What are good compression numbers for a m30B35?

[thespeedfactory]

Looking for someone to chime in with their numbers from compression test of stock engine.

Thanks in advance

[ArnZ]

I had about 160psi +- 2psi across all cylinders on my engine.

[strad]

I got 170 out of four of my cylinders and 165 out of the other two. Test was done at 2000' asl and I did not correct for that -- sea level compression would be higher if I had. Engine had 130k miles on it at the time of test.

[Mr Project]

170-180 on my B35's usually IIRC, didn't correct for altitude either, I'm not that far above sea level here in NE.

Remember you're not really looking for the number, per se, more looking for a variation in the numbers between cylinders. You don't want to see more than 5-10% variation across the 6 cylinders, I wouldn't get too worried if they were all 150-160 vs. 170-180 as long as they are all within the 5-10% range of each other. There are too many variables in the test to get too worked up about the absolute psi number. Unless they're all at 90psi or something. :)

[stoney85]

i have used different guages on the same engine with a variance of 30psi, so also take into consideration, worn or incorrect reading guages

but as mentioned, what you are looking for is uniform readings across all cylinders, however if they are uniform at 60psi, then look a bit harder into it

also, larger cams with more duration/overlap can affect compression readings

[5senses]

Well this is new news to me... how is a compression test performed for the weekend at home mechanic? Special tools? Hours? Can this be done on any engine using the same method?

Thanks

Wes

[moroza]

A compression tester is basically a pressure gauge with a peak-hold feature and a sparkplug-sized nozzle. They cost $15-25 for a basic one. You remove a sparkplug, replace it with the tester, crank the engine (preferably with fuel cut off), and it registers and holds the highest pressure it read while you cranked. Best done with a warmed up engine. The peak-hold feature means that cam lift and timing would not affect numbers - during the compression stroke of the Otto cycle, all valves should be closed no matter what (Atkinson cycle is a different story, but I digress).

If you can change sparkplugs, you can check for compression. It's that easy.

How they check for compression on diesels I have no idea...

[Jbaroli]

I had never seen such good compression values in my engine (having 134000 miles) and cylinder 2 showing 0 compression, no matter the number of strokes and cranks...
I think I have a blown valve :( (I hope instead of having broken piston rings).

Just ensure to disable fuel pump (removing its relay).

[stoney85]

also, remove all plugs and have the throttle open, and fuel disabled when doing the test

all the plugs out allows the motor to spin freely without any resistance except for the cylinder with the guage in it

the open throttle allows air into the manifold, so there is no chance of any vacuum to lower the readings,

and the fuel disabled, prevent the unburnt fuel from washing the cylinder walls of oil, which will prevent the rings from sealing as well as they could, the last cylinder will have the lowest reading as you have cranked the engine so many times and the fuel will just pool around the piston

also have the car warmed up and running when you unplug the fuel pump relay, that way the car will use what fuel is left in the lines

[5senses]

A compression tester is basically a pressure gauge with a peak-hold feature and a sparkplug-sized nozzle. They cost $15-25 for a basic one. You remove a sparkplug, replace it with the tester, crank the engine (preferably with fuel cut off), and it registers and holds the highest pressure it read while you cranked. Best done with a warmed up engine. The peak-hold feature means that cam lift and timing would not affect numbers - during the compression stroke of the Otto cycle, all valves should be closed no matter what (Atkinson cycle is a different story, but I digress).

If you can change sparkplugs, you can check for compression. It's that easy.

How they check for compression on diesels I have no idea...

Good new 2 my ears... thanks

Wes


also, remove all plugs and have the throttle open, and fuel disabled when doing the test

all the plugs out allows the motor to spin freely without any resistance except for the cylinder with the guage in it

the open throttle allows air into the manifold, so there is no chance of any vacuum to lower the readings,

and the fuel disabled, prevent the unburnt fuel from washing the cylinder walls of oil, which will prevent the rings from sealing as well as they could, the last cylinder will have the lowest reading as you have cranked the engine so many times and the fuel will just pool around the piston

also have the car warmed up and running when you unplug the fuel pump relay, that way the car will use what fuel is left in the lines


Thanks for the extra info..

Wes

[ArnZ]

My old e21 m20 had 150psi across 5 of the 6 cylinders, one cyclinder had very low compression, turned out one of the valves was incorrectly adjusted. Fixed the problem and compression went straight up to 149psi. So something as simple as that can cause bad figures.

[M Quick]

A compression tester is basically a pressure gauge with a peak-hold feature and a sparkplug-sized nozzle. They cost $15-25 for a basic one. You remove a sparkplug, replace it with the tester, crank the engine (preferably with fuel cut off), and it registers and holds the highest pressure it read while you cranked. Best done with a warmed up engine. The peak-hold feature means that cam lift and timing would not affect numbers - during the compression stroke of the Otto cycle, all valves should be closed no matter what (Atkinson cycle is a different story, but I digress).

If you can change sparkplugs, you can check for compression. It's that easy.

How they check for compression on diesels I have no idea...



You never tought of that an incorrectly adjusted valve could cause different numbers, eh? But if everything is set up right, sure, i agree with you. However you are wrong, as you always have to put in the factor of that something may be wrong. "is that easy" SA..

[moroza]

:rolleyes Ok, if there are mechanical problems with the engine, like valves that won't close because someone adjusted the lash too low, all bets are off. I wasn't referring to that, and either way, the measurement would still be correct - still tells you you're low on compression. Up to you to find the reason.

Yes, it really is that easy. Productively applying the result to diagnosis and subsequent repair of whatever you're fixing... is not necessarily that easy, and out of the scope of this thread.

[M Quick]

:rolleyes Ok, if there are mechanical problems with the engine, like valves that won't close because someone adjusted the lash too low, all bets are off. I wasn't referring to that, and either way, the measurement would still be correct - still tells you you're low on compression. Up to you to find the reason.

Yes, it really is that easy. Productively applying the result to diagnosis and subsequent repair of whatever you're fixing... is not necessarily that easy, and out of the scope of this thread.


An unadjusted valve is just an example.

Q: Why do you use a compression tester?
A1: To see if all is OK.
A2: To see if something is wrong. (like bent/unadjusted valves / bad/damaged piston rings / leaking HG etc etc.. )

If you're gonna assume that you always get "correct" readings (as you said, and i quote you).


the peak-hold feature means that cam lift and timing would not affect numbers - during the compression stroke of the Otto cycle, all valves should be closed no matter what

As you're also using the compression tester for A2, you cannot assume that the otto cycle is trouble free.