Post up your compression #'s and how you do it

[AwesomeJ10]

Ok I'll start.

I always leave the throttle plates CLOSED and don't hit the gas at all. Some prefer the throttle plates to be open, but I have personally not seen a difference.

I sometimes perform the test cold, but usually like for it to be warm so the system is oiled. All plugs removed.


AMC 360 in my J10. It has about 30K miles on a mostly stock rebuild. Summit 8600 cam. This engine has been ran twice with no oil pressure for a total of about 2 hours (long story) and has sank in a lake (turned off engine just before it went down). I use dyno oil in the engine. 5k feet in elevation (does elevation make a difference?) I'm using an old made-in-usa compression gauge.

#8 140 #7 130
#6 130 #5 139
#4 130 #3 139
#2 127 #1 140

[wedgetail]

This engine has been ran twice with no oil pressure for a total of about 2 hours (long story) and has sank in a lake (turned off engine just before it went down).

wow, sounds like your motor has had a bad run of luck... Am I reading correctly that it ran for two hours straight with no oil pressure? Twice? x_x How can it be still turning free let alone have any compression?

But seriously, I've always done it with the throttle open for max air flow. I haven't noticed a difference in vehicles but in chainsaws it makes heaps of difference (like 10-20 psi)

[Stuka]

Back when I had my Cherokee it had 149-157 psi across the cylinders. That was with 157k on the vehicle, no clue on the engine. It ran strong though.

My J10 back when I got it had 116-123 I think across all 6. Its quite a bit lower than that now I think.

As for how to test, I have seen no difference with throttle, and no difference with other plugs in or out. I just spin the engine over a few times, and see what comes up.

Elevation can make a difference, but not quite as much as you might think, unless you are testing up at 10k feet or something. I used to have a chart that showed the difference for a given altitude someplace. Manifold vacuum however can really change for a given altitude.

[whsii]

I think that plugs in or out, throttle open or closed, high altitude or sea level, warm or cold, is irrelevant, as long as you check the cylinders all the same.

One thing that could help when you see a low reading on a cylinder, squirt a table spoon or so of oil into the spark plug hole, and re check. If the compression raises, then the rings are getting worn, or were just dry. If it makes no difference, then the valve is causing the drop in compression.

Just my thoughts...

[carnuck]

Before I compression test, I run a trickle of water through while revving.

[AwesomeJ10]

Before I compression test, I run a trickle of water through while revving.

Ok..

Can you explain this procedure?

[Stuka]

Before I compression test, I run a trickle of water through while revving.

Is this to help get rid of carbon deposits or something?


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[carnuck]

Before I compression test, I run a trickle of water through while revving.

Is this to help get rid of carbon deposits or something?


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Exactly. With the motor warmed up, I pour a little down the carb as I slowly rev the motor 2-3000 RPM (not redline!) Not enough to stall it quite. I've compression tested before and after this. Some motors jump as much as 50 lbs compression afterwards.

[whsii]

An old school mechanic told me to do this on an old sluggish V8, He told me to get a quart mason jar of water, and pour it into the carb revving the engine between 2500 and 3000 rpm.

He said that the water would quick chill the carbon on the valves and it would shatter.

I have done it on several vehicles and have seen a marked improvement. I did it once on a tear down, just before I pulled the heads. The valves had no carbon build up.

Just my thoughts...

[AwesomeJ10]

I wonder how many times people have done this and cracked some valves, a head, etc?

[Stuka]

The steam does the cleaning from what I know. Kind of like how when you get a blown head gasket and pull the head the leaking cylinders are spotless.

I typically use a spray bottle to regulate how much water is going in. Lot safer. I have just never done it before a compression check.


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[carnuck]

I wonder how many times people have done this and cracked some valves, a head, etc?

The only problem I ever saw was somone dumping WAAAAAY too much water in at once and hydrolocking their motor, bending a rod. I'm talking a 2 gallon pail dumped in at low RPMs, like doing a water crossing without a snorkel. Several hundred motors and the only trouble I've had was temporary heavier oil consumption.

[lindel]

I wonder how many times people have done this and cracked some valves, a head, etc?

The valves survive temps around 1200F to 1500F, so they're not likely to suffer any damage from steam. Same thing with the heads. There won't be enough of a compression increase (unless you hydro-lock it like Jim said) to break/bend anything either.