Burping the 912

[Ehfox]

Hello,All...how many times are we suppose to spin the prop to "burp" the engine before checking the oil or starting it?. When I watched Av8r3400 do it @ Oshkosh,you could hear his oil tank actually make the sound....but I must have turned my prop 15 (1/2 turns) and it still would'nt make a noise...Thanks, Dan

[Dorsal]

Sometimes more, sometimes less but it will always make the sound. Can definitely take more that 15 1/2 turns at times.

[Peteohms]

Make sure to remove the oil cap. You can turn it over with the starter to speed things up then maybe it will take fewer turns.

[Av8r3400]

I've had to turn mine 20 full turns or more to get a burp if the motor hasn't been run in a while...

[Iceflyer]

How necessary is this procedure really?

I have never done it. I just check the oil level after flight or after taxi to the fuel pumps. Have never had to add oil between oil changes either.

[sdemeyer]

I burp my 912 only before the first flight of the day. Before I changed to a "normal" oil cooler, I used to get drastic changes oil level readings resulting in me overfilling the oil tank. I don't know what difference the cooler made but now the oil level never changes on a before or after burp. Even though I don't think burping is necessary, I still do it because that's the habbit I got into.

To answer your question, it takes me an minimum of 18 turns, sometimes more to get the burp.

Scott

[Dave S]

Good afternoon,

Regarding the necessity and reason for burping the 912 - It is recommended by the manufacturer - the manufacturer usually has a good reason for its recommendations- The Manufacturers stated reason in this case is to assure an accurate check of how much oil is in the system. If the oil tanks were always mounted low enough so all of the oil would drain to the oil tank following shutdown; and, there were no loops in the oil line to interfere with the flow - burping might not be necessary; HOWEVER, since installations vary from aircraft design to aircraft design; and, all installations are not identical even for one aircraft model - particularly homebuilts, the engine manufacturer is likely recommending this procedure to assure accurate oil level measurements given all the installation variations they cannot account for.

Other rumors/legends from the field as to why burping is a good idea include assuring that oil has not backed up past the rings so it could cause a hydraulic lock - not sure that happens - I am pretty sure if any significant amount of oil (more than 1/4 ounce or so) got past the rings - you'd know it on startup anyway because the engine would probably belch smoke and fire like a radial engine does. I have yet to see a 912 do this on startup.

It is also true that # of prop flips needed to burp a 912 will vary from installation to installation and from day to day because of a number of factors including: 1) differences in the amount of drainback occurring because of different relative heights of the tank and the engine (the oil wants to seek its own level between the crankcase and tank); 2) oil viscosity - thick oil burps slower than thinner oil because it flows slower; hot oil drains back quicker; 3) temperature of just about everything in the system - hot oil flows faster than cold oil - air leakage past the rings varies with temp and when the engine was last run affecting compression to some extent. Probably a ton of minor effects. In any case - it is perfectly normal to observe differences in the number of prop flips necessary to do this - yup - oil cap off, turn it till it gurgles and forget about counting blades. Well...maybe it it took 800 something is up.....

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

[Av8r3400]

Excellent explanation and information, Dave. Very good stuff...

[Rudix]

If you are turning your 912 15-20 turns to burp it you are doing it wrong. The secret is to turn is slowly up to each compression stroke, then linger there before going to the next. I seldom have to turn it more than 4-5 turns (on several different planes).

The reason this works better than the fast turn is that the whole idea is to get the air to leak past the rings into the crank case blowing the oil back into the oil tank. If you turn it fast very little air escapes past the rings on each stroke.

If you turn it with the starter you stand a chance of bending something if you had oil in the combustion chamber, I have seen that happen......

Fly safe,
Rudi

[DesertFox4]

Thanks for the good tips Rudi. Makes good sense. I'll try doing slower next time. I find about 40 "blades" on my 3 blade Ivo gets a burp. Also agree that you should only turn by hand until you are sure oil is not in the bottom cylinders. Hitting the starter before clearing the oil will be very expensive.

Dave, great info. Thanks.

I also check my oil after a flight when engine is hot. Never found either of my 912's to burn a drop between oil changes. Amazing engines.