Compression

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Jul 21, 2004 | 10:01 AM

DeanW

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From: Bristol PA

Compression

I have a 69 350/350 with 11:1 compression ratio. What should the PSI on a compression test read?

Jul 21, 2004 | 10:18 AM

MILO

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From: Mooresville NC

I don't believe it works that way. Cam duration and lift are addition factors. The numbers I hear is that the compression readings should be within 5% of each other.

Jul 21, 2004 | 10:22 AM

comp

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From: eville in

Jul 21, 2004 | 10:23 AM

DeanW

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From: Bristol PA

I understand the numbers should be close, but , what should I expect to see? 150-200?????

Jul 21, 2004 | 10:23 AM

ddecart

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Exactly what Milo said. The cam duration makes a huge difference. Compression ratio is only half of what determines cylinder psi.

Jul 21, 2004 | 10:28 AM

comp

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From: eville in

if you have a stock to mild cam if mine i would guess 150 at a low side

Jul 21, 2004 | 10:29 AM

zwede

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From: Plano TX

Quote:

Originally Posted by DeanW

I understand the numbers should be close, but , what should I expect to see? 150-200?????

Yeah, somewhere in the 150-200 range. The actual reading depends on so many factors. As already mentioned the cam makes a big difference, but also the cranking speed (shape of starter and battery), ambient temp, humidity etc etc. With the high compression and factory cam I would expect atleast 150 PSI, though. That's about as far as I'm willing to go in predicting actual results. The important part is that the readings are even between cylinders. You want to see less than 5% variation. Over 10% is rebuild time.

Jul 21, 2004 | 10:36 AM

MILO

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From: Mooresville NC

I did mine last weekend. The engine is not original for my '79. Actually the engine ID numbers put it back to a 1972 165HP, 2barrel, Camaro/Nova/Chevelle. The heads are 1976 smog heads. To guess my compression ratio is 8.5 to one IF the engine was just swapped in. My compression test revealed a range from 143 to 155, average was 150.

I don't know if that will help. You want the numbers to be inline with each other +/-.

Good Luck.