912a compression readings

[Keith W]

Hi can anybody tell me what sort of compression readings you could excepect from an 80hp 912 and also how much variation between the cylinders, thanks Keith.

 

 

[ruffasguts]

Hi can anybody tell me what sort of compression readings you could excepect from an 80hp 912 and also how much variation between the cylinders, thanks Keith.

dependant on hours, minimum 60/80 , most i have done are better than 75/80 with hours ranging from 200 to 1300 hrs

 

Mick W

 

 

[old man emu]

Kieth,

 

It is not normal to talk about the compression readings of an engine in he same way as you do for a car.

 

The normal way of checking whether or not a cylinder is holding pressure as designed is to do a "leak down" test. To do this, you introduce compressed air to the cylinder and measure how much of the pressure is lost before the pressure in the cylinder stabilises.

 

The usual introduced pressure is 80 psi. An engine that is in good nick will stabilise somewhere between 72 and 80 psi. Lesser stabilised pressure readings indicate that air is getting past rings or valves and this is a sign of wear in those components. What one likes to see are the stabilised pressures being nearly the same for all cylinders. You can have the situation where one cylinder is way down compared to the others, and that cylinder needs attention.

 

You can't do a compression test on an aero engine like you can a car. There's a big heavy whirling thing on one end that can get in the road. Besides, the pressures in an operating engine will be a lot higher than 80 psi. the level of 80psi is used for safety. 80 psi in a cylinder can still turn a prop forcefully enough to cause injury.

 

Old Man Emu

 

 

[planesmaker]

Just to add to OME's post, leak down should be done with engine warmed up. Cold testing could have quite a bit of variation. Tom

 

 

[geoffreywh]

Quote "You can't do a compression test on an aero engine like you can a car. There's a big heavy whirling thing on one end that can get in the road." What nonsense is this? The dynamic compression test is a significant part of engine condition diagnosis. Don't leave it out. With at least one plug per cyl removed you screw the meter adaptor into the plug hole get in the aeroplane and hit the starter for ? about 8 revs. The reading should be around 150 pounds sq. in/ ( Jabiru.).. Got plus / minus 10% deviation ? Then do a leakdown test .... A compression test will tell you if there's a leak , the leakdown test will tell you where.... Warm engine of course..... Note the 150 number is arbitrary and depends on the compression ratio.. so long as they are all nearly the same. Look at the numbers together with leakdown result.... Get used to your own engines' numbers and look for changes.

 

 

[facthunter]

IF you were going to record something in an engine log book I think the leakdown would be the one I would use. By listening in the exhaust port and inlet port and checking the breather, you can work out where the leak(s) are. IF you think you are on the limit ( or anything like it). Investigate it rather than just record a figure that is in limits , if you happen to get one. An engine that has just been run is a more indicative situation of how the motor is, though there is no need to rush to do the motor when really hot. Once you are familiar with your engine ( how the compressions feel when normal) You can get a good indication from turning the prop over on a pre-flight. Use some caution even though the engines are not supposed to fire at low revs.

 

Exhaust valve seating can get worse fairly quickly with more use. If you have that indication remove the tappet cover and check clearance on the earlier motors( solid lifters) and check exhaust valve guide clearance, (for wear)... Nev

 

 

[old man emu]

What nonsense is this?

Dynamic compression and leak down are two sides of the same coin. Both will indicate that the combustion chamber is not sealed properly.

 

Quote: "The reading should be around 150 pounds sq. in/ ( Jabiru.).. Got plus / minus 10% deviation" End quote.

 

Can you indicate where this value is to be found in the Jabiru's, or any other engine's, documentation? Not that it doesn't seem to be a reasonable value, but it will vary considerably depending on whether the engine is high or low compression.

 

How do you calibrate your pressure gauge? It is possible to calibrate both gauges on a leak down tool (although it is actually more economical to throw the old one away and buy a new one as the cost of a new unit is about a quarter or one third the cost of calibration.

 

Finally, there is the safety aspect of turning over an engine in the workshop. Some twit might walk into the propeller arc, and Murphy's Laws says that one time, somewhere, an engine will fire while the test is being done this way.

 

Yes. The test delivers reliable results is the engine has been allowed to cool down a tab from operating temperature. This saves burning the LAME/L2's fingertips. However, studies have shown that apprentices are not affected by handling spark plugs removed from an engine immediately after it has been shut down.

 

Old Man Emu

 

 

[Keith W]

Thanks for all the feedback, but has anyone done a compression test on a 80 hp rotax and what readings should you get thanks Keith.

 

 

[facthunter]

Some of my comments relate to jabiru more than the 912a. These (912) engines have very few valve problems. The only ones I know of relate to flying on AVgas when sometimes a build-up of lead on the seats will become a problem when some of it detaches. This causes a loss of compression that sometimes recovers when the engine is run on mogas for a while. You can't totally rely on this happening but I am aware of several instances when it did and have heard that claimed many times. If you are going down this road monitor the engine till it improves. You should pick this up when you are turning the prop over to get the oil to the oil tank for level checking at preflight.

 

IF the motor has been stored (after being used) It is worthwhile inspecting the exhaust ports for rust on the valve stems. This is about the only area these engines show corrosion internally, unless they have been under water. Nev

 

 

[geoffreywh]

When I did a compression test on a 100hp rotax a couple of weeks ago I got in the region of 155 per cyl. which is very good, considering it had 2000hrs on it.(average 75-80) on the leakdown.) .....I can't say exactly what compression reading you will have in your engine as some factors are unknowns. Valve timing , compression ratio, general wear influence readings. BUT what you can know is if you have a weak cyl. Get used to your own engine. A compression meter does not have to be calibrated. You are doing a comparison test. If your meter reads 150 pounds sq.in. on 3 and 130 on one, then you MAY have a problem. My advice would be to refit the plugs, run the engine briefly and check again. The valve may have had a bit of carbon trapped under it ( I have seen that in maybe 10% of tests I have done over 50 years) The carbon is liberated when plugs are removed. If at the retest, the number is still low , a couple of squirts of oil into the cylinder will seal the rings, retest , if still low you have valve problems in that cyl....... Generally compressions are 140-170 even my 0-200 reads 140 and that at 7-1 compression BUT with very "slow" valve timing. IF you test a "hot" motorcycle engine with heaps of valve overlap you may still get a "lower number" reading.. I would look for the 1 ( or more) "low" cyl. reading to identify a problem...of course If all the readings are low (80-120 ) then your engine is probably buggerd and probably uses heaps of oil... Get to know your own engine. Look for changes...Thats why I LOVE solid lifter motors ....Sorry to go on a bit.......Geoff

 

 

[Guest Maj Millard]

In the US the FAA is happy with 50/80 psi on leak down testing, as a minimum for serviceability . CASA requires a minimum of 60/80 for serviceability. Shouldn't be much variation between cylinders to maintain engine smoothness however 5 psi difference is routinely the norm..Refer to the Rotax Maintenance manual for their recommendations on 912s, although you won't get much drop down over the TBO life of those engines in my experience... Maj...

 

 

[geoffreywh]

The edit button is U/S... the last bit should read............If all the readings are low (0-100 ) then your engine is probably buggerd ... AND I refuse to be intimidated or concerned by expressed fears of an engine starting with the plugs removed or someone walking into a prop arc while I'm doing a compression test. Murphy's Law belongs with the fairies at the bottom of my garden...On the other hand a performing a leakdown test on your own can be quite interesting at times. It should really only be done with two people!

 

 

[facthunter]

The edit button has a time limit on it. Oil in the cylinder to seal rings is better applied with a spray with the piston at BDC at the time. nev

 

 

[Guest Maj Millard]

There is no need to spray oil into the cylinders........not normal procedure with GA testing, as the piston moves down the cylinder it leaves a film of oil on the cylinder wall so the seal is already there...must be done warm to hot though, (operating temperature). if cold your wasting your time. .Spraying oil into the cylinders may have been necessary with worn out Holdens in the past !................Maj...

 

 

[old man emu]

... AND I refuse to be intimidated or concerned by expressed fears of an engine starting with the plugs removed or someone walking into a prop arc while I'm doing a compression test.

Please advise the location of your workshop. I want to be able to give it a wide berth as you clearly have no concept or care for workplace safety.

 

Old Man Emu

 

 

[AVOCET]

Doing a leak down and introducing 80psi into a jab cyl , can cause an uncontrolled prop spin in any direction and can inflict enough peripheral impact force to the head to kill , or seriously injure !!

 

It's definatly a two person job , with both knowing what their doing ,and strict safety protocol and communication .

 

It's as dangerous as handling a loaded gun

 

Mike

 

 

[geoffreywh]

abso bloody lutely.........

 

 

[old man emu]

Never a truer word said.

 

OME

 

 

[CrayonBox]

Thanks for all the feedback, but has anyone done a compression test on a 80 hp rotax and what readings should you get thanks Keith.

I did mine early this year, and after warming up the engine, I had 110-115psi. The engine had 1200 hrs. I cranked until the pressure stabilised. (approximately 3-5 seconds).

Hope that helps.

 

 

[facthunter]

Reading directly depends on the rotational speed of the engine. This can vary depending on the compression of the preceding cylinder, and the battery state of charge and the accuracy of the gauge. It is OK as a comparison between cylinders. I would recommend removing all plugs and where necessary short out all HT leads. It is not good to open circuit the secondary at cranking speeds. Nev

 

 

[geoffreywh]

Dear F/H .........Wouldn't leaving the mags switched off do? Or should the plugs still be earthed??? you also mention the compression of the preceeding cylinder... IMHO you ABSOLUTELY MUST have at least one plug per cylinder and plug caps removed.. The previous poster ( mr. Crayon Box) mentions that he saw approx 110 psi on each cylinder. I would have expected to see more. If the number should jump quite a lot ( up to, say, 150) after a couple of squirts of oil in the cylinder (at any crank position ) I would recommend a leak down test. You may have worn rings.....The engine may well still be operational for some considerable time ...........but showing wear

 

 

[facthunter]

Geoff I don't trust the earthing of the IGN switches. There are ways of holding the points open on conventional magnetos but I didn't want to get into details because they are all different. IF all plug leads are earthed there is no chance of anything firing in there and the magneto coils are protected from an internal leakage/discharge. Atomised oil or WD could fire in the cylinder, too. Nev