How do you test compression??

socalplanedoc

socalplanedoc

Senior Member
OK, you guys, come clean. Exactly what method should I be using to test the compression on my A? (no jokes) :)

I've asked a couple of time in threads and sent a couple of PMs but no one is divulging the secret code.

Been trying to get a handle on my engine.. wont idle, runs rich. yada yada. so today I got new plugs and headed out to check the compression...
So the thread is way bigger than my usual tester so I whipped out my handy-dandy universal tapered-rubber-nozzle universal test-anything compression tester..
Compressiontester.jpg



I jammed it home into the #1 hole and yanked on the started rod.
next thing you know.. Kah-BAMM! you'd thought an M80 went off..
I went over to the yard and retrieved it.. put the face bezel and plastic glass back on.. then this time I got the engine spinning and proceeded to push it in the hole. I cant hold it in! I saw the needle swing past 75 twice and gave up on that one.. the other 3 were the same way.

Now, I've heard people saying things like 50 and 60 were typcial numbers so, what's the deal?
At least they're all the same...
Come'on now, tell me the secret, let me in the club :)
 
I guess you will need to get an adapter from one of the parts suppliers or hold the one in the photo very tight. Maybe there's more to the story if you are getting 75LBS compression then you are telling. You do have all the plugs out when you do your test? There is no secret.
 
I would have thought that 75 lbs is excellent for a stock head

No secrets - that's the way I have done it, but I hold on tight!

Mines a bit higher, but with a repop Winfield head. Others will tell you that different compression testers will give different readings, but this one reckons my Dad's well worn B engine in his Cabriolet is 50 across the board, and 60 for the spritely A engine in the Fordor.

I'm just happy if the readings are the same for all cylinders

Does anyone remember seeing a chart showing compression ratios and tester readings? I'm sure I saw it on this site but can't find it now.
 

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Take atmospheric pressure at sea level and multiply X the compression ratio. In other words. 14.7 X 4.2 = 61.7 Lbs is the theoretical pressure in the cyl. It may vary a little. It is more important to have all Cyls within 10 lbs of each other.
I think you're hedging around the "Leak down" test that is performed on aircraft during an annual. While very common in aviation, I've never seen it done on a car. This is a much more comprehensive test that tells you more about the state of being with the rings etc than just holding the gauge in a spark plug hole.
Terry
 
ya actually in aviation we use a differential pressure test but anyway...

I ate some Wheaties and went back out there to use the rubber nozzle tester.. all plugs out, ran it maybe 3-5 compression strokes.. all cyls are 100.

I got this car from the estate of a guy who's kids told me he was always trying to make this one go faster (he had three). it has a Chandler and Groves 2bbl downdraft carb with a pump (and now a regulator). I wonder if it has a either a high-compression head or if he flycut it or something? It had a couple of plugs in the trunk that he had welded some hoops on - the classic sign he was doing head work at some point...
I may find out.. when I was doing the test I thought I detected some fluid mist come out of #3 and it didnt smell like gas.. no sign of water in the oil or steam coming out of the exhaust though..
by the way, it's 12V so the engine was turning with great gusto during the test.
It ran OK when I first got it although it wouldnt idle. I put a different carb on it but it's probably not setup right, stumbles and backfires off idle.
I was hoping to get it running good enough to make the 4 mile trip to work over the holidays since my boys will be flying home and they're sure to takeoff with my modern - might have to break out the Schwinn.
 
The 5.5 Snyder head compression runs about 95-110 pounds. It probably has a high compression head of some kind.
 
My compression testor came with an air hose adaptor - female end. I took an old 7/8" base spark plug and removed all the inner core parts. Then welded in the male end of an air hose connector. I screw the adaptor int o the spark plug hole then snap the testor to the "plug" --- No more blowing the gauge out into the yard.
 

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I have a tester that looked like that. I cut the bottom part off. Take an old spark plug,break the top and guts out of the threaded base. Get some 5 minute epoxy and bond the two together. Mine works well,also on my 5.9 head,your readings will be more accurate. Cheers
 
well, I got this mega-nickle lathe and a wife that makes a comment every now and then about how much room it takes up so, if it's all the same to you guys, I think I'll just machine up something...
She's sweet enough to make gaskets for me with her crafty-cutter machine so I better keep her...
 
I always hold the throttle wide open so the cylinders can breath in air,
that should give a more accurate test.









.
 
Hi, this is Martin in Ohio.
See post #7 regarding used an old spark plug.
Like Ken, I took apart an antique A two piece plug, took out the porcelain part and screwed back in the upper holder. This narrows down the available hole and thread size enough to fit a standard screw in adapter
which is then connected to my compression dial by a short high pressure hose.

I stand on the left side of the engine (igniton OFF), all plugs out,
compression dial / spark plug set up screwed into one cylinder and push the starter rod by hand (wearing a glove). That lets me see the dial at work and stop turning the motor when I get a true reading. Works for me.

Let me know any questions or comments. I'll leave the compression spec details to others more knowledgeable.

Regards,

Martin Mindling
 
If you have a high compression head, you may also have a "hot" cam which might account for the poor idle. Mine will not idle as low as it did before. Just a thought. Ron W
 
Yep,, when we speak of compression testing,, its a different animal from an aircraft engine..
I hear all these responses of 100# reading.. Wow.. I'm beginning to think my poor old roadster with 45# readings shouldn't be able to pull itself..
 
I have a old style mechanical gauge I picked up a while back and it came with some adapters. MY engine was about 60


Comprission_Gauge1.jpg
 
As the others have stated on how to do it, plus I've seen posted that the carb needs to be held fully open.

FWIW
Paul in CT
 
Patrick L. said:
Yep,, when we speak of compression testing,, its a different animal from an aircraft engine..
I hear all these responses of 100# reading.. Wow.. I'm beginning to think my poor old roadster with 45# readings shouldn't be able to pull itself..
Click to expand...

I agree that the tribally accepted method is different, but not that the engines themselved are all that different - at least as far as compression testing goes. I emphasize that in hopes to avoid a lot of sideways threads tracks about the differences in air vs water-cooked, etc...
Both are still reciprocating ringed pistons inside a cylinder with intake and exhaust valves.
Aviation has a well-defined standard method for testing the health of a cylinder/piston assy - auto, not so much. Differential pressure testing identifies the sealing quality of the piston/cylinder fit and valve sealing so a determination can be made if it's within specification. If it's within spec, it will create the needed compression,. Autos are measuring the ability to create compression directly.
So why doesnt aviation do that? Would you want to stand that close to a club flinging around with lethal forces? me either.
Why dont cars use differential method? you need to position the engine precisely - impractial when you cant get your hands on the crankshaft.

Differential testing does provide more opportunities to identify issues sooner. Recently I was checking the engine on a Cessna Skylane which has a Continental opposed 6 cylinder engine with 471 cubic inches (BIG cylinders!) Annual Inspection - no complaints about the engine were made at all.
While under pressure I moved the piston on the #5 cylinder along it's compression stroke up and down slowly and noticed that the as the piston went up, the reading took a dip about 1/3 of the way up.. that's not supposed to happen. the pressure at the top was right were it was supposed to be. The cylinder head and exhaust gas temps were right on the money during the flight test earlier that day... I pulled the cylinder.

KurtPiston.jpg


the piston between the #2 compression ring and the oil control ring was missing for about 1/3 of the circumference and had signs of parts beating the piston.. the ring was shattered.

Never would have found that in a car until it was way too late.. :)
 
I agree that the tribally accepted method is different, but not that the engines themselves are all that different - at least as far as compression testing goes. I emphasize that in hopes to avoid a lot of sideways threads tracks about the differences in air vs water-cooked, etc...
Both are still reciprocating ringed pistons inside a cylinder with intake and exhaust valves.
Aviation has a well-defined standard method for testing the health of a cylinder/piston assy - auto, not so much. Differential pressure testing identifies the sealing quality of the piston/cylinder fit and valve sealing so a determination can be made if it's within specification. If it's within spec, it will create the needed compression,. Autos are measuring the ability to create compression directly.
So why doesnt aviation do that? Would you want to stand that close to a club flinging around with lethal forces? me either.
Why dont cars use differential method? you need to position the engine precisely - impractial when you cant get your hands on the crankshaft.

Differential testing does provide more opportunities to identify issues sooner. Recently I was checking the engine on a Cessna Skylane which has a Continental opposed 6 cylinder engine with 471 cubic inches (BIG cylinders!) Annual Inspection - no complaints about the engine were made at all.
While under pressure I moved the piston on the #5 cylinder along it's compression stroke up and down slowly and noticed that the as the piston went up, the reading took a dip about 1/3 of the way up.. that's not supposed to happen. the pressure at the top was right were it was supposed to be. The cylinder head and exhaust gas temps were right on the money during the flight test earlier that day... I pulled the cylinder.
end quote

Yep, I know.. Been working on both for more than 50 years.. Leakage tests have been far more common on automotive stuff over the past 50 years,, but,, an old compression gauge is fine for an old Model A.. Very seldom are leak or compression test performed on autos today,, anything happens and you pull off on the shoulder, stop and call for a hook.. When your motoring along with nothing but air under you,, having an engine quit,, pulling over on the shoulder and stopping becomes a bit more problematic.. I've had 4 aero engine failures, 3 in-flight fires, and at least 2 dozen gear extension failures[ I always was able to get it down though].. I digress, as usual..
And,, what model/series 0-470 did that piston come out of ??
 
Patrick L. said:
And,, what model/series 0-470 did that piston come out of ??
Click to expand...

Cont 0-470-R C-182P that cylinder had 1700 hours on it, the last original one on the engine.
temps were good, mag check fine, compression low 70s and bore scope looked textbook perfect..
still, they shouldnt have that dip.. :)

those 470s are amazing..

and tell me about having one get quiet...

bellancaFL.jpg


14FEB2011, 5 1/5 minutes after it got quiet at 10,500 50 miles east of El Paso.
 
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