Re: No one knows compression psi?... - Saab Sonett Bulletin Board - Saabnet.com
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Re: No one knows compression psi?...
Posted by kansas (more from kansas) on Mon, 8 May 2006 17:17:30
In Reply to: No one knows compression psi?..., Chaz, Thu, 4 May 2006 23:28:04
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Hi Chaz, I think you probably have got the information you need from numerous previous posts, but I'll chime in anyway and hope to clarify the matter a bit.
Chad: "...I guess no one knows [about compression ratio], as I cannot find the info in the factory manuals either."
I have looked a bit and also do not find a specific number. Some mfrs publish a number, others do not. Those who do not have the better idea IMHO, because testing under different conditions will yield pretty different numbers. Numbers will vary according to 1) Altitude 2) Spark plugs in or out 3) Throttle open or closed 4) Battery state of charge 5) Starter motor condition 6), 7), 8), ask and I will think of some more.
Oh, 9), this is critical, accuracy of gauge. Unless you are using some high-dollar equipment, like SnapOn, different gauges can easily vary by 10# or more. So absolute numbers even if the mfr provides one, are very problematical.
The better measure is measurement across cylinders with the same gauge and the same conditions (throttle, plugs, etc), looking at differences in readings from highest to lowest. Difference over 15% highest to lowest may indicate an existing or incipient problem. Repeated testing to establish a clear pattern and eliminate random variations is highly recommended. Results should be correlated or confirmed with other testing.
Compression testing alone may have been the best measure of some things back when dinosaurs roamed the earth, and I was there then. Nowadays it is seldom done because there are better methods, even on old engines.
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