direct vs differential compression check and a valve job

I have only done direct compression checks on my engine and they were always on the high side (160-200 psi). I took the plane to a IRT @ 50 hrs to look it over and asked for a diff check but he didn't want to as it was too "involved"

-8 years later, 512 hours on the engine-
Lately I have been having a few concerns with the engine performance, too many blades to burp the engine, slightly low static run-up (50-100 rpm), subtle vibrations at full power, max to min direct compression nearing the 29 psi limit (#1 @ 190 vs #2 @ 160).

I took the plane to another IRT (great guy) who did the differential test. Turns out the test is easy (as I am sure many of you already know) in fact now that I have my own tester I find it easier than direct. That said the results were not good, 74, 50, 68 and 58 over 80 :eek:

I pulled all the heads off, cleaned them and lapped the valves which looked a bit pitted. I replaced the intake valve on #4 as it looked like a re-grind was used when built :mad:

I now get a static run-up ~100 rpm higher than I have ever seen and the diff-comp numbers are much better (79,78,79,76 / 80). I have only flown for an hour but it feels smoother and now "burps" quicker.

This engine sat for ~7 years before being installed and run, I wonder if the valves and seats suffered some corrosion during that time.

In conclusion, if you don't already have one, get a diff-comp tester ($100) they are easy to use and, in this case, highly informative.

Valve pics to go with the re-grind comment, top is #4, bottom is the way all others looked.

I think your thoughts on the #4 valve were right on the money.

It doesn't show in the pic but the edge on that valve was sharp enough to cut, happy to have it out of there. Overall doing the valve job was pretty straight forward, I was worried not having done one before.

Hi Dorsal,


I think you did a great job on getting your engine up to spec.


Spent some time thinking about your observations.


Some observed variation will always be present for a couple reasons. a) Analytical variation of the test method - for instance 77, 78 and 79 might be the same number. b) cylinder head volume versus displaced volume can be different from cylinder to cylinder, yielding different results on the direct read method. The Differential method actually factors this out which is likely why your final numbers are so close once the valves were sealing correctly.



#1 - the valve with the non-existent margin (distance between the top of the valve and the angle of the valve face), in my opinion, should never have been installed in an engine in the first place. Had that been an exhaust valve, the issue would have made itself known in a very few hours as that would have caused a hot spot and burned through the edge of the valve in fairly short order. Looks to me like someone in the rotax factory was trying to salvage something they should not have.



Both compression testing methods tell you something; however, I think the differential method is more precise.The reason for the lower readings on this method is due to the fact this it is a controlled bleed test while the direct read method measured maximum cylinder pressure. The direct read method will for certain tell you what already might be the case with a burned exhaust valve. Neither will tell a person everything.



FWIW - the direct read method on a 912ULS should yield pressures in the 180-190 PSI range +/-. One trick in doing a direct read test is to observe how many strokes it takes to get the maximum pressure indication. If one cylinder takes 4 compression strokes to get to 190 psi and another takes 9 strokes to get to the same pressure, the latter has a leak problem.



Curious what fuel and oil you have been using??? - the photo of the one valve looks quite clean.


One of the bad things about about 100LL is its tendency to gunk up valve stems and leave deposits that get caught between the valve and seat.


Another source of crud, particularly with high time worn valve guides, is oil sucked past the guide tends to carbonize on the back of the valve and break off in little chunks with time.

When you had your engine apart, what did you find in the way of combustion chamber deposits?

Your experience kinda makes me think a good mid time maintenance procedure on this engine would be to pull the top apart and clean/lap the valves after a few hundred hours.

Dave,
Thanks for the helpful insights, this has been a good learning process. I run Aeroshell Sport-4+ and the valve pictured had been cleaned. Below are some pics of what I found initially. I cleaned the heads with a small wire brush on a Dremel tool, I also cleaned all the piston heads and valves.

I remember somebody from the group writing about an engine out due to a stuck valve from lead build up, glad to remove that worry (warranted or not).

The lead oxide build up on two of the exhaust ports was over 1/8" thick, had to (gently) chip it out.