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Re: Compression Ratios

To: "Peter J. Thomas" <pjthomas01@snet.net>
Subject: Re: Compression Ratios
From: Berry Kercheval <berry@kerch.com>
Date: Thu, 26 Mar 1998 19:49:01 -0800

> Bought my first compression guage.  It is calibrated in psi.  How do I
> determine the compression ratio? Ie. how do I convert  x psi to n:1?

Well, atmospheric pressure is 15 psi (close enough for this calculation) so
10:1 compression ratio will give you 150 psi.

However, only a perfectly sealing engine will give you 150.  figure at least 
10% leakage, which would be 135 psi.  If you can get 130 on all cylinders of a 
10:1 engine you're doing fine.

The calculation of absolute expected pressure like this really isn't useful 
except as a gross check that the gauge is working more or less OK.  It's more 
important that you get repeatable, uniform readings across the cylinders.  If 
your 10:1 engine gives you 115, 120, 120 and 125 that's pretty decent and not 
cause for worry.




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