How To Test Your Compression

[carl]

Originally Posted by Dave

140 LBS? Try a different gauge, seems too coincidental for it to be that low and so linear on all 4.

thats 100 psi lower than my car or Joe's we used 3 different gauges all within 5psi. i think the range is 200-250. carl

[robains]

As per my other engine thread. I'll add my compression test results to this one:

190 - 185 - 180 - 190

This is with Mahle 9:1 pistons.

Also noted that with all plugs OUT, throttle open/closed made no difference in the readings. Clutch in/out also made no difference.

I think what could cause the biggest variance in readings is how well one secures the compression tube in the spark plug hole. Small dab of oil on the rubber seal is pretty good at ensuring a good seal and more accurate readings.

5 or 6 cranks worked well for me.

Rob.

[goldengatehornet]

Most of the old manuals say to stop at three pulses and use that reading. A concern would be if the first pulse is low, even though it gets up to pressure by the third or fourth. I'm lucky to have a compressor and leak down guage. The only way we measure for leaks in the chip manufacturing industry is by the leak rate over time. Did anyone mention to have the engine at running temp first?

[robains]

Interesting, the procedures I've read and follow for compression testing are those provided by the manufacturer of the testing tool and/or service manual -- they suggest to never remove spark plugs when the engine is hot because of potential thread damage to the head -- hence why cold torque specs for spark plugs is only 13 ft/lbs.

Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.

[goldengatehornet]

Originally Posted by robains

Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.

A Viton O-ring is good for over 200 degrees C. A Teflon one is good for over 500. On my race car, I start with a hotter plug, warm the engine to temp, then change to the colder plug for the track. This eliminates the problems of fuel fouling. This is a common practice. Antisieze on the plug threads is good for even street engines.

P.S. work hard in Cupertino. I need my Apple stock back up to $150. Typed on my Powerbook, as my HP is half the speed.

[robains]

Golden,

Thanks for the info, much appreciated -- I didn't know.

Cheers, Rob.

[goldengatehornet]

Welcome. Apple reported well today. Much of the money for my Elise came from Apple stock options.....................