initial timing?

pntrbil · 04-01-2002, 09:42 AM

I didn't mean to mislead you guys on compression but to some extent we're talking apples and oranges here. The biggest factor to consider when selecting an octane rating is your compression ratio, not the cranking pressure you obtain form a compression test. Two different things.

The cranking pressure is affected by the compression ratio but it's also affected by cam timing. If you have a big rumpity-rump cam in there the intake will be staying open for a longer period of time and the piston can't develop any pressure while the intake valve is open. So you get less cranking pressure on a compression test.

For a comparison, I had a 10-1 motor developing 215 psi on a compression tester because it had a stock cam. Now that motor is at 12.5-1 but because I have more cam it's only cranking 155 psi.

Assuming a fairly stock cam 122 psi would indicate a lesser octane is fine and the higher octane gas probably won't make any difference at all. I'd try both tho.

Good Luck. Hope I cleared that up.

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04-01-2002, 09:42 AM	#6
pntrbil Registered Member Join Date: Jan 2002 Location: Hesperia, CA Posts: 19	I didn't mean to mislead you guys on compression but to some extent we're talking apples and oranges here. The biggest factor to consider when selecting an octane rating is your compression ratio, not the cranking pressure you obtain form a compression test. Two different things. The cranking pressure is affected by the compression ratio but it's also affected by cam timing. If you have a big rumpity-rump cam in there the intake will be staying open for a longer period of time and the piston can't develop any pressure while the intake valve is open. So you get less cranking pressure on a compression test. For a comparison, I had a 10-1 motor developing 215 psi on a compression tester because it had a stock cam. Now that motor is at 12.5-1 but because I have more cam it's only cranking 155 psi. Assuming a fairly stock cam 122 psi would indicate a lesser octane is fine and the higher octane gas probably won't make any difference at all. I'd try both tho. Good Luck. Hope I cleared that up.