Compression Test, PC- An exercise in mental masturbation Options

[SLITS]

Scenerio: You buy an engine and do a compression test. Cylinders measure 135, 130, 130 & 85 - BUMMER. So, since this engine has not run in upteen years, you throw it on the lift and pull the valve cover on the 3/4 side (#4 cyl is the low one).

Lo and behold, you notice that the #4 exhaust valve rocker has no adjuster screw or lock nut and there is a v-shaped piece of aluminum missing from the rocker box beneath the #4 intake valve. Futher probing (Aaron would like that) with a magnet reveals the missing parts.

Great story, but not the point of the post.

Problem:

If the exhaust valve is not opening to relieve the pressure, allowing kinetic energy to pull a larger charge of air into the cylinder when the intake opens, it will not build cylinder pressure like a normal compression test of a cylinder that is working. This is my theory anyway.

I have already been told by one mechanic that I am full of shit and another that agrees with me. No I haven't redone the compression test since replacing the screw and locknut.

This is your chance to mentally masturbate and tell me if I am full of feces or not! Spew away.

[phantom914]

A closed exhaust valve could "possibly" cause a lower compression reading, but I am not sure about your reasoning for it. I would think that normally, with overlap and due to very slow cranking speeds where the inertia of the air is not a factor, that during the overlap period on the intake stroke, you would normally pull air through both valves until the exhaust valve closes. So possibly, less air gets drawn on the bad cylinder with the stuck exhaust valve. But that is a lot of difference you are talking about. Are you sure that the valve is sealing?


Andrew

[SLITS]

Not sure of anything - like I said, it is an exercise that I can't prove until I do another compression test. The rest of the theory would be that since the cylinder is already pressurized, at overlap the pressure would be released into the intake tract (much like a reversion wave) decreasing the intake charge volume.

Hey, I had nothing better to do than think of this or I could go back to hijacking threads. (IMG:style_emoticons/default/laugh.gif)

[phantom914]

Not sure of anything - like I said, it is an exercise that I can't prove until I do another compression test.  The rest of the theory would be that since the cylinder is already pressurized, at overlap the pressure would be released into the intake tract (much like a reversion wave) decreasing the intake charge volume.

Hey, I had nothing better to do than think of this or I could go back to hijacking threads. (IMG:style_emoticons/default/laugh.gif)

Could be. But just doesn't seem like it would make as much difference as you're seeing. Don't forget also that at cranking speeds, the inertia of air is not near the factor it is at running speeds, so reversion and scavenging may not mean anything. But who knows? Let us know what the 'after' results are.

Andrew

[Aaron Cox]

probe this :finger2:

and

Problem: .... that I am full of shit...and another that agrees...and tell me if I am full of feces or not!

yes, in fact you ARE full of shit (IMG:style_emoticons/default/biggrin.gif)

[ChrisFoley]

The inertia of the airmass at cranking speeds is definitely not a factor. If this engine has a stock cam, overlap is definitely not a factor.
I say do a leakdown test on cyl 4. If you don't have a leakdown tester, just pump air into the cylinder through the spark plug hole while the piston is at tdc on the compression stroke. If something is wrong you will hear the air rushing out somewhere.