4g93t compression test

[mistamidget]

I have two 4g93t engines where the performance of one isn't as good as the other so as part of chasing down what's the difference I started with a compression test.

Both engines were cranked until the needle no longer advanced with the throttle wide open.

The first engine is a standard factory engine with 75,000km up. Every cylinder measured 125PSI on the dot no deviation.

The second is also a standard factory engine but with 170,000km up. I got
1. 185
2. 175
3. 180
4. 180

The high km engine feels stronger than the low km engine as the test seems to bear out.
Can anyone suggest why the readings and performance wouldn't be the other way around.
Cheers,

 

[4piknup]

very interesting comparison. i must check mine for comparison too
maybe the higher km motor has more carbon build up on the pistons?
what about a wet test with a teaspoon of oil down the bore? that may show the low km engine has slight ring damage, maybe from overheating or something.
what about vacumn at idle? if they idle the same speed, which has more manifold vacumn?
the higher compression engine should have more vacumn as it will have higher torque,

 

[Smutt]

I have some figures on record;

4G93T Standard compression ratio (8.5:1) = 180psi across the board (x4)

4G93T Custom build compression ratio (8.0:1) = 160psi across the board (x4)

There is obviously something wrong with the motor that your getting 125psi (x4).... have you done a leak down test

An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.

A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.

If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.

If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).

 

[nwr31]

^great reply.
mine has done 220k kms & still has 175 across all 4.

 

[4piknup]

i built my own leak down tester and use it fair bit. i love that it can test the condition of a engine that is out of car and not able to be cranked.

 

[mistamidget]

Thanks for the pointers, very good info. I'll do a leakdown test and see where I go from there.

 

[BMGTZ]

in the continuing saga of the rattle trap I have buily I did a compression test today with a hot engine. 180psi across all 4......these are supposed to be 9:1 pistons so I was expecting higher.

 

[Smutt]

I agree Brian...
Does anyone else run 9.0:1 compression ratio?
Please post up your results for comparison..

 

[nwr31]

180 sounds about right imo