Compression Test Results

[roys33]

i just did a compression test ,(all plugs out throttle body wide open)

these are the results

1=120

2=120

3=125

4=125

5=120

6=125

whats your thoughts?

and results?

[zebra]

It's fine

The numbers mean nothing, it's the spead or variation that really matters

Or your engine has done 500,000ks

[MrStabby]

Also - since you didn't mention it - motor is supposed to be at running temp

[roys33]

yeah thats what i thought and there was one at 122 and the others more more like 123-4 so it was pretty close i was thinking i had blowby but i wouldve had a bigger drop wouldnt i?

[roys33]

yeah it was warmed probably not exactly temp though

[The Dan]

The results are low but even. Being even is more important than low. The gauge isn't necessarily accurate either.

[roys33]

cool thanks everyone once again

[AngryRB]

sounds like that motor has had alot of floggings in its life..

i have 170000ks and still have 170psi...

its like dropping your compression ratio down to 7.6:1, like dropping in some massive cams and loosing the bottem end. why would that be ok? even if they are even

Edited August 24, 2012 by SliverS2

[The Dan]

its like dropping your compression ratio down to 7.6:1

How did you calculate that number?

[AngryRB]

not scientifically, more estimating...

eg. my rb25 has 9:1 compression and it has 170psi

my old rb20 supposed to have 8.5 and it had 150psi

my old cordia had 7.6:1 and had 150psi , then after cam drop in , went down to 120psi

so estimating from above, all engines were healthy with moderate km's, 120psi must be quite low

must be a big difference between 170psi and 120psi..

[zebra]

[AngryRB]

teach me wise one... 2 rb25's one has 170psi , one has 120psi

where did the 50psi go? if you were going to buy a motor for your car.. which would you be buying?

[zebra]

did he magically use the exact same comp tester as you did he?

[The Dan]

teach me wise one... 2 rb25's one has 170psi , one has 120psi

where did the 50psi go? if you were going to buy a motor for your car.. which would you be buying?

I'd buy the one with 120psi coz I can jam 60 pound of boost up its arse more easily

[AngryRB]

yes i sent it to him with my magic wond and magic hat..

obviously there is some error in the instruments, but i would like to think that all these sorts of equipment are tested to be within certain limits .. 50psi would be a rather MASSIVE error wouldnt it

[AngryRB]

I'd buy the one with 120psi coz I can jam 60 pound of boost up its arse more easily

i thought you would run from that cause it would be ready to blow up .. never heard anyone recommend to push a motor with more boost if its on its last legs.. usually tell you its a bad idea..is that so you can rebuild it for some $$$ hahaha

[dkbcleaner]

What if your engine has between 180 and 190 psi??

[The Dan]

i thought you would run from that cause it would be ready to blow up .. never heard anyone recommend to push a motor with more boost if its on its last legs.. usually tell you its a bad idea..is that so you can rebuild it for some $$$ hahaha

I'll explain it better in a minute when I'm not driv.....umm....doing stuff

[The Dan]

When you get a compression test, the results are taken statically. That is why you will hear a lot of people refer to them as static compression results.

Three guys get a compression test done, Guy A has 165psi even on all 6, Guy B has 125psi on all 6 and Guy C has between 140-170psi on all 6.

The engines for both A and B are healthy engines. C is a dud.

What happens when a compression test is done is the engine is turned over by starter motor and is not being moved by its own power. This static result can give you an indication of how healthy an engine is but it does not give you any indication of what it will do when the engine is running. A hundred more things happen dynamically when the piston is in motion and under power of combustion. The rings will seal A LOT better and everything is moving MUCH faster so leak down past the rings is virtually no more than when it was standing still.

The reason C is a dud is because one cylinder has 170 and another has 140. The variation is far too high so therefore you can determine the engine has a problem.

If all results are even but low, it can be due to a number of things. Worn exhaust valve guide, carbon build up on valves or ports, lower tension on rings, piston to bore wear etc etc. It shows the engine is aging but still healthy because there is no variation. If its a worn exhaust valve guide, on a static comp test the valve will seat in a totally different position every time and your low results could be due to that. It doesn't mean the engine is going to explode.

We have seen RB26's with 120psi on all 6 making 600hp at the wheels on 30psi boost and 2 years later.....still going strong. Still has 120psi on all 6.

Of course we have to set some limits in our mind about how far is too far for a low result to become a bad result. For a straight 6 that's 8.5/9:1 comp ratio generally I use 120 as a lowest limit before telling someone the engine is had it.

For an Rb20 that was the lower comp ratio you might accept 100-110 as a lowest result.

Back on the worn guide thing, the valve will seal much better when in fast motion or if the spring is improved it could actually clear away any carbon or make itself a fresher seat. That is possibly why you saw a higher result when fitting your larger cams. Camshaft size won't effect static compression results unless the cause of your comp low result is due to valve train deficiencies.

[The Dan]

Oh and I forgot to add that 50psi is nothing compared to the 1500-2000psi your combustion chamber sees at peak power and cylinder pressure.

Engine A might have 2000psi whereas Engine B might have 1850-1950psi due to his lower static result. Both cars will make within 5-10 HP of one another and potentially live just as long as one another.