20 Valve Compression readings.

[Jorrs]

ok ive done a bit of a search but cant find what im looking for.
my friend is doing a compression test on a silvertop 20 valve motor down in melbourne for me in about an hour or two and i need pretty urgently to find out what the compression results should be, ive only had experience with 3S type motors and found that the compression should be around 170 across the cylinders, is this the same with this type of 4age motor as well ?
any help would be appreciated, sorry if there was a thread already bout this, but i didnt see it.
thanks guys

[BigWorm]

I imagine it would be fairly high, up towards the 200 region, but more importantly is the difference between cylinders. As long as all 4 cylinders are within 5 or so psi you know it should be a pretty healthy motor.

[RobertoX]

generally as long as the readings aren't too low and all within 5-10% of each other this indicates the motor is alright.
The readings can differ depending on what the motor has been doing, yesterday I tested a motor that had been sitting around for a while (6months or so) and got 170 across all 4, then ran it for a while and then got 180 evenly as the bores got some oil through it and the engine got a bit of heat into it.


according to the 4age book at 250rpm the reading should be 179psi standard with a variance no greater than 14psi between cylinders. The minimum limit is 142 psi. This is for a bigport though and the 20valve might be higher or lower depending on compression ratio and cams etc.

edit^^ he beat me to it

[Jorrs]

ahhh cheers boys, yeh its just pretty much this reading may decide whether i part with my hard earned and travel 20 hours south to buy it ! hope it goes well, will no in a few hours

[Jorrs]

cold was 150 across all 4, and after a spin around block and motor hot, was 150 again across all cylinders ?
anyone no what this means ? nothing at all wrong with car that can be seen.
perhaps the oil wasnt on the bore inside the cylinders and didnt give a proper reading, because the drive wasnt long enough. ?

[BigWorm]

Was the airbox & AFM & airfilter & everything all attached? This could result in lower readings.

[Jorrs]

yeh was all attached, but i dont think it was run in enough for the drive they went on for the oil to get around the bores, so i think im pretty good

[Trekka273]

According to the blacktop engine manual, the compression reading should be around 193psi or more, but no less than 156psi with the engine warm with a differnece between cylinders of no more than 14psi.

Getting 150psi on the motor when slightly warm might be a little bit of a concern.... I haven't heard of the airfilter, etc being attached to the engine making any difference to the compression test, as far as my understanding goes of this it shouldn't make any difference whatsoever, but I may be wrong.

[Jorrs]

im not to worried, the car ran perfect didnt miss a beat, and all fluids, oil checked out fine.
ill prob give it another test once i drive it.

[Trekka273]

Fair enough. I am sure that it will be fine. I just thought with a 20 hour drive to get it and the cost, etc...

Maybe get your friend to do the test again, this time letting the engine warm up fully, just to be sure. A small test now could save you some money/hassles later on.

[BigWorm]

Last time I did a comp test (a while ago) the instructions said to pull off the air inlet at the throttle body & hold the TB open. I'm sure with the AFM (especially if it's a flapper type, not sure about the 20v?) and stock airbox etc it would affect it.

[Trekka273]

There is nothing in the blacktop manual to indicate that you should do that. It simply states to have the engine warm, remove the plugs and then test the compression with the throttle open fully.

[allencr]

cold was 150 across all 4, and after a spin around block and motor hot, was 150 again across all cylinders ? anyone no what this means ? nothing at all wrong with car that can be seen. perhaps the oil wasnt on the bore inside the cylinders and didnt give a proper reading, because the drive wasnt long enough. ?

It means the compression is consistant, but poor because of bad rings and/or bad valve seal(NOT stems), uncalibrated compression gauge or hot/wild cams. Doe it have aftermarket cams?

It has little or nothing to do with oil & bore & cylinders & drive not long enough!!!!!
The throttle being closed will affect it SLIGHTLY, 5psi max, even though it is always published everywhere that it should be opened fully, for some reason.
It sounds old & worn & tired, but may last a long time if it isn't thrashed.
Wet test????
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On second thought, getting the same exact reading on all 4 is pretty rare, and I'd really suspect that the gauge isn't accurate.

[vtrfirestorm]

Jaws,

A lot depends upon the gauge that you use and how it is fitted. I have found a big differnce between having a long hose compared on your gauge comapred to those old rubber nose type units that you simply push into the plug hole. A long hose increases volume above the piston and in essence gives the effect of lower comp ratio.
cheers peter

[cambelt]

You cant comp test any motor thats been sitting for a while as flaking carbon will make it read much less , and also when comp teating any motor with spark plugs that point to the sky , when u remove them if any debris like dirts fall down the smallest peice can get caught under a valve and that will give a poor reading.
Dave