And in theory on a fixed camshaft engine, or even a moveable cam:

If the factory timing marks are setup up in advance from TDC at the factory (per their design) and you were to have a tooth jump, and the camshaft timing was off, and jumped towards true TDC then I could see how it could raise compression.


But that's a hypothetical situation. I don't personally know of any engines that are set up with advanced timing as your timing marks

 

Cam timing is a compromise setting. That is why engines are going to variable valve timing so the timing is closer to ideal for different engine speeds. The cam timing is most likely not the ideal setting for idle speeds in a non variable cam timing engine. The compression pressure read at idle could be increased by changing the valve timing to the ideal setting for a lower engine speed. We all believe and understand what we think is correct but it's been a good discussion anyhow.

 

N

I agree with that.


I always enjoy sorta taxing the ole grey matter, trying to figure out what would happen under circumstances these machines were not designed to cope with.


This thing started out causing me to chuckle over the three out of eight cylinders (involving both banks) that were supposed to have higher than normal (whatever that means) compression readings - not just higher than the others, and its because timing jumped a couple of teeth (somewhere). That one still draws a odd grin on my face.


EDIT: Normally I would have expressed that as the other five being LOW.

 

An old hot rodder trick was to advance cam timing to gain more low end hp. In fact a replacement timing gear was readily available for that purpose for the Chevy small blocks. Turns out retarding cam timing reduced emissions and it was one of the things the manufacturers did to pass the emissions tests but it also reduced low end torque. It was better for high rpm hp. So, it is reasonable to assume that if advancing cam timing increased hp at the low end, it would also increase the measured compression pressure.

Yep, when the compression is significantly different in different cylinders, that's a problem.

 

Not if engine Revs aren't up so some benefit can be realized from the inertia effect of moving air can 'pack' a little more air into the cylinders on the down stroke. Same with the valve overlap racing cams. That's where the added hp (and yes, increased compression also) comes from. But that would have a negative effect at cranking / idle / AND compression testing RPMs because advancing cam timing will cause the intake valves will close earlier also, negating any hope of aspirating any additional air.

 

I still disagree with you on the cam timing possibly causing high compression test results.
The following thread is one of the reasons I disagree. The owner had a 4.2L F150 with 270k miles on it. Ran terrible and finally found out the keyway on the balancer was worn throwing the cam timing off. Compression readings were in the 200 psi range which is very high for that engine with that mileage. post 30 of the thread has the comp readings.

https://www.f150online.com/forums/v6...nna-try-2.html

 

There's certainly nothing wrong with that. Nor does it mean you're not right.

I read the entire link you provided - and found it most interesting. My take away from it, unfortunately, did not resolve our differences for me. A couple of observations that bare heavily on that are; first, the visual wear on the cam gear keyway would effectively 'retard' cam position - (although - very few degrees, maybe 1-2). THAT alone, (if nothing else were involved) should not have the suggested effect. Second, the OP's list of compression readings were said to be "After he learned how to do a compression test correctly (but did not describe how that was). He did, however, state that he first screwed the tester into one cylinder and started the truck - resulting in a reading of 150'ish [significantly lower than any of those readings done "CORRECTLY" ??????.] Hummmmm???

Lastly - if the engine manufacturer could grind cams in such a way as to increase the compression developed on an engine with a 9.2 to 1 ratio that much - I believe it would have come from the factory with that modification. It would perform much better. I believe @glc was close close close to figuring the OP's issue out. Something else was wrong in addition to the cam gear - probably with ignition timing - maybe somehow making IT way out of proper advance/retard??, or that unknown sensor.


Anyway - thanks for making me exercise that ole grey matter. It's what keeps me going.

 

Ignition timing is computer controlled.

When designing an engine, performance has to be balanced off with emissions and gas mileage, also accounting for production tolerances. That's why aftermarket custom tuning can be very beneficial.