Date: Fri, 15 Mar 2002 08:32:58 -0000
From: "Andrew Hookins" <andrew'dot'>
Subject: Re: compression numbers?

Consistancy of the numbers is good, but I agree with KeithG that I slightly higher figure may be in order. Unfortunately its too simplistic to say that the measured pressure is atmospheric pressure multiplied by compression ratio. Gas law tells us that as you do work on the gas (i.e. compress it) its temperature will rise and if the temperature rises then so will the pressure (since the volume here is fixed). So this effect will tend to give a higher reading. BUT, other effects counteract this: The inlet valve(s) don't close at BTC, they close later which at cranking speed means that some of the air drawn in gets pumped back out again (This is why torque at low engine revs is low, but at higher engine speeds there is insufficient time for the direction of air-flow to reverse and so more air gets drawn / stays in). Also the pressure gauge will add a small un-swept volume that will reduce the effective compression ratio. As KeithG stated, if the throttle is closed then less air can get in anyway, and lastly, a higher reading will be obtained with the engine at its normal operating temperature than if it is cold. AndyH "KeithG" <> wrote in message news:a6rvm1$glp$ > I would say yes. It is important that the numbers are close (<5% between > values for the cylinders). The absolute values should be the compression > ratio times the atmospheric pressure. This generally works out to > 140-150psi. It may be that he kept the throttle closed or that the gage is > not properly calibrated which causes the values read a bit low. How does the > car run? Does it burn oil? > > KeithG > > "Geo" <> wrote in message > news:a6rqt4$adn$ > > I had a mechanic look at a 93 9000 CSE. He says the following compression > > results are good: 110, 110, 115, 110. Are they? Just double checking. > > > > Natural Light Black and White Photography > > > > -George- > > > > > > > > > > > >

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