Gernby

I checked my cylinder compression about 20K miles ago after a dealership rebuild, and all cylinders were between 250-260. I later did a "performance valve adjustment" to about .002" below spec. Then I checked my compression about 10K miles after that, and compression was about 230 on all 4 cylinders. It was still making similar power, so I just brushed it off as a weather thing. However, I checked them again today, and they are all 230 again. It made me wonder if maybe the tighter valve clearances reduced the dynamic compression enough to drop the reading from 250 to 230 PSI. I believe it would have to reduce dynamic compression some, but I don't have any idea how much. Does anyone know what the duration and overlap is of the low cam lobes?

Gernby

Correction: After pulling my plugs, disconnecting the injectors, and doing a better compression test, they are all hitting 240 PSI. So it seems that they've only dropped about 15 PSI, but I'm still curious if that could be caused by the longer duration and overlap given by the smaller valve clearances.

chris_barry

Do the experiment and change one of the cylinders back to your original clearances and retest.

We expect the written paper on your results tomorrow.

Gernby

As much as I'd love to do exactly that .... I have way too much on my plate to adjust my valves twice in a day.