Can loose valve adjustment cause low compression?

dang

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I know tight valve clearance can cause low compression but at what point would loose clearance cause it? This topic came up the other day after I took off the head on my E3. No visible issues but when I turned it upside down on the bench and poured mineral spirits on the chamber is leaked out slowly on the exhaust side port. The head is getting checked out at a shop, but while I had it off I noticed that on the low compression cylinder one of the rockers was too loose, I think it was the intake valve, can't remember now. I did a non-gauge hand check while it was still on the engine and the clearances all felt the same and normal, but on the bench I found that one of the rockers was "notchy" and felt normal until I pushed really hard and it got more loose, maybe two or three times the normal clearance, I didn't have a gauge. This brought me to my question...

Dan
 

Mike Goble

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Loose valve clearance will raise the compression because you are closing the valve earlier in the cycle. Compression doesn't start until the intake valve closes, so by closing it earlier you will be compressing the mixture for more degrees of crank rotation.
I tested my M30B35 motors - 165# or better on all 12.
 

dang

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Loose valve clearance will raise the compression because you are closing the valve earlier in the cycle. Compression doesn't start until the intake valve closes, so by closing it earlier you will be compressing the mixture for more degrees of crank rotation.
I tested my M30B35 motors - 165# or better on all 12.
Don't you have to "fill" the chamber with as much air as possible to get higher compression? If the intake valve is not open as long it seems there would be less air intake and lower compression. The valve would open later and closer sooner. Less air. This is assuming the valve closes before the stoke is done sucking the air in.
 

Mike Goble

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There are effects in a running engine that can greatly affect the amount of mixture in the cylinder, pressure waves in the intake manifold and exhaust scavenging that take place in certain rpm ranges while the engine is running. The intake valve opens before TDC, and at that point there is no suction due to the motion of the piston. All motion of the mixture is caused by the suction of the exhaust wave at the open exhaust port (scavenging), and the pressure wave present at the now opening intake due to the design of the intake manifold. None of these effects are present when you are testing for compression, and the gains in compression due to the early closing of the intake valve will far outweigh the loss at the start of the cycle. If you look at the valve timing cycle picture here, if you had a tappet loose enough to cause a 20° delay in opening and 20 ° early closing, there is much greater displacement of the piston on the end of the intake cycle than on the beginning. If you were to calculate the motion of the piston from 10° BTDC to 10° ATDC it moves about 0.8mm up and 0.8mm down. The same 20° at the end of the cycle has a piston motion difference of about 11.9mm. The piston moves about 15 times as far on the compression stroke due to the delayed valve opening, and this effect will be very noticeable.

 
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