using a compression guage

General engine tech -- Drag Racing to Circle Track

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rebelrouser
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Re: using a compression guage

Post by rebelrouser » Wed Nov 01, 2017 11:55 am

our text book at school says 4 strokes is the industry standard, and they want the throttle open, during testing. You will get less compression with the throttle closed. When I teach compression testing, I show them with the throttle closed, throttle open, running compression test, and a wet compression test. All give different readings, all tell you different things about the engine. Also battery charge and starter condition affect cranking speed which again gives different results. If I do a compression test throttle open and closed and do not see a difference in results. I suspect a flat cam or excessive carbon build up. Running compression can show up worn valve guides, wet compression worn rings. ETC. I tell my students main issue is do the same on each cylinder, or you get in trouble real quick. Newer engines with computer controlled throttles and idle air, make the throttle closed and open test not reliable. Of course performance engines with different overlap camshafts give different readings than a stock engine. The testing I lean to now is using a scope or scan tool. You can measure the amperage draw of the starter and find weak cylinders by simply using an amps clamp on the starter wire. Lots faster than taking plugs out, lots of engines you now have to remove the intake manifold to get to the plugs. Ford for example has Cylinder balance and cylinder contribution tests built into the ECM and scan tool.

I much prefer a cylinder leak tester to diagnose engine mechanical issues. Less things to consider when making sense of the results, Plus I can leak an engine on the engine stand.

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bigblockmopar
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Re: using a compression guage

Post by bigblockmopar » Wed Nov 01, 2017 6:41 pm

Upto how much psi you think a push-on compression tester can still be 'somewhat' reliable?
Or this is probably more a question of how much muscle you have in your arms...

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Re: using a compression guage

Post by hoodeng » Wed Nov 01, 2017 7:13 pm

The screw in tester with the check valve in the chamber tip takes the variables out of the test.

Cheers.

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MadBill
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Re: using a compression guage

Post by MadBill » Wed Nov 01, 2017 9:39 pm

The ID of a 14 mm plug thread is ~ 12 mm or ~ 0.470", = ~ 0.176 ²in. Therefore 200 psi would create a force of 200 x 0.176 = 35 lb.
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Re: using a compression guage

Post by Truckedup » Thu Nov 02, 2017 6:18 am

hoodeng wrote:
Wed Nov 01, 2017 7:13 pm
The screw in tester with the check valve in the chamber tip takes the variables out of the test.

Cheers.
Are push in testers still used?
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Re: using a compression guage

Post by hoodeng » Thu Nov 02, 2017 11:47 pm

I haven't used a push in since the 80's but that does not mean it wont give you the information you need ,and if you have been doing your testing the same way for years and getting readings you can relate to the job in a positive way there is no need to change.
There were a number of reasons i went the screw in path, some of the race engines we tested had very high cranking pressures that would wheeze and fart around the rubber,it was difficult to hold the tester as the plugs were on the same side as the grinder[starter] , also and probably better for me it enabled me to do testing without an assistant.

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Re: using a compression guage

Post by CharlieB53 » Sat Nov 04, 2017 9:01 am

statsystems wrote:
Tue Oct 31, 2017 11:48 am
Stan Weiss wrote:
Tue Oct 31, 2017 11:33 am
Was he using a starter or by foot?

Spinning it more times than needed will not cause any problems, but not spinning it enough will not show the correct number.

Stan
Ha! I didn't think about him kicking it over. I'm old school but as old as I am now, if that has a kicker and no giggle switch to make it start I ain't kicking it.
My old Shovelhead is a kickstart. It does have the old Prestolite electric starter. Even clean, good connections, and a new battery it is marginal. I only use the button when checking compression. Kick each cyl over to prime, turn on ign, pops off and runs on that next kick.

Mercury Marine somewhere has published a minimum cranking speed of 350 RPM. This wasn't indicated for compression reasons, rather as a minimum RPM required for the flywheel alternator to generate sufficient ignition voltage to operate. Many hard to start/won't start outboards have been cured simply cleaning battery cable connections, starter commutators.

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Re: using a compression guage

Post by Schurkey » Sat Nov 04, 2017 4:19 pm

A typical V-8 will stop with the flywheel in any one of four positions. If you look at a well-used V-8 ring gear, you'll see four areas of heavy wear, evenly spaced around the circumference. I suppose a single-plane crank will stop with the flywheel in either of two positions...but I never really looked.

So you stuff a compression tester into a cylinder, and hit the starter. Maybe the flywheel stopped in position that the piston will rise on compression right away. RPM is low, so the compression reading is low on the first "hit", but rises on subsequent hits. Or the flywheel is positioned so that the starter motor has 3/4 of a turn to get the RPM of the crank faster before the piston rises on compression, and the first hit shows excellent pressure.

I never cared what pressure the first hit produced. I know others do.

I crank the starter until the gauge reading stops increasing. If it takes some exceptional number of compression strokes, I note that, but I don't actually count the number of compression strokes--that is entrusted to my subconscious. I suppose five or six would be typical...but I can't say for sure.

I speed up the process by using two identical compression testers. I know one reads about five PSI higher than the other. An eight cylinder engine takes four cranking cycles. I test "companion" cylinders so the starter has equal time to raise crank RPM before the next tested cylinder's piston rises on compression. Two-at-a-time seems to be easier on the battery.

My compression testers are occasionally compared to each other and the gauge on the "shop air" regulator. It's not a comprehensive test, but prevents gross errors due to failed compression tester.

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When doing things with a 'scope and amp probe, ideally you use a two-channel scope and two probes--an amp probe on the starter power wire and an ignition probe on #1 plug wire. (Kill the fuel supply to prevent the engine from starting.) Then you have the correlation between cylinder and starter draw. Sun and Snap-On have this provision on select models of automotive 'scopes, and I suppose it's fairly common. If the 'scope "knows" which momentary increase in amperage is aligned with #1, the other amperage bumps follow in firing order.

Snap-On made an accessory for their Counselor II series of automotive 'scopes. There was a hand-held probe with a wheel on the end. You'd hold that wheel against the torsional damper while cranking the engine. It would read the speed variation of the crank based on the resistance to movement created by cranking pressure. Again, this was correlated to individual cylinders via a probe on the #1 plug wire. For whatever reason, this was recommended for four-poppers but not eights, which limited the usefulness of that particular probe. I expect (but have not tried to do) the same thing using the crankshaft and camshaft position sensors on a modern engine--looking for speed variation of the crank correlated to #1 during cranking.

Yet another variation on a theme: Correlate the MAP sensor output to #1 ignition firing while cranking, with the throttle at normal idle position. The variations in vacuum among the cylinders should be small. Cylinders with lower vacuum than the others may have problems such as poor valve sealing or poor ring sealing. I've never done this with a typical MAP sensor, but I've used a Snap-On vacuum sensor accessory for my Snap-On 'Scope. I expect a MAP sensor would do the same thing.

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