Differential pressure test

[Mark 1]

I've just done the tests on my engine (permit to fly aircraft) and the results have come out similar to those for the last 3 years - 64,68,76,74 respectively.

Normally low 60s would be a bit of a worry.

But reading AC43-13 it states that the orifice for testing bores over 5" diameter should be 0.060", my tester has the smaller 0.040" orifice.

The engine is an O-320 with 5.125" diameter bores.

So presumably the larger orifice would give "better" compressions.

My questions are:
How much difference is changing the orifice likely to make?

Should I be concerned about the consistent low readings on the front pots?

The engine runs very well and performance is spot-on.

Thanks for any advice.

[Vortechs Jenerator]

Well, the same orifice that measured the 76 also measured the 64.

(I usually prefer a smaller orifice but we digress from Engineering!)

[ericferret]

I doubt that the orifice is making a difference.

All four cylinders are being measured together and you have a big split between the highest and lowest.

78 over 80 would be the highest I would expect to see. 76 would be normal.

I think one and probably two of you cylinders require some investigation.

[f8peter]

Yep, time to investigate.

Since you're getting these results for the past 3 years now,
I would pull those 2 cylinders and perform a fluorescent dye check, to see if there are any cracks around the valve openings and sparkplug hole to begin with.
If you've got cracks its time for new cylinders.

[SNS3Guppy]

First of all, compression tests are very misunderstood, and given far too much credence. These do far less to tell of an engine's health than they do to diagnose a problem in real time. The actual numbers you're seeing don't mean much. With respect to the numbers, the general rule of thumb is 75% of the input value; traditionally where 80 psi is used, this means you want to see at least 60 psi. However, what you really need to look for is the trend; just like oil analysis, you're looking for a number trend, not at the actual numbers themselves.

The differential compression numbers will vary with the test unit used. They will vary with the person doing the test. They will vary with the temperature of the engine. Often they'll vary if you do the test twice in a row. They'll vary significantly with the position of the propeller (and consequently the crankshaft, camshaft, piston, and valves in the cylinder).

What the test does first and foremost for you is allow you to see an obvious problem, and hear a leak through the inductionor exhaust. You'll hear all kinds of wild, incorrect ideas about why compression might be low (my favorite one is the notion that the piston ring gaps lined up will cause a low reading, which is utterly ridiculous). Ignore them.

Your compression has been consistant for the last three years. Be happy with that. It's not particularly high. So what? Are you seeing significant oil burn? Having vibration or power problems?

If you were to have cracks in the cylinder significant enough to cause compression loss, you wouldn't need a dye penetrant check to find them; you'd hear it. Trust me.

So far as large orfice vs. small; large bore orfices are meant for large engines, such as R-2600's, etc. Your 0-320 is a small engine. If you hold the prop steady at it's high compression point (and you can get an inaccurate, artificial reading by messing with the prop while you do the test, as you may have noted), and give it a moment, you won't be hindered by the orfice test. For small engines, Continental (not lycoming) requires a calibrated orfice.

I get the impression that you're not a mechanic, but for some reason you're doin the test on your own. I get this impression because you aren't very versed in what you're doing. I bring this up for two reasons; first, diagnosing on your own may not be your best bet, and second, if you're not careful that prop can really hurt you during the test.

Why are you referring to AC 43-13 instead of the manufacturer maintenance manual?

[good spark]

you could try a squirt of oil, same kind that is used in your engine, into the top plug hole and let it run around the bore, if the rings are worn you will see an improvement in the results- no improvement means valves/seats worn standby to spend money.





gs

[ericferret]

To have a trend you have to be flying hours. If you are only doing 20 a year you have not got a trend just a snapshot. Our 0-200 gives 72/76 on all four every year based on about 100 hours flying. Except last year (see below).

As has been said diff comp testing doesn't always tell the full story and really it is a diagnostic tool.

Last year we had our 150 done. It had no reported loss of power yet on diff comp one cylinder was 8 over 80. Exhaust valve guide totally shot.

I did an R22 and got 0 over 80 on all four!!!!!!!! After checking the equipment and getting someone else to do the check we had the same result. All four exhaust guides shot to buggery and no reported loss of power!!!!!!!!!!!!!!!!!

[SNS3Guppy]

You'd have to have a massive leak on the valve guide to get zero differential pressure...and at that you'd have a hard time loosing the pressure if the valve was closed...air doesn't leak past the valve guide when the valve is closed you see. Not if the valve is seated properly. When air does leak past the valve, it goes out the induction or the exhaust. If it were to be leaking past the valve guides, you'd have to have a sealed induction and exhaust to know it, and a very leaky valve cover gasket.

What this means is that the test was very improperly done. If you've got valve guides so badly shot that the compression is zero, even if the compression test could show that, you'd have fould plugs right away, considerable oil loss, and smoking cylinders.

The valve guide seals weren't leaking. Perhaps your valve guides were bad and not allowing the valves to seat properly, or to seat inconsistantly. Again, here the actual pressure doesn't tell you anything. Listening at the exhaust allows you to hear a hiss of air; you know to start looking at the exhaust valves. A hiss of air in the induction lets you know to start looking at the intake valves. A hiss of air from the engine breather or through the dipstick/oil filler tube indicates blow-by on the rings.

[Mark 1]

Thanks for the replies chaps.

It's a homebuilt (RV-4) on a permit to fly and my inspector is happy for me to do the checks myself. I'm not a mechanic, just an engineer, and have been helping out with 50 hours etc. for over 20 years and am very carefull with props and pressure in the pots.

As SNS3Guppy says, I wouldn't pull a pot based on one test. The last test was done cold for various reasons and the pick-up strainer and filter elements closely examined (both were clean). The engine runs smooth and achieves it's "book figures" for climb rate - 1400 ft/min at gross and 2000 ft/min solo.I would expect a .060 orifice, at over twice the area to have a much lower pressure drop and give results nearer to 80 as a result.

Just wondered if anyone had compared results with both orifices

[SNS3Guppy]

On small engines I have, and it hasn't seemed to make any significant difference.

[ericferret]

Didn't realise you were present super guppy when we did this work!!!!!!!!!!!
We did the test wrong did we!!!!!!

As I said due to the results I had another engineer repeat the check and we also used another diff comp tester as we didn't believe the results we were getting.
Given that we had a fleet of over 25 piston engined aircraft comp checks were a daily occurrence.

The cylinders were removed and you could move the valves around in the guides like a belly dancers ar*e.
New cylinders fitted and we were back to 76/80 all round.

When carrying out the test on the R22 0-320 the valves were closing very slowly under hand rotation and were failing to seat properly as the were hanging so far down in the guides.
Air was pouring out through the exhaust. We took the exhaust off and were able to watch the valves closing and what was happening. Under full speed there would be enough dynamic energy to slam the valves closed onto their seats.

Exactly the same problem on the 0-200. Air leaking past the valve seat due to the guide being shot. Again not showing under dynamic conditions.

[Ladytech]

The early O-320 is a great engine. If it is an "H" model not so good(lousy camshafts). Above 60 you are "legal". If you are concerned screw in the tester,have a freind hold the prop for each cylinder and listen for the leakage at the oil filler, muffler and carb to determine what is the leak. (Muffler= exhaust valve,
oil filler= rings, carb= intake valve. If it's rings try a little oil spray in the plug hole and rock the prop for a better reading. If it's valves, remove rocker cover and hit the valve rocker with a SOFT mallet with the air on, and see if the compression
comes up. AT ALL TIMES MAKE SURE THE PROP IS HELD STATIONARY!
Install a new rocker cover gasket. Personally,I wouldn't worry too much unless the engine is over TBO.