very high comp... good engine or worn rings?

[Vios-GT_07]

hi

atm i've got what has confirmed to be a JDM smallport motor in the car. It makes good power and revs hard to the redline, no issues whatsoever.. no oil leaks that i can see yadda yadda.. i did a compression test on it a few weeks back and got 215 psi, more or less 5 psi across all cylinders.

Now i've got mixed reactions about this. I myself don't feel that the motor's tired or about to give way, but some people have said it's been rebuilt with the wrong pistons rings, it's just a newish JDM motor, it's got bigger pistons, yadda yadda.. but today someone mentioned to me it could be because the piston rings are worn, and is using the oil to hold compression, hence the extra high figures. Unfortunately i'm too old and have a really bad memory to remember exactly word for word what he said, but key points are worn piston rings, oil, check breather hose (i did, it's not blocked), and blowing rear mail seal first..

is there anything i should know about this ubber high compression? anything i can look out for to indicate the rings are fried, or the motor just has very high compression and nothing is wrong with it?? it's not missing any oil, and has cracked the oil cap which i got with the car..

[ed]

thats not uber high compression

oil will not fasely elevate compression readings because its worn. the effect of oil will be standard accross worn or unworn engines in terms of sealing the rings.

breather and blown main will do SFA in terms of altering your compression test. but will be an issue if youve got major blowby.

if youre worried, bore-o-scope the block and have a look at the hone, or take the head off and measure the lip wear.

otherwise, aint broke, dont fix

[Vios-GT_07]

4A-GE manual reckons compression should be 140 to 180 psi...

ah.,. okay.. that's what i thought.. it's not as if i'm using thick oil or anything currently it's got 10W-40 magnatec oil.. engine doesn't make any noise, and is smooth at idle..

blowby? hmm.. there's no blue smoke or anything.. any way i can check it?

yeap.. not into fixing anything, just want to be sure i don't have to fix short of a rebuild in the near future

[love ke70]

itll be fine
leave it alone
you dont have blowby, you have high compression.
when you have low compression you have blow by.

[Vios-GT_07]

hm

i'm not going to do anything to it...

i'd really like to know how it's come to have this compression though, it might not be high by your standars, but 40 psi more than brand spanking new standard motor has a little shock value to me... so now my question is it possible to achieve this compression with bigger pistons, thinner HG etc etc?

[love ke70]

if it has been built with over bored pistons, they could have decked the block to match compressio height and also a thinner hg would also help.
it will also depend on valve clearances.
keep the exhaust valves shut longer and you will get more compression.
adjustable cam gears?

[schnitzel]

I would be getting another compression tester and redoing the test, sounds a bit like the gauge is out of whack.

[Vios-GT_07]

i did the test with another tester.. 3 more in fact.. they're all very accurate more or less 2 psi from the original gauge tester...

and the original gauge tester i used was fairly accurate, was giving proper results for 3 other cars i've seen and probably a few dozen more i didn't..

the cam gears are stock, ignition timing is a steady 10 (made sure with timing light)..

performance wise does not indicate oversized pistons.. feels like any other smallport except a lot smoother and much newer.. not faster or slower than any other smallport 4A-GE.. even so, how much do you have to bore and what size pistons, rings, HG etc u'd need to get 40 psi compression more than stock? is it even possible?

thanks for the feedback opinions and advice guys

[love ke70]

checked valve clearances?
might be just one of those freak jap motors, you lucky boy
if your really interested i would be checking your valve clearances

[oldcorollas]

if the guage is accurate and the numbers are right, 215 is a bit high.. higher than should be expected. regardless if the number gauge got for others cars is in ballpark, if it's not accurate.. it's not accurate

an extra 3 or 5cc of oil or water in bore would bump it up that much, but that seems unlikely.
alternatively, you have bigger pistons, smaller combustion chamber etc, or, just higher comp pistons.

why not look thru the sparkplug hole at the piston crown and compare it to stock pistons?
please do that first.

next, your cams could be out. doesn't matter if they are stock gears, if the inlet cam is advanced, it will close earlier, and give a higher compression test pressure.
check the cam position by measuring the lift itself vs crank position...

[love ke70]

or just check the cam via the cam marks and crank pulley at tdc like you would when setting the timing normally.

[IN 05 NT]

if you have crazyu blowby.... you can tell by opening the oil fill cap while the engines running, and you will feel pulses of air through the oil fill point....

nothing wrong with high comp tho..... leave it as is i say,

[jeffro ra28]

massive carbon build up! lol.
I always wondered if a totally caked up piston and chamber would affect it.
But seriously now
Did you check to see that the compression gauge insert wasnt protruding far into the cylinder, or did it resemble the same as a spark plug?

Perhaps you have an aftermarket or really good starter motor and it winds the engine over a few revolutions quicker? this can affect it also.

[oldcorollas]

if you have crazyu blowby.... you can tell by opening the oil fill cap while the engines running, and you will feel pulses of air through the oil fill point....

nothing wrong with high comp tho..... leave it as is i say,

i don't think you can tell that way.. cos...

in the crank case, each piston displaces the bore volume of air every time it goes up and down.... that air is rushing around the crannkcase AND the head.. so the pulses you feel are not pulses of air coming out, but the sound of the pistons going up and down..

there would be buggerall blowby happening at idle or low throttle, compared to high throttle...

i guess for a compression test, you are testing leakage, not blowby.. since there is a totally different situation with the combustion pressure pushing the rings out during combustion events.... anyway...

yeah, carbon can have an effect, but you need a fair amount of carbon...
40psi is 20% extra compression.... thats not small...

[Vios-GT_07]

checked valve clearances?
might be just one of those freak jap motors, you lucky boy
if your really interested i would be checking your valve clearances

how do you do that?

if the guage is accurate and the numbers are right, 215 is a bit high.. higher than should be expected. regardless if the number gauge got for others cars is in ballpark, if it's not accurate.. it's not accurate

the gauge is accurate. It gave proper numbers for a range of engines including Nissan and Isuzu, and yea their numbers reflected the compression readout. The tester i've used in all cases was either a plug in type, so push it straight in, or screwed into the spark plug hole, same results..

starter motor is standard, in fact, the battery is smaller than what's meant for the car..

i've checked the cam pulleys against the crank, and got a 2nd and 3rd opinion, the marks line up, so the engine is indeed square. it's right on the dot.. even if it's not, and say theoritacally, it's a few degrees out .. okay let's say it's the intake side and advance by a few teeth, can i still get this high compression??

i'm not going to "fix" it, not going to touch anything.. i just want to confirm there's nothing wrong with it.. or even get a definite answer as to why it's high, and put those negative thoughts to rest..