Direct compression 912S

[Dorsal]

Just finished my condition inspection FWF and did a compression test, the results are 190 182 190 204 psi on cylinders 1-4 respectively. These seem high relative to the Rotax stated range, perhaps because it is still a young engine (100 hrs). For others who do this test what types of numbers do you get?

My other issue is that my Dynon fuel pressure sensor indicates 2-3 psi while flying with 2 being slightly off the low end of the range. I will be checking the system with a second gauge but am wondering what others see for fuel pressures at ~5200-5400 rpm cruise?

[MotReklaw]

I used a differential pressure gauge on my 912 and got readings of between 78/80 - 80/80 with about 110 hrs. That's with the engine at normal operating temperature.

[Dorsal]

Thanks Tommy, I only have a direct test setup though it appears they prefer the differential method. The documentation says I should get between 130 and 172 psi.

[Dave S]

Dorsal,

Compression varies around 169 - 178 on our ULS (150 hours)
Fuel pressure about 4 psi when I had a direct read gauge plugged into the engine for static testing. I don't have a panel gauge for fuel pressure.

The direct compression readings for a 912ULS should be in the 175 psi range. I'd question your gauge before I would question the engine condition - particularly with a higher than normal reading. Please stick with me....

What I have found in this world regarding a "consumer grade" measuring instrument, such as a compression gauge, temperature gauge, fish scale, bathroom scale, check weights is that 1) they are not calibrated 2) they have been manufactured to some standard which includes an unknown variation from what the number is on the instrument face. In other words, the reading will normally be biased + or - something and you won't know how much unless the instrument is compared to a calibrated instrument or it is actually certified to a standard in which case you will have a sheet of paper declaring the instruments calibration to a primary standard (and the stupid thing would cost a fortune).

So my bathroom scale reads 92% low (nice) - My fish scale reads16% high (also nice) - .......something like that can happen with compression gauges too.

The numbers you are looking at may be correct in relation to each other (which is what is important); but, they may all be biased to the positive. Looking at the numbers you have - the highest and lowest are only + or - 11 psi from the average - not bad at all when dealing with an engine with a CR of 10.5 to 1.

When I speak of bias - it is going to be a percent of scale - always reading high or low by the same %. Considering your average of the 4 is 191.5 psi; and, an expected value may be 175 - the compression gauge may be reading 109.4% of true - it would only have to lie to you by reading 9.4% high to give you those numbers. But then if you were doing a check on an engine which normally gave 70- PSI - the gauge would read 76 - which would not seem weird at all because the absolute difference is not that much in spite of the fact the % of scale is the same.

Another clue that the compression may not actually be over 200 PSI is the engine has obviously not exploded.......

Almost no instruments - panel instruments or tune up instruments - are free of bias - LIke the Airspeed indicator - they give you an "indicated" value - not a calibrated value. The AC the FAA provided for homebuilders speaks to the issue of calibrating panel instruments, but, nobody does it - they are all installed and that's it. No instruments tell you the absolute truth directly.

A second gauge like you already have decided is exactly where this would lead a person. I know the thought is a bit hard to stomach; but, the gauge which is right on is the exception.

Sincerely,

Dave S

912ULS Warp

[kitfox2009]

Hi Dorsal

My 912UL (80HP) reads 142 to 145 at 240 hours. Not sure about the gauge accuracy though. It is new.

Don

[Dorsal]

Thanks Dave, now that I think about it it would not be too hard to calibrate my tester and scale the results accordingly. As you point out the engine has not exploded (in fact runs great) so if these are unusual numbers the gauge is certainly suspect. I suspect the same is true for the fuel pressure reading though I would feel pretty stupid sitting in some farmers field with a failed fuel pump. Out of curiosity does anyone know how well these engines run on just the head pressure from the wing tanks?

[Dave S]

Don,

With the lower compression ratio of the 80 horse, 142 to 145 is about where it should be for a direct read compression test.

Hope I didn't carry on too long in my earlier post...

Dorsal - I didn't speak to the differential pressure test since I haven't used it; but, my understanding is the DPT gives a person a more precise measure of cylinder leakage - which is what a person is trying to determine. The fact that your engine runs smoothly says it is healthy. You are spot on thinking critically about the fuel pressure - might be OK and might not - but best to know. Almost hate to admit it but I once flew a high time rental warrior which went to zero fuel pressure one time at full throttle - throttling back brought it back up as the weak fuel pump was able to keep up to the engine...arrrrg.....(another reason for going to a new homebuilt).

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

[Dorsal]

Good news on the fuel pressure, replaced the sensor and all is good, 4psi steady. Will probably move the sensor back to the firewall to get it further from vibration.

[Dave S]

Dorsal,

Great news that you have the fuel pressure deal resolved - nothing wrong with a little peace of mind.

BTW - I have the quad gauge and one of the first sensors to die was the one for the oil temperature (which is the same sensor as for the CHT on the quad gauge) - it went intermittent at first then died completely and rested on the peg - so far the replacement has held up.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

[kitfox2009]

Hi Guys
I do not have a fuel pressure gauge and are just relying on the mechanical pump to do what it is supposed to and if that should fail the gravity pressure from the tanks should do the trick.
If I was to install a gauge though, what type and sender would you recommend? Also is there a reasonably priced (and reliable) fuel flow/ pressure gauge combination that you would suggest.

Thanks for your input.

Don
912UL Vixen IVO IFA