of 210 to 220 PSI. There are so many factors that are involved, that there is no clear answer. C/R, mixture, timing, octane, cam, all affect performance. Alot of guys shoot for 150 PSI cranking compression. What is your cranking compression reading?

 

[Meep-Meep]

Yeah, like Gary says, good luck. I have a 426 hemi mystery motor with 220 PSI and I believe a stock cam. Very unhappy engine. A cranking compression test is a good start and perhaps judging by the idle characteristics you can guess the cam. Get a 5 gal grog of 110 and start with one gallon per 10 or 15 of 91 and work up from there. You can calculate the average of the two octanes to get your mixed number.

 

[STREETREBEL]

Thanks.
Sounds like good advice!

 

[Garys1969RR]

Yeah, like Gary says, good luck. I have a 426 hemi mystery motor with 220 PSI and I believe a stock cam. Very unhappy engine. A cranking compression test is a good start and perhaps judging by the idle characteristics you can guess the cam. Get a 5 gal grog of 110 and start with one gallon per 10 or 15 of 91 and work up from there. You can calculate the average of the two octanes to get your mixed number.

I also have a 451 in a 65 Belvedere. It has 210 to 220 PSI cranking compression, but lacks good power output. It has a near stock cam, and it too is not a happy motor. Also has Edelbrock Perf RPM heads, 10 to 1 C/R, but the mild cam doesn't allow the motor to breathe. In other words, the heads have alot more potential for performance than the cam is allowing. The max lift is only .480".

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But the mild cams early intake closing point makes the cranking compression numbers go way up. So the 13 to 1 C/R motor with the 509 cam has the same cranking compression as the 10 to 1 motor with a mild cam. They both require at least 97 octane fuel at 5200 feet.

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The 13 to 1 C/R motor is producing at least 100 more horse power than the mild cammed 10 to 1 motor. They are both 451 Cubic inch.

 

[nm9stheham]

Going with the cranking compression test is not a reliable indicator of the compression ratio unless you have an idea of the cam. With stock or near-stock cams (not long duration), my rule of thmb for the compression pressure vs CR is multiply CR times 10 and then subtract 20 psi. But, any change in cams will change that whole lot; you can see that in the numbers above with 2 engines giving the same 210 psi range of compression pressure, but with 10:1 and 13:1 CR's.