what do you look for in a compression test?

[2000impreza]

when doing a compression test do you look for consistant pressure from piston to piston? or what about higher or lower numbers compared to similar cars/engines?

i'm still a newbie at this stuff.... :nut:

[cocoabrova]

There is usually min-max #'s that are given as a guide, and as long as your compression is falling somewhere in between those two #'s and aren't too far off from each other, your engine should be good. I imagine if the comp. was the same straight across it would mean a little more power than having uneven comp. :thumbsup:

[speedracer]

Your looking for compression differences between each cylinder. I would say no no more than 5 - 8 psi difference between each cylinder across would be :thumbsup:

When you do run compression test make sure you plant your foot on the gas before you crank.

[shadowz]

Constant compression throughout each cylinder

[redline]

Originally posted by shadowz
Constant compression throughout each cylinder


yep this correct and there is lots of factors that effect the final numbers, weather, how the test is done etc...

as a general rule no more then 10psi difference between cyclinders. Make sure that you crank each cyclinder the same amount. ie 5 cranks and the first crank should be about half the final number.

[2000impreza]

a buddy and myself compression tested our engines yesterday. cars were similar same with engines. we DID NOT plant our foot on the gas. but we did cranked it till the guage did not move anymore so it probably didn't make a difference. 8-9 cranks.

for numbers, both the cars was very consistant from cylinder to cylinder. first car was around 180psi. the second car had lower number of around 160psi but again very consistant. both cars was tested in the same conditions...

[redline]

You must make sure you crank the car the same amount for each cyclinder, doing it till the gauge stops is useless. Crank the car 5 times, make sure the TB is open cause this can give you screwy numbers. You can hold the TB open with a screw driver if you wish..

[2000impreza]

crap. i think we did it worng. so 4-5 times with the throttle open? you have to warm up the engine first right?

[speedracer]

Yup, you want consistant readings and the throttle open each time.

[redline]

pick a number between five and eight and make sure you crank it the same number of times for each cyclinder.

how are you cranking the car over?

[2000impreza]

ok this is what we did.

1. run the engine up to temprature
2. turn off the engine and take out all the wires/plugs and unhook the coil pack
3. hook up the compression tester
4. crank it 8-9 times
5. record compression reading
6. repeat with the other cylinders

what we forgot to do was hold the throttle down. but i really don't think it could have changed the numbers that much.

the cylinder to cylinder compression was very consistant on both the engines.