Measuring compression without disassembling motor

[ChrisBennet]

Tomorrow I'm helping someone figure out what he's purchased for a motor. It's supposedly a 2.8 that used to have carbs. I've got a plan for determining the cam but I'd like a way to figure out the compression and displacement without filling it full of liquid.
I think race tech inspectors have some way of doing this but I don't know how.
Here are some crazy ideas I have:
(A) Measure displacement by measuring the volume of air displaced on the compression stroke. I'm thinking a "U" shaped tube partially filled with water plumbed into the spark plug hole such that air in the tube would force water out the other end of the U tube. I could then catch and measure the water displaced.
(B) Measure compression by measuring the difference in TDC/BDC air pressure with a manometer. I'm thinking the air might leak out too fast for this to be practical even with a little oil to help things seal.
Any ideas?
thanks,
Chris

[304065]

Chris, do a search here for "whistler" and you'll see the thread on tech inspection at the track.

[viejopatron]

Hey Chris,

I used to test centrifugal pumps and had to volumetrically measure shaft seal leakage. We'd run a tube from the leakage port into an upended measurement flask/pan arrangement. Remember those school science experiments where you made a barometer? Anyway, as air was displaced by the seal leakage, bubbles (equal to the displaced volume) were collected in the upended flask. I can't remember if there was a data correction for pressure.

[snowman]

Actually you can measure the air pressure and determine the compression if the rings and valves are in good condition. Just use a normal compression tester. Not highly accurate but could be good for determining if its 9:1 vs 11:1.

Since the cam can mess up the reading comparing the numbers with known good engines could still solve the problem.

I think the Nascar stuff only gives the displacement, not compression ratio.

And if the engines not in a car,, why NOT put a piston on TDC, compression and fill it with a light weight oil from a calibrated syringe?

[ChrisBennet]

Hi guys,
I wasn't successful in measuring the displacement. My plan had been to use the engine like a syringe to pull water out of a jug and measure how much was removed. The hose was hopefully long enough to allow the sucked water to reach the motor. The overlap of the cams was such that it wasn't possible.
Jack, in retrospect we should have just filled it with oil. It would have been a little trickier than you might think. The plug hole would need to be vertical which would require the motor to be tipped to one side AND tipped back some which we could have done with his "hoist" (tractor bucket loader).
-Chris