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blktopbig28
07-28-2005, 01:05 PM
Hey guys, I posted this in the Skyline GTR section, but more traffic on this one so apologize to those who are reading it for the second time.

Recently swapped RB25 into my S13. Did a compression test a couple of days after driving. Here are the results:

140 127 137 137 130 125

Pretty bad. Called the shop that got the engine and did the swap, they said bring it in and they'll check it out. After a few days I brought the car in and they did another compression test, it appears they tested it as either 140-145 perfectly even straight across all 6 cylinders.

The car runs AWESOME, really smooth and pulls well. It's just that when I searched the net, info I got was compression should be standard = 185 psi, minimum = 142 psi.

Question:

1) Is it common to have compression raise after a week of driving? I admit I am a bit skeptical, and will do a test myself in a week to verify.

2) If compression does turn out to be 145 across, should I be worried that the number is barely above the minimum?

3) What factors affect compression numbers? I'm assuming temp, altitude (I'm located quite above sea level), condition of engine... Just wanted to get your thoughts.

Five10PacksDE
07-28-2005, 01:36 PM
Altitued does not affect compression #'s. Those #'s are cool casue there is not a big vcariation, but low. SO that could indicate a worn engine. Did you do your compressoin test when it was warm or cold? WOT?

When u get low #'s like that , you should #1, add a lil oil to every cyl and see how much the compression goes up, if it goes up significantly, then the rings are worn, If it does not go up significatnly, then the valves are probably leaking.
The best engine test is a CYl leakage test, it can pinpoint intae valve, exhaust , valve, ring, or gasket leaks. With proper diagnosis of course.
To me I rather have your compression #;s than mine. 1)180psi, 2)170psi, 3)150psi, 4)135psi. There might be a headgasket leak, dunno

asad
07-28-2005, 01:40 PM
Altitued does not affect compression #'s.



Since when? If you've got lower ambient pressure, you'll have lower compression numbers.

Asad

nissanfanatic
07-28-2005, 01:56 PM
Compression gauges can vary readings. A leak-down test will tell you much more about the condition of your engine

blktopbig28
07-28-2005, 04:00 PM
How is a leak down test conducted and what are the advantages over a compression test?

blktopbig28
07-28-2005, 04:01 PM
Asad,

does higher altitude = lower ambient pressure?

asad
07-28-2005, 05:43 PM
does higher altitude = lower ambient pressure?



Yes, absolutely. That's the whole reason it's harder to breathe at high altitudes, baseballs carry farther at Coors field, and engines make less power in the mountains.

The pressure follows a roughly exponential curve, with 50% pressure at roughly 19,000 feet IIRC.

Since the engine just compresses the air that's available to it, lower ambient pressure in = lower pressure out.

Asad