Re: Compression Numbers - Saab Performance Bulletin Board - Saabnet.com
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Re: Compression Numbers
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Posted by Dean [Email] (more from Dean) on Wed, 7 Jun 2006 13:37:39 Share Post by Email
In Reply to: Compression Numbers, DrewP, Wed, 7 Jun 2006 11:29:41
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Compression tests are low speed and some cam behaviors may not do as expected. Temperature can have a big impact on readings. If the fuel pump is not disabled, you will also be measuring the effects of fuel vapor pressure as well. Sometimes the carbon bits released as plugs are removed can get to the valves and reduce readings. One can crack plugs loose, re-tighten, start briefly to get oil circulated and cylinder walls oiled, blip throttle to blow out carbon particles from plugs, shutdown. Engine should be quite cold after that brief event. Remove plugs and pull fuel pump fuse. Crank engine briefly to purge fuel from intake valve area, then do tests.

Your pressures seem quite high. As pressures can vary with temperature etc, the fact that the pressures are so close together is perhaps a greater indicator of things been right.

Not all compression gauges would read the same on the same engine under the same conditions.

When cranking, it is best to disable the ignition system as having the coil(s) fire against open circuits can create abnormally high voltages that can puncture the insulation in the secondary coil that may lead to problems later on.

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