[richdolby]

OK, so to quote Dave Allen - 'I'm being a real thicky here, but........ '


I am familiar with the automotive compression tester, where you connect a gauge to the spark-plug point and crank the engine to produce a reading on the gauge........

But how does the engine compression differential test work?? what is the process, and from the results what can be determined??

http://www.aircraft-tool.com/shop/detai ... UCT_ID=2EM


Many thanks in advance!!


Rich D.

[a British pilot abroad]

Hi Rich,

I will be doing a compression test on my Pitts in the near future if you want to come up to Staverton and give me a hand and see what I do.
Else we could talk over a beer that would be a lot easier than typing many words.

Cheers, Al.

[Flying_john]

So the theory is that when you pressurise a cylinder with air with a constant pressure on the input side (80psi) via a regulator and then through a small orifice (40thou) connected to the cylinder monitored by a second gauge, any leakage in the cylinder will result in a lower pressure on the cylinder side pressure gauge.

Aha - you may ask why the gauge doesnt just show the regulated input pressure of 80psi. Well the reason for the air being fed via a 40 thou orifice is so that as the air leaks out of the cylinder there is a resultant pressure drop that is only being replenished via a very small air feed (40 thou) and the air flow on the cylinder side will cause a pressure drop according to Mr Bernouli's theorum.

[stickandrudderman]

In the automotive world we use something called a "cylinder leakage tester". I think it's name gives a clue as to what it does and it's the same principle in the tester used in aviation.
It's a much better way than simple compression testing of determining mechanical condition because the escaping air can be heard, thereby indicating a location of any leak and in some cases such as an incorrectly adjusted valve the fault can be demonstrably eliminated.
The best indicator of engine condition though is manifold vacuum.

[damhead]

EAA has a good range of instructional videos.

http://www.eaavideo.org/video.aspx?v=2363274331001

[Charles Hunt]

The purists will shoot me down, but when a friend had a badly performing engine we did a quick test simply pushing on an automotive type tester. (Watch out for the rotating prop!). Compressions looked sensible and equal, so we moved on, and with a bit of work on the magnetos everything was fine.

[Trent772]

The differential test shows a snapshot on the day, it does not give reason to do more and sometimes major work.

There may be other issues in play and a single low reading should be treated as an indication, not a diagnosis of something dire.

If you want a better look - get a £50 borescope from fleabay and have a look inside the cylinders, particularly at the valves. You should be seeing no scoring on the walls, no pooling of oil and no hotspots or burn marks on an edge of a valve.

Superior do an excellent 'Lycoming 101" publication available here:

http://www.superiorairparts.com/about-u ... ement-101/

May take a few days to reply and if they don't, email me off forum and I will forward a copy.

[Dave W]

Thanks for the pointer, @Trent772.

I requested a copy this afternoon and the link dropped into my Inbox an hour ago. I'm a little way through it and can already see how useful it can be.