Rotax Compression Tests

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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designrs
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Rotax Compression Tests

Post by designrs »

Recently I noticed a change in prop pull pressure when burping the engine when cold. One blade feels very light on compression, especially if the airplane has been sitting for 10 days or so. After running the motor the prop pull feels even again. Motor runs fine.

Recent compression test show one cylinder lower at 76/85 when COLD.
If the compression test is done with the motor WARM all compression is uniform and normal. I was advised that Rotax recommends doing the compression test when the motor is warm and this phenomenon is pretty common on all motors.

Thoughts and comments?
- Richard
Sport Pilot / Ground Instructor
Previous Owner: 2011 SportCruiser
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MrMorden
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Re: Rotax Compression Tests

Post by MrMorden »

designrs wrote:Recently I noticed a change in prop pull pressure when burping the engine when cold. One blade feels very light on compression, especially if the airplane has been sitting for 10 days or so. After running the motor the prop pull feels even again. Motor runs fine.

Recent compression test show one cylinder lower at 76/85 when COLD.
If the compression test is done with the motor WARM all compression is uniform and normal. I was advised that Rotax recommends doing the compression test when the motor is warm and this phenomenon is pretty common on all motors.

Thoughts and comments?
I think you are good to go. A little slop when cold would be normal if the recommendation is to do the test warm. Expansion could easily seal up a minute gap that exists when cold. I'm not even sure 76/85 would be a cause for concern in any event. That's 11% off and limits for most engines allow cylinders to be within 25%.
Andy Walker
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roger lee
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Re: Rotax Compression Tests

Post by roger lee »

If an engine had a poor differential pressure check I would always run it before I made a final decision on a low cylinder reading.
Yes a warm engine is always best, but......

Typically a Rotax engine compression test is very close or the same warm or cold because of its tight tolerances. That said it could be different if left over long periods and would need to be run a few minutes to get oil and sealing surfaces up to their optimum. I have been checking this over the last 10 years. A warm engine versus a cold engine that just sat overnight or for 5-8 hours. I do at least 50 compression test a year on Rotax engines. Test results are usually the same, i.e. 87 over 83-85 psi is pretty normal. Normally if it loses anything it may be 1 psi more if done on a cold engine. I have seen a few engines over the years that had a low pressure test on one cyl., but usually turning it over a number of times fixes it. I had 1-2 engines over time I actually had to run it. My guess is a sticky valve. Using 100LL all the time may get a lead clunker under a valve seat.
Rotax like many other engine MFG's use that 25% loss as a marker for serious concern, but in class we will teach there is a problem even with a 10 psi loss.
Many people also don't take into consideration gauge errors of 1-2 psi. This is actually quite common. When I buy gauges for pressure or vacuum (carb sync) I usually buy 4-6 at a time. I put them on a pressure or vacuum system and keep the ones that read the same and pair them up. I don't want a gauge being 1-2 psi or inches of vacuum off from one another. This is extremely important to have good gauges for the carb sync.

The other thing I never believe is when I see a logbook entry of 80/80 psi. You need to get your gauges checked. The cylinders have to leak a small amount of air into the crankcase to help push the oil back to the tank. That's why you always hear a small bubbling in the oil tank when doing a compression test. No cyl. can be absolutely air tight with no air bypass to the tank. Normal would be more like 80/79 or 80/78.
If I saw one that said 80/70 I would be concerned. This isn't an air cooled loose tolerance Cont. or a Lycoming.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
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MrMorden
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Re: Rotax Compression Tests

Post by MrMorden »

So Roger, what would you say about a cold test that showed 76/85, if it came up to more normal pressure once warm?
Andy Walker
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Re: Rotax Compression Tests

Post by 3Dreaming »

Roger, to be fair a large percentage of Lycoming engines will test with very little leak down, just like the Rotax. I had one yesterday that showed very little leak down. Continental on the other hand normally has a bigger differential. They also have a SB covering compression checks that requires a test orifice pressure. That number on my tester is 41.5/80. If the compression is anything better than 41.5/80, and it is not going past the valves they consider the engine to be OK. For both the Lycoming and Continental they require air pressure for the rings to fully seat against the cylinder walls.
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Re: Rotax Compression Tests

Post by roger lee »

I would think it was okay, but not what you would normally see. The longer it sits the more chance this will show up. I see most of mine right when they fly in and within 2-3 hrs. do the compression test. Then when I was doing some research I would do it the next day and it would be the same. So long periods of sitting could affect it. I would always run it before maint. if it had sat longer than a couple of days. This would help with the gearbox check, the oil drain and the compression test.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
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Re: Rotax Compression Tests

Post by drseti »

One of the things emphasized in Rotax maintenance training is that compression checks should be done with the engine warm. I wouldn't pay much attention to cold values (and wouldn't waste my time checking the engine cold in any case). As for the reference pressure, Rotax (as I learned in their Heavy Maintenance courses taught by Eric Tucker) recommends 6 bars. Since a standard atmosphere (one bar) exerts 14.5 PSI of downward pressure at sea level, six of those equals 87 PSI. Checking at 80 is the norm for Continental and Lycoming engines, and I see a lot of mechanics who have not been Rotax trained erroneously use this reference pressure on our engines as well. So, 85 PSI applied pressure is closer to (but not exactly equal to) what Rotax recommends. In order to be able meaningfully to compare compression figures from one annual to the next, it's really desirable that all tests be done at the same reference pressure. In my shop, that figure is 87 PSI. (Check your logbook, Richard; I think that's what I used last year.)

Note to other Rotax mechanics out there: note that I cite my source. Other maintenance instructors may disagree.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
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AvSport.org
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designrs
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Re: Rotax Compression Tests

Post by designrs »

Thanks for all the good insight on this issue.
Paul, I assume that we did the compression test warm?
I'll reference the exact numbers again shortly.
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Re: Rotax Compression Tests

Post by drseti »

You assume correctly.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
[email protected]
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US
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Re: Rotax Compression Tests

Post by roger lee »

Hi Paul,

Rotax used to be only 87 psi, but a few years back stated that 80-87 psi was okay. It is in the Line Maint. Manual, Rev 3 Sept. 2012, sec. 12-20-00, page 13 step 5 (80-87 psi). I'm still old school and use 87 psi. I never saw a reason to switch.
Using a warm engine is always best and recommended, but real life testing on 40+ engines warm versus cold I can tell you it rarely ever makes a difference and when it does it's only 1 psi. This isn't worth worrying over since most people's gauges are off that much.
This is predicated on a cold engine that was at least started and run within the last 48 hours.

I have had the pleasure of doing a lot of research for LSA MFG's and component MFG's. I usually always give my feedback to Rotax.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)
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