80hp 912 compressions

[chrisag]

A few questions for the experts out there. Have noticed that one pot was a bit soft on a pull through. I have just purchsed a new leak down tester,the model specified by Floods, and tried it out today. This motor has 900hrs since new rings and a valve grind. When pressurising the cylinders we got a reading of 80/79 on 3 pots and on the other flat one 80/75. The minimum numbers are 80/60 as per Rotax. These figures seem too high but did the test per instructions.Have we done it correctly and the donk is perfect or have I stuffed up somewhere. The other problem was an increase in oil use. I think I have sorted this out after talking to Floods. I need to stop using Shell Aero Sport and go back to using VSX 4. Floods stopped using it for that reason.

 

Would be very interested on other readings that people are finding on their 912.

 

J.C.

 

 

[bones]

The other problem was an increase in oil use. I think I have sorted this out after talking to Floods. I need to stop using Shell Aero Sport and go back to using VSX 4. Floods stopped using it for that reason.J.C.

Ohhhh really when did this occur? damn was about to buy another 12lts

 

 

[chrisag]

Thanks for that info Dexter,I will try the higher pressure but I don't think it will make much difference. Yes, the pistons were TDC. Mate who assisted me has done this job hundreds (he tells me) of times but on much larger aero donks,the process is the same I believe.

 

Bones.. I only found out today RE the oil. I spoke to Floods about increased oil useage and they said they had stopped using it for that reason. It was the oil use that got me into gear to do a proper compression test.

 

J.C.

 

 

[gregrobertson]

Hi Bones.

 

Also my experience with Shell Aero Sport. On a trip (32 Hours) up to Birdsville, Longreach and back to latrobe valley last year I burned 3/4 liter of oil. I have changed to Shell Advance VSX 4 and had no further problems. VSX also seems to hold slightly higher oil pressure.

 

Greg

 

 

[Tomo]

Regarding the leak down test, Rotax's (I believe this is correct) have tapered rings, so no matter how old they really are, the way they are designed they will pretty much have good compression for quite some time - when compression or pressure is given to the top sides of the piston.

 

Also, depending on how much oil is on the piston bore, it will change things dramatically, more oil, more compression, less oil, or dry bore, obviously less compression. (to a point)

 

Sorry that doesn't help you much, but, turning a motor over by hand has a lot of varibles to change it a lot - turning it slower will seem to have less compression; faster feels more; jerky, is, well not really checking anything....

 

Turning over by hand will only pick up large amounts of compression loss, or unless you know exactly what you are looking for, if you know what I mean?

 

 

[bones]

Hmmmm ok well come to think of it, i had to put oil in for the first time ever the other day, just didnt think too much about it, i been running it for about 150hrs now, might have to throw it out too then. thanks so much for the info.

 

 

[chrisag]

Thanks for the input Tomo. What has got me stumped is the fact that 1 pot seems noticably less than the other 3 but when tested produced good results. The motor does run well with no noticable reduction in power but when something happens gradually perhaps its hard to pick a drop in power.

 

J.C.

 

 

[Tomo]

One thing you could do, is pull a spark plug out of each cylinder and look at them, (don't mix it up, until you've checked them!). That is 'one' way of seeing 'if' there is a low compression in one pot.

 

 

[old man emu]

When pressurising the cylinders we got a reading of 80/79 on 3 pots and on the other flat one 80/75. The minimum numbers are 80/60 as per Rotax. J.C.

JC,

 

Those numbers ar quite OK. You've got a 5% variation between the best and worst. If we got them for a Lycoming, Continental or Jabiru engine during a 100 Hourly, we'd have no concerns. There are engines out there that are quite happily chugging away at 80/72 and such.

 

They show that each cylinder's rings are working as designed, and there is no burning of the valve faces or seats. As long as the range of values is not wide, you're OK. On those figures, if the low one was 80/68 or such, I'd be suspicious of that cylinder, and would first start looking for a speck of carbon between the valve face and seat. If you ever got to 80/60, then you would start to make further investigations. The engine will still run at 80/60, but its efficiency would not be at its best.

 

What you've got to remember is that this is a Leak down test, not a compression test. When you do the test, you pressurize the cylinder to 80psi and see if the pressure will hold at 80psi. You expect some variation in the readings from the guages as the system settles down. You also need to accept some variation in the accuracy of the guages (although they should be calibrated every 2 years)

 

Old Man Emu

 

 

[Guest Maj Millard]

When I purchased my current Lightwing with a 912 ULS, (250 hrs TT) I found that my cruise figures wern't what I had experienced in similiar planes, with the same engine. Also the engine could have been smoother allround. Plug checks showed #2 cylinder (fwd left) always blacker than the rest. I suspected that cylinder wasn't pulling it's weight for some reason.

 

After a chat with Wal I decided to pull that cylinder to check it out. This cost a gasket kit and a special Rotax ring compressor. (Only one to use by the way). Upon removing the cylinder I found the bore nicely glazed. You cannot hone them, (Nikasil) so I roughened up the bore to get rid of the shine with a scourer (new green one) per Wals advice.

 

On firing up the engine it was obvious I had fixed the problem, and the engine sounded much happier and smoother. I gained an extre 5 kts cruise speed, so I was correct about it not pulling it's weight. It has run perfect ever since.

 

Wal suggested that since the gearbox oil return is right by the base of #2 cyl, that it may be getting a double dose of oil so to speak, on the cylinder wall, which may have caused the glaze especially during run in.

 

I would not suggest that everybody go out and pull #2 cylinder and deglaze it. I work in the industry and had a slow day, so I did it. I did it also for my own education as much as anything, as I am supposed to know all about these engines being a Level 2...................Maj...

 

 

[chrisag]

Maj., Interesting that you say that # 2 plugs being a bit blacker than the rest. What I can't work out is that the leak down test was fine on this cylinder. # 1 cylinder is the one with the marginaly lower readings with good plug colour, but pulling the prop through you would think that the test figures would be lower. As I explained in a earlier post, # 2 has felt a bit flat for a long time, the engine runs well, and I only started looking further because of the extra oil burn. This is probably because of the oil being used.( shell aero sport)

 

Thanks for the input, It's all interesting stuff.... J.C.

 

 

[Guest Maj Millard]

I do GA leakdown tests weekly and have done for years. You can read them pretty well and if you get a low reading, you'll retest, but generally it'll be low for sure. Mostly carbon/lead buildup on the valve seats, or simply worn rings. All compression testing must be done right after the engine has been run, and therefore HOT !. If you do them cold you are wasting your time. There are half a dozen different metals on an aircraft cylinder, and they all cool at a different rate. I even leave the hot oil in the engine until I have done compressions, to slow the cooldown.

 

I started doing compression checks on 912s years ago on bush Storches used for mustering. These are generally engines that get used daily and work hard.

 

I got all sorts of odd readings for no particular reason. I can't really say I've had any real bad readings on a 912, unless it's really old and abused. Basically I don't do them any more unless specifically requested, which demonstrates the total faith I have in Rotax Nikasil cylinders and piston rings. Even if I do get a low reading, a quick valve lap generally puts it right again, so probabily just carbon build-up on valve seats again. These engines aren't bullet-proof, but I can tell you they are pretty damn close !...............................Maj...

 

 

[facthunter]

Operating Observations.

 

There is a view that operating with 100LL in the 912 constantly can cause the valve sealing to deteriorate due to lead deposits accumulating on the seats. This is considered to be the result of the heads, (being liquid-cooled) running a little bit cooler in the internal surfaces than optimal. A period of running on Mogas can restore the sealing but you can not guarantee that some seat detioration will not occurr at the time.

 

Nikasil cylinders are about the optimal as they are as hard as, expand with the alloy so can run closer clearances than with steel (or iron), and don't CORRODE.

 

New engines after a few hours running have such good sealing of the rings that "burping" the engine takes quite a while as only the leakage past the rings forces the oil back to the tank (Dry sump scavenging of engine for oil level check.). High time motors do it after a few compressions. You get better effect by turning the engine over slowly. Remenber it is geared ,so the engine turns over a lot more than the prop .

 

While this is not a precise check, if you are aware of what feels "right", then it is still valid as an alert to do a more precise assessment if you are not happy. When you are priming and propswinging Austers and Tigers you get an opportunity to check for "soft" compressions that you never get with electric starters. Nev.

 

 

[Kev]

JC,

 

The differential pressure test should be carried out iaw the Rotax Manual. The engine should be at operating temperature (oil temp 50-70 degC) and the supply pressure should be between 80 and 87psi. The maximum allowable drop is 25%. Leakage is expected past the piston rings (detectable by oil tank burbling). Any leakage past the inlet valve will see air coming out of the relevent carburetor and leakage past the exhaust valve will see air exiting from the exhaust. The correct equipment must be used. Smaller engines (including Rotax) require a 0.04in restrictor in the test equipment. Larger engines have a larger restrictor - take note if you ever choose to purchase one.

 

Values or comparative results from other types or makes of engine are not relevent.

 

Your results are within limits. :thumb_up:

 

Safe flying

 

Kev