Compression tested my engine!!!!!

[simers]

It's not good... I am getting stupid variations in my readings... I am getting readings of between 150-170psi.... What should a good engine be reading bearing in mind I've only rebuilt my engine two months ago... It's still burning oil too at a rate of knots... My head is melted.... Advice please!!!!!

[stupink]

How are you testing?    is engine warm, are all plugs out, is throttle opened, is fuel pump stopped...

150 is too low if all of above is true, but it depends how many miles you've done potentially..  maybe the rings are still bedding in if the engine has only been used once for 500meters...   also what was rebuilt??   the important things are the piston rings, and valves.. was the cylinder honed or rebored? were the valves lapped in?

[mrzwei]

Mattperformance fairly recently quoted 190 - 220 with no more than a 15% varience. Obviously take account of what was said above.

[simers]

Plugs were out when being tested...
As regards the rebuild it was a full one... Everything from big ends to valve stem seals... The mechanic that built it is now blaming me for when my radiator burst... I have done over 8000 kms since the rebuild and have serviced twice

[stupink]

what happened when your rad burst? sorry, not seen your thread..
overheating could cause issues, but if a rad bursts, and you turn the engine off, then thats not going to cause a problem, if you drive it for a while after with no coolant system yes it could cause issues like warping/glazing of various components.

[normanh]

I have to be honest cylinder pressures baffle me as taking atmospheric pressure at 14.7psi compressing 10:1 yields 147 psi, I dont believe it can increase much more and on that basis the values dont look that bad, using the formula p1v1 = p2v2 maybe its to simplified as a part of the gas laws equations?


norman

[Anonymous]

It 10.5:1 Norman never know how much that .5 will make  s:lol: :lol: s:lol:

Also its the compression of the gases as well I think.

As for the op question. To me the readings are low and the biggest question was what did your mechanic do to fix the oval bore???

[simers]

It 10.5:1 Norman never know how much that .5 will make  s:lol: :lol: s:lol:

Also its the compression of the gases as well I think.

As for the op question. To me the readings are low and the biggest question was what did your mechanic do to fix the oval bore???

Being honest with you I have no idea... I have him the engine with a full kit without pistons to do the engine and this is the way she running now.... As for what happened when my rad burst I switched her off the very second I saw temp rise before it hit red line

[Anonymous]

My guess is he over looked the fact the reason your engine burned oil before was due to oval bore. Now new rings do help for a bit but it still don't fix the problem that you got a round piston in an oval hole. Get him to give you a full brake down of what he did, because if he did miss it then he would of dug his own hole.

[mrzwei]

I have to be honest cylinder pressures baffle me as taking atmospheric pressure at 14.7psi compressing 10:1 yields 147 psi, I dont believe it can increase much more and on that basis the values dont look that bad, using the formula p1v1 = p2v2 maybe its to simplified as a part of the gas laws equations?


norman

I think that is a flawed argument. If you keep blowing up a baloon then eventually it will burst.
Why not just trust the data from the experts? 150 is low, 175 is what you are looking for.

[Anonymous]

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[stupink]

You could also try a wet test, by putting a small ammount of oil down each cylinder, it helps the rings seal, (and nothing else) so if you see a significant improvement then you know the ring seal is at fault. just a cap full.. not looking to fill the cylinder at all, with bowled cylinders it can help to put it in through a needle/straw so you can fire it at the cylinder walls/piston edge. then re-do compression test as before..  no change- valve or gasket problem,  small change =  valve or gasket problem, big change= ring problem

[simers]

You could also try a wet test, by putting a small ammount of oil down each cylinder, it helps the rings seal, (and nothing else) so if you see a significant improvement then you know the ring seal is at fault. just a cap full.. not looking to fill the cylinder at all, with bowled cylinders it can help to put it in through a needle/straw so you can fire it at the cylinder walls/piston edge. then re-do compression test as before..  no change- valve or gasket problem,  small change =  valve or gasket problem, big change= ring problem

Might just try that today... Thanks mate... My plugs are also white so does that mean I'm running too lean??? Because she is a japper should I be using octane boost to bring up the RON as Irish fuel only averages at 94RON and jap fuel is 101RON???

[simers]

Another note... Should I have changed my pistons when I done the build???

[stupink]

no set guides for a "rebuild" you can only decide what to replace if you do the rebuild yourself, otherwise you have to trust the builder...  only they will know the condition of parts. if the pistons are not worn, but the cylinders are, you can re-line the cylinders, or you can rebore them and fit larger pistons. either is perfectly acceptable as a rule, i dont have any particular mr2 experience to know the exact personality traits of this engine.  with worn pistons you tend to get a nice little knock on idle if they are bad. dink dink dink dink dink dink dink.. like lifters but slightly less clacky.

[simers]

I don't have any of them problems for the minute anyways so that's not too bad I suppose

[dcod]

A leak down test would be a good idea. Should help to find out if it's rings, valve seals or gaskets.

[simers]

I don't fancy paying to get it rebuilt as mechanic won't stand over it

[stupink]

if the oiled rings doesnt help a leakdown doesnt involve removal of anything substantial... oiled compression test is just a poor mans leakdown.

if you put a cylinder to top dead, and then pump air in, you can see how much air the cylinder flows, it would be nice if it was zero, but its always something. too much is bad, but if it is leaking you can hear where its going by listening to the crank vent, the inlet and the exhaust and engine externally to see where the air is going.

rebuilding or not rebuilding, worth knowing whats wrong.. 150psi is not good.. but it could well plod on for a while yet  s:) s:)

[simers]

rebuilding or not rebuilding, worth knowing whats wrong.. 150psi is not good.. but it could well plod on for a while yet  s:) s:)

After forking out 1200€ approximately you wouldn't want it to just plod on for a while... You want it right...

[normanh]

I dont think our engines expand like a ballon and if they do we have serious problems but then every time the valves open the pressure is released so at 10.5 compression thats about 157 psi, have you checked the accuracy of the gauge before you go to the expense of an engine change?

A wet oil test is a good choice!

as Scotty once said to Jim do you want me to cheat the laws of thermodynamics? This law was good enough for Mr Boyle et all

norman

[mrzwei]

 s:lol: :lol: s:lol:   Thanks for the slap on the wrist Norman, on re-visiting Mr Boyle et al it does make a difference that the cylinder doesn't expand!

Good point though, I looked out my old compression tester and that was showing green (as in ok) at 150 / 157 and at the time that was sold the average compression ratio was probably around 10:1, maybe a tad lower.

The main conclusion from the stuff I've read is that equality of readings across the cylinders is more important than absolute readings. As stated by others, temperature, altitude (I think) and valve overlap are other variables.

Good call!

[stupink]

Hi normal you are not taking into account temperature so your comments are not relevant. as the compression is happening fast there is a great deal of heat created. this leads to a higher pressure on top of the compression alone..  think finger over bike pump=hot.
so yes you can (and do) create a pressure higher than the static compression ratio of the engine. considerably so, but a lot of that escapes through valves etc. with a perfect/digital valve timing it would be even higher again, plus, you dont even end up with a cylinder full at 1 atmosphere at bottom dead, the volumetric eficiency of the engine isnt perfect so it will be at a slight vacuum. so that takes a bit off also..   hopefully that makes sense.

simmers, i just thought you said you didnt want to rebuild.. but yeah if you're going to get him to fix it fine  s:) s:)  150 suck..

[normanh]

On a quick crank over pressure the only heat will be from the gas compressing which on a few revs will be negliable as the engine wont be at operating temperatures. The equations are valid as there is another which will account for any temperature rise in the gas.


norman

[Jon_G]

On a quick crank over pressure the only heat will be from the gas compressing which on a few revs will be negliable as the engine wont be at operating temperatures. The equations are valid as there is another which will account for any temperature rise in the gas.
norman

The heat from the gas compressing (adiabatic heating) will be significant, even with a cold engine. But shouldn't the test be conducted with a hot engine?

I don't think it's a simple as applying Boyle's (and/or Charles') law, as it is a dynamic situation.