Decided to do a compression test on my engine. As far as I know, its a stock 70 olds 350. Cylinders 1, 3, and 7 are at 130. The rest are at 140. I know they are good as far as consistency, just was wondering if it is good for what engine it is. As far as I know, it was a 2 bbl motor.
Compression test results
- Thread starter dan2286
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yeah...thats what mine tested at....130 right after the build...might be a bit higher now. I think a #6 head 350 engine is about 9.0 to 1 if Im not mistaken
I may be wrong here ... but I don't believe that cylinder pressure has anything to do with a static compression ratio :idea:
Agreed. Cam timing and overlap will probably have more effect than actual static CR. A cranking compression test is more about consistency and history. Your 130-140 numbers are fine unless it used to (historically) be 170-180. Then it would indicate a problem. A leak-down test gives a more accurate view of engine condition.RITTER said:
Sure, cam selection and combustion chamber size all come into play.
On any given engine, a higher static compression ratio has to show a higher cylinder pressure when compared to the same engine with a lower static compression ratio. You are stuffing the same amount of "air" into a smaller volume combustion area.
Camshaft plays also by increasing or decreasing dynamic compression etc. thus the need for small combustion chambers when using large camshafts.
However he did say it was a stock engine.
His readings are on par with what I have seen and would expect from an 8 or 8.5:1 CR engine with a stock or mild cam. If he were running a 10:1 engine I would have expected more compression. Just my opinion fwiw
On any given engine, a higher static compression ratio has to show a higher cylinder pressure when compared to the same engine with a lower static compression ratio. You are stuffing the same amount of "air" into a smaller volume combustion area.
Camshaft plays also by increasing or decreasing dynamic compression etc. thus the need for small combustion chambers when using large camshafts.
However he did say it was a stock engine.
His readings are on par with what I have seen and would expect from an 8 or 8.5:1 CR engine with a stock or mild cam. If he were running a 10:1 engine I would have expected more compression. Just my opinion fwiw
Sounds a bit low to me but not bad. A 70 350 with 2 barrel was rated at 9:1 CR but were always lower than that ( though not 8:1 ). The stock cam is very small too. So I would expect the psi to be a tad higher but since they're all pretty close then I would say it's fine. It's not like they're like mine; 180, 179, 160, 160, 130, 80, 40, and 'unknown'. Now that's a HUGE concern!
So assuming 180 on my 2 good cylinders is what they should all be at, and the fact I have less than 9:1 CR with a very small cam, I would think yours should be a little higher but it's hard to say since my cam is vastly different than a stocker ( though still very small like a stocker ).
So assuming 180 on my 2 good cylinders is what they should all be at, and the fact I have less than 9:1 CR with a very small cam, I would think yours should be a little higher but it's hard to say since my cam is vastly different than a stocker ( though still very small like a stocker ).
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