Low compression

[Jimbo2000]

recently got spark plugs changed and did a compression test at the time to find out that compression was 125 across the board!

what could be the cause of this and what does it mean?

better head gasket?, forged pistons (have no way real way of telling do i?)

thanks boys

[T0nyGTSt]

Wet test?

Rings, lotta blowby.

T.

[meggala]

yeha jim your car could have a metal head gasket if its not blowing smoke and expecially if the figures were similar across the board let the boost up begin

[Zman]

i got 120 on all cylinders with a dry test.. and so did a mate of mine with same engine.

are engines suppose to have any blow by at all? coz mine seems to give out alot with the oil cap open.

[Jimbo2000]

whats blowby? smoke from exhaust or engine?

[Steve]

Getting alot of blow by is a sure sign the engine is a bit tired. Having said that, it could last for ages yet.

[T0nyGTSt]

I would urge you to install a catch can and see how bad blowby is. Could be bad on 10 year old cars.

T.

[Jimbo2000]

again whats blowby, i thought it wasnt too bad to blow a bit of black smoke out the exhaust, thought it was just cause of the big bore piping that these cars have

whenever i'm at the workshop watching a car on the dyno waiting outside for mine to go on theres always a lot of smoke coming out under full load.

thanks

[blind_elk]

blowby - correct me if I'm wrong - is the combustion gasses leaking past the rings and pressurising the sump.

[4 Doorz]

how much does a test cost?

[Jimbo2000]

what i had done cost about 20 bucks and took all of 2 minutes, done while the plugs were out.

[Steve]

Blow by is the gases going past the pistons during combustion, these gasses are circulated to the rockercover where they then get redirected, one way or another, to eventually be burnt again in the cylinders (if you dont fit a atmo vent catch can of filter).

If the rings are starting to wear a bit, blow by will increase, and it would be noticable when opening the oil filler that alot of air seems to be getting forced out. If there appears to be no air being forced out, 'usually' means the rings are ok.

Jimbo2000, the smoke you see from a dyno run is because the AF ratios are intentionally richer during power runs, which helps control detonation.

[Zman]

would a brand new engine (sr20, or rb2X) have any blow by?

[Steve]

shouldnt have a noticeable amount. My RB25 has nearly clocked 60,000 and still doesnt seem to have any. But I would imagine it would be impossible for an engine not to have any at all, esp when boosting:)

[wrxhoon]

how do u know that the compression was 125?

are u sure that the gauge is correct ? if they are all 125 the gauge mynot be right or the engine not normal temp .to check if its bottom end prob ( warn rings or pistons ) do a compression test note the comp on all cil then squirt a small amount of oil in the cil crank a few times then check comp again if it raises say by 10 psi or more u have bottom end prob ( warn rings more than likely )if it doest rise it means warn top end ( valves or valve seats ). the other way is to do a cil leakage test , this is how u do it : spark plug out install i tube with a threaded bit at the end that screws in to the spark plug hole car in gear handbrake on then put compressed air in to cil (150 psi ) and listen far air leaking ( bottom end or top end ) .

note most of the time if engine runs good makes good power doest use a lot of oil and u find low compression ( but all cil the same ) its your gauge or the method u are using not correct so dont panic if all the above are right .

[4 Doorz]

what does 125 mean? Is it 125 psi out? (obivously not) or 125 ml/min leaking? What units is it measured in?

[strutto]

4 Doorz: 125 psi is the amount of pressure the cylinder can make and hold at low revs. Usually cranking revs.

32 GTR RB26DETT Specs are 171 psi @ 300rpm standard, 128 psi @ 300rpm limit, 14 psi variance between cylinders.

Not sure about the RB25DET.

[Jimbo2000]

what they did when they did my test (wasnt paying too close attention so didnt get a good look at what the instrument actually was) they had the plugs out and inserted the probe thing with guage on top into each spark hole (real technical! ) and turn the key for about 2 seconds.

wasnt 125 exactly on each one but pretty close.

[wrxhoon]

standard compression for rb25det is 1.2mpa (12.2kg/cm2)or 171 psi ( apr) limit .9 mpa , 9.2 kg/cm2 or 129 psi . thats at 300 rpm lower rpm = lower comp.variance is 14 psi . rb26 is slightly lower .like 2% lower as it has a lower comp and runs much higher boost . jimbo if urs has a true 125psi u wouldnt have much power unless u run very high boost but i dont think it would be that low .

it would be hard to start if it was that low too ( like taking a few cranks to start it .

[Jimbo2000]

so are you saying it could be a bad thing to have lower compression?