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Re: '49 Cub compression

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George Willer

02-24-2003 21:36:36

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In real numbers the atmospheric pressure will be multiplied by the compression ratio. Sea level pressure under standard conditions is about 14.7 PSI. Multiply that by the compression ratio of 6.5, and you get a theoretical pressure of a little over 95 PSI for a perfect engine. It can go no higher...and engines are rarely perfect.

Sometimes, a combustion chamber is reduced in effective size due to deposits, so the reading can be a little higher.

I think gauges sell better if they read on the high side, so gauges are most useful for comparing the cylinders to each other.

George Willer

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02-25-2003 05:13:02

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 Re: Re: '49 Cub compression in reply to George Willer, 02-24-2003 21:36:36  
Thanks for explaining that George,
Now with that being said, how do your nice fleet of beauty Cubs stack up to the numbers?
I'm lacking power on my '49, which is probably a common complaint when one considers the age. I'll get back to you with the ratio I find.

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George Willer

02-25-2003 07:24:38

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 Re: Re: Re: '49 Cub compression in reply to EZ, 02-25-2003 05:13:02  

My gauge will show somewhere around 90 PSI on a fresh engine...but that tells you more about the gauge than the engine. The important thing is how the cylinders compare to each other.

For diagnostics, a leakdown test can be even better than a compression test. It can tell you whether a problem is rings, intake valve, head gasket, or exhaust valve.

George Willer

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