Always Read the Directions AFTERWARDS

[red03evo]

.... So you can ask dumb questions when you're all done.

When doing a compression test, the directions that I read said to "hold the throttle down all the way" while doing the test.

I didn't ,... so how would that have skewed the results? Would they likely be higher or lower if the throttle had been been held open.

I know I could test it again tomorrow (and probably will, but first I'll have to get another tester because the rubber cone on the straight tester, started slipping up the tube.)

... but you know how it is when you just want the answer :-)

So,... higher or lower?

Thanks
Dean

[Wazzelby]

IIRC the numbers will be higher.

[TheJackal]

Lower, allowing more flow out of the intake port, through the manifold and out into atmosphere. But on the flip side, If its hard for the engine to build compression (hence cranking it 5 times or whatever) it would allow the air to fill in through the intake ports more quickly.

I never have the TB open then I do my tests. I would be curious to see the difference between open and shut. Different engines could do both depending on piston compression, and the ratio to the port sizes.

[Mirage]

Lower? It would be higher.

It will allow more cylinder filling than when the butterfly is closed.

Personally, I wouldn't worry about it too much. Yes, the numbers will be higher, but not too much higher.

Marcus

[red03evo]

Yesterday the cylinders were about 163-165 all the way across.

Today I couldn't get #3 to read above 145 .... unless I put oil in the cylinder and then it only went to 150

I (hopefully) might have a bad tester, because I quadruple checked everything yesterday and #1 showed weak at first, and then it came up to the rest when I pushed down really hard on the tester (it's the 7-8 " straight metal tube with a rubber cone on the other end.)

The screw in fitting measured 5 psi lower. I don't think I was getting it screwed on all the way.

Anybody have a reliable tester? I can have it "test ready" at any location, (at a parking lot, or on the street, etc...) in about 5 minutes, (after it cools for about 10 nimutes).

BTW, the numbers were very close to yesterday - too close to really say one way or the other, because of the mixed results.

South or East part of town are best, but beggars can't be choosers :-)

[Toybreaker]

I like to do it with the throttle open.

As the piston travels down on the intake stroke, (throttle closed) it will create a vacum in the intake, and you may not get a full volume of air in before the intake valve closes to begin the compression stroke. Remember, the only way for the air to get in with the throttle closed is thru the idle bypasses (biss, iac, and a very little around the throttle plate itself.)

You can learn a lot about the condition of the motor by watching the intial readings as the motor is cranked. It should get close (ish) to it's final reading on the first pulse on a healthy motor.(with the throttle open) It may increase`a schosche on subsequent turns, but the initial pulse will tell the tale about the rings sealing capability.

Opening the throttle enables the maximum amount of cylinder fill, per revolution. This larger volume of air draw in on the intake stroke will give the most volume of air to compress on the compression stroke. This will give the highest pressure to seal the rings against the chamber walls and pistons on the compression stroke. This will give the most accurate picture of the health of the rings.

Eventually, I would imagine it would get to about the same reading, throttle open or closed, so it probably doesn't matter much. I just do it the same way every time, that way, it's a repeatable test.

Warm motor, fully charged battery, disable fuel/ignition (see below), all the plugs out, throttle open, crank and watch...(please) record readings. Repeat on all cylinders in a time efficient manner.

Beer is then required for a proper evaluation of the readings :D

The main reason the old school guys do it with the throttle open is because carburated cars would draw fuel into the intake from the idle circuit with the throttle closed. (With the throttle open on a carb, there's no vacum to pull the fuel out of the idle metering circuits.) That fuel will then wash down the oil on the cylinder walls, killing the ring seal, and will result in artificially low numbers.

On a fuel infected late model car, I like to unplug/disable the crank/cam sensor. This will disable fuel and spark. Fuel, so I don't wash the bores, and spark, so I don't damage the ignition ignitor/coil/capacitor etc by having the spark leaping all the way from the plug wire center to ground. (This can require insane firing voltages, and may damage the electronics...) The downside to disconnecting cas/cam sensors is a code stored in the computer. Disconnecting the battery, or erasing it with a scanner makes everything good again.

I like to use the thread in style tester, as it eliminates leakage around the cone/end of the tester, making the test more accurate/repeatable.

And, oh yea, hold the throttle open :D

If you suspect an issue with the health of the combustion chamber, I would strongly suggest you follow up the compression test with a leak down test. It will pinpoint the quantity and location of any ring or valve leakage.

I have the neccessary equipment. My schedule is a little fubar, but there's usually time to get stuff like this done at the end of the day. Shoot me a pm, and we'll see if we can hook up and get it done.
John

[red03evo]

John, that all makes sense *** Except One Thing *** I may have added the Beer too early on in the test!

I ha-ate id wen I du that

I'll pm you on the leak down test

[Outlaw]

higher