Compression question

[idratherbeflying]

Hey all, a little unfamiliar with the compression ratings for normally aspirated piston engines. I have seen lots of numbers in the 70's written on the cylinder heads, but I don't know what the maximum is or how it is calculated. Any help would be appreciated.

Thanks.

[Edo]

You take one plug out and screw and adapter into the hole. Set the prop near Top Dead Center the adapter is hooked to a compression tester and an air line. The tester has 2 dials one to measure air supply applied and one to measure pressure in the cylinder. Wiggle the prop back and forth the highest number is at TDC. 80 pounds is usually applied and any number below that reflects how much blow by the rings have. Same test for super or turbocharged pistons. The numbers you see are maybe 78/80 or 2 psi loss. Run away if the numbers are in the 50's

[tired of the ground]

Maximum compression is 80. It's measured using a differential pressure tester. It tells you how much air pressure you're putting in and then tells you how much your cylinder is holding.

Most people start to look a little closer when compressions start hitting below 60ish. Actively looking for metal in the filter and other things to see where that air is going.

If you charted the compressions over the life of an engine it would look like a parabola. Starting off lower, hitting close to 80 then declining till the engine is done.

Once you decide the engine is done it's either new cylinders or engine overhaul time.


Edit: You beat me to it!

[idratherbeflying]

Thanks for all the help!

[Cat Driver]

The leak down test is a quick way to determine if you are losing compression and where.

Listen for the air leak.

If it is coming from the crankcase breather it is worn or broken rings.

If it is coming from the exhaust pipes it is an exhaust valve.

If it is coming from the carb intake it is a intake valve.

[KISS_MY_TCAS]

Run away if the numbers are in the 50's

Not true, depends on a lot of circumstances. Cold engine leakdown pressure is a lot different than a hot engine, differential pressure checks MUST be done on a hot engine, it makes a difference of about 20PSI on average from experience. 50/80 is no reason to run away but is indicative of a problem on a hot engine, carbon buildup on the valves is fairly common and can sometimes be rectified by staking the valves with pressure applied to the cylinder. I have always pulled the jug at 45/80 if I cannot get the pressure up through staking, but I do know Continental published a service letter a few years back that stated readings as low as even 40/80 are not necessarily reason for rejection. In addition, if the tester being used is not calibrated it is useless. We calibrate ours before every engine check which requires a calibrated oriface that is attatched to the tester to confirm it's accuracy. Without it the tester is pretty much useless.

[Ballsssssss]

Cat's is the simplest, best description. If I recall correctly, Continental is the only company that uses the fixed orifice calibration technique; it allows your compressions to go low into the 40's or 50's provided that it is a static leak (piston not moving) and is not leaking through a valve.

[Hedley]

not leaking through a valve

And that's the most important thing. You will probably
never see an intake valve leak in your lifetime. They
are cooled by the incoming air/fuel mixture.

Exhaust valve leaks are far more common (they
operate at MUCH higher temps). Problems with
the exhaust valve are MUCH more serious than
problems with the rings.

On the other hand, you can have badly worn or
even broken rings and still continue to fly (I know
guys that do this). Your oil burn will certainly increase
but as long as you keep pouring oil in the crankcase
it will continue to run.

As . points out above, it's childishly easy
during the leakdown test to figure out where
the leak is.

[niss]

I am planning on building a leak down tester and the question I had was this: Is there a requirement for an orifice between both gauges or can I just put in a valve?

If I have a valve, I open the reg to 80psi with the valve open then shut it and see what the downstream gauge is reading?

Methinx it should work fine. Thoughts?

[Hedley]

Just get one of these:

http://www.aircraftspruce.com/menus/to/ ... nders.html

[niss]

Just get one of these:

http://www.aircraftspruce.com/menus/to/ ... nders.html

That seems like more money and less fun.

[iflyforpie]

A valve won't work. You need continuous airflow to do a leak-down test

The first gauge should read regulated PSI and the second should read line pressure, just like a high pressure cylinder regulator. There should be no orifice between the two since you want the airflow to reflect the leakage of the cylinder. An orifice would mask a huge hole in the piston or a stuck valve.

It is counterintuitive, but more airflow (because of blow-by/leaky valves) means a lower pressure reading on the second gauge and less airflow will mean the second gauge will be higher. I always test the second gauge by dialing the reg up to 80 and reading the second gauge with the line disconnected; they should be equal.

[niss]

A valve won't work. You need continuous airflow to do a leak-down test

The first gauge should read regulated PSI and the second should read line pressure, just like a high pressure cylinder regulator. There should be no orifice between the two since you want the airflow to reflect the leakage of the cylinder. An orifice would mask a huge hole in the piston or a stuck valve.

It is counterintuitive, but more airflow (because of blow-by/leaky valves) means a lower pressure reading on the second gauge and less airflow will mean the second gauge will be higher. I always test the second gauge by dialing the reg up to 80 and reading the second gauge with the line disconnected; they should be equal.

http://www.easterntech.com/e2a.html

The tester on the Aircraft Spruce link Hedley provided show a .040" Orifice. I seem to recall reading somewhere that FAA cert. testers require the orifice so you have a constant to calculate percentage.

Now I am confused.

[iflyforpie]

The 0.040" calibrated orifice is installed downstream of the second gauge and is used as a benchmark for doing leak downs on Continental engines. My tester doesn't have one, so I have an external one that threads into the spark plug adapter. This usually tests just above 40/80 and this is what you are allowed to go down to provided there isn't evidence of exhaust valve burning.

In my experience, Lycs are pretty much always tight so use the standard 60/80 limitations and keep an eye on the exhaust valves using a borescope if there is any blow by.

[niss]

The 0.040" calibrated orifice is installed downstream of the second gauge and is used as a benchmark for doing leak downs on Continental engines. My tester doesn't have one, so I have an external one that threads into the spark plug adapter. This usually tests just above 40/80 and this is what you are allowed to go down to provided there isn't evidence of exhaust valve burning.

In my experience, Lycs are pretty much always tight so use the standard 60/80 limitations and keep an eye on the exhaust valves using a borescope if there is any blow by.

So I don't need to dick around with orifices? I can just put a reg then a gauge and I'm ready to rock?

[iflyforpie]

I think so, but I'll add a disclaimer that you should check it against a proven tester first...

...and of course under the supervision of your friendly neighbourhood AME.

[niss]

I think so, but I'll add a disclaimer that you should check it against a proven tester first.

There's no time for checking! I have things to pressure check! Definitely, UBC has an annual coming up so I will check my beast against his.

Merci beacoupe a tout le monde.

[Strega]

There is an orifice between the two gauges on your tester, Details can be found in AC.43.13, it actually shows 2 sizes, one for small engines, and another for "big" (think round) engines.

you need a regulator, one pressure guage, the orifice, and then another pressure gauge, then the hose to your cylinder.

The idea is to measure the pressure drop across the orifice. to determine how much air is leaking out of the cylinder through the valves, and or rings.

iffp,, dont know what orifice you are talking about that screws into the cylinder...

[iflyforpie]

Strega, I stand corrected. Good info in AC43.13!

This is the orifice I was talking about...
http://www.skygeek.com/ats-646953a.html

[niss]

Okay dokay,

So I will be picking up a reg c/w gauge, male - male tube, T, Gauge, quick disconnect adapter for hoze.

Fill the male-male tube with some jbweld and drill it out with a #60 (0.040) bit.

Screw er all together and I got myself a leak down tester?

[Strega]

iffp,

the orifice you linked is a calibration orifice,, to ensure your orifice and gauges are used properly it is never intalled into the cylinder

http://www.tcmlink.com/pdf2/SB03-3.pdf

[iflyforpie]

the orifice you linked is a calibration orifice,, to ensure your orifice and gauges are used properly it is never intalled into the cylinder

The 0.040" calibrated orifice is installed downstream of the second gauge and is used as a benchmark for doing leak downs on Continental engines. My tester doesn't have one, so I have an external one that threads into the spark plug adapter.

[flying4dollars]

The leak down test is a quick way to determine if you are losing compression and where.

Listen for the air leak.

If it is coming from the crankcase breather it is worn or broken rings.

If it is coming from the exhaust pipes it is an exhaust valve.

If it is coming from the carb intake it is a intake valve.

Or the soap test. Do mechanics still use that method??

[niss]

No need to build one.

I just picked up one of these used from my old Mechanic for $20.

[Dr. Mcillicuddy]

Im sure that it has a calibration cert with it.