compression test with plugs in ?

[nissan_star]

I realize the general consensus is to pull all of the plugs out when checking compression, but i didn't do this .
I tested a s14 blacktop (VCT) and got 125 +/-2psi, I went about 4-5 cranks and stopped. The needle might have still been going up but i didn't think to look.
The plugs looked clean and I got to drive it before hand and it seemed ok, but i have never driven a turbo car before so... wtf do i know
I didn't put oil in because someone said that the numbers were fine, I was looked them up later to make sure.

My real question(s) is: What is the consequence of not removing all the plugs during the test? is it just harder to crank? does it take longer to pressurize?

thanks for reading!

[Sugar]

First time I did a compression test I only had the plug out the cylinder being tested. No real negatives other then the numbers will probably be a tad higher, I think I got 185 all across. Without the plugs I got 174.

120's are really low. Usually 150 to 200 is what I see people getting.

Do it again and crank the engine until the gauge stops climbing. Usually 8 to 10 cranks. Be sure to have the throttle down too (WOT).

If its only varying by 2 psi or so you are probably fine, but get it under more pressure to be sure.

[Justin35ll]

Take all the plugs out so that it's less strain on your battery and easier for the engine to crank over. As he said 8 - 10 cranks, but usually easier to just do it with another person. And hold throttle WOT

[nissan_star]

Thanks for the help. I will test again to make sure its ok before I buy it, even though i am almost positive its a good motor.

btw, the FSM states compression to be normal from 128psi to 155psi, how are people getting up to 200psi? bad tester?

[PyR0NiAk]

it should NOT read 200 psi, although the numbers in the FSM are for a warm engine. Keep this in mind. Cold = a higher number...

[nissan_star]

it should NOT read 200 psi, although the numbers in the FSM are for a warm engine. Keep this in mind. Cold = a higher number...

When we tested, the engine was a little too hot, i guess it could result in lower numbers based on your statement above.

[PyR0NiAk]

You are correct.

[nissan_star]

wait, why wouldn't pressure go up with temp?

[Blown240sx]

I would think also that the pressure would be more with the engine hot. Since all the metal parts would be full expanded.

[PyR0NiAk]

Cold air = denser...

[nissan_star]

I was thinking more along the lines of PV=nRT.

i can see how this doesn't really apply though

[PyR0NiAk]

Ideal gas law is more suited for low pressures...

[zachattack_5491]

it should NOT read 200 psi, although the numbers in the FSM are for a warm engine. Keep this in mind. Cold = a higher number...

Well I'm no scientist here, but when the engine is cold the rings aren't fully expanded, which allows air to seep through and cause lower compression. Colder air is indeed more dense than warm air, but the change in air density would have a much smaller effect on compression numbers than the rings.

When you put new rings on a piston you gap them to compensate for expansion when they get warm.

[nissan_star]

Ideal gas law is more suited for low pressures...

low pressures? I would say 200psi is low pressure

[nissan_star]

Bump: I tested my compression the right way (warmed the car up, let it cool so it wasn't too hot, pulled the cas and cranked @ WOT)

#1 #2 #3 #4
143, 145, 143, 145

I am stoked, esp. since its my motor now in my car now

[PyR0NiAk]

Thanks for posting the update, along with proving my point.

[nissan_star]

ha, yeah i guess i did sort of prove it. I think they were also low before because we pulled one plug out at a time, within like 2 turns the pressure was higher then it was on the last test.