Venomous S

Has anybody here actually run a compression test at a a predetermined valve clearance, and then for example made the valves tighter by like .001" and then ran another compression test. I'm just curious to see about what that .0001" tighter clearance difference made in cranking compression in PSI.

Obviously no 2 cars will be alike but i'd like to see whos done this and what they got.

xviper

Until someone who has actually done it comes along and answers, I like to make this supposition ...........................

When doing compression testing, you are cranking the engine over at the speed of the starter motor. This is very slow. Any valve clearance setting that are ZERO or bigger (withing the acceptable range of the specs), will still create a situation where all the valves are totally closed. Whether they are closed with the actuator at ZERO clearance or at 0.011", they will still be closed. Technically, compression should not vary.

Now, let's hear from someone who has actually done it to tell me I'm full of it.

Greenlight

The camshaft timing plays a role in cranking compression. If the intake valve closes sooner, the cylinder will "trap" more air to compress and thus increase the pressure.

With that being said, the rate of lift of the camshaft, on the closing ramp, is about 0.003" per crankshaft degree. So, increasing the valve lash by 0.003" would be equal to advancing the camshaft 1 degree and reducing the lash by 0.003" would be equal to retarding the camshaft by 1 degree.

I have personnally changed my camshaft timing by 4 degrees and seen the cranking compression change by 10 psi.

In summary, I don't think changing the lash by less than 0.005" will make any difference.

dhayner

[QUOTE=Greenlight,Apr 28 2006, 08:51 AM]The camshaft timing plays a role in cranking compression. If the intake valve closes sooner, the cylinder will "trap" more air to compress and thus increase the pressure.

With that being said, the rate of lift of the camshaft, on the closing