Compression tests. - PeachParts Mercedes-Benz Forum

Deni · #1 10-03-2007, 04:55 AM

If I perform a compression test, how do I know which compression is good, which one is not? Can a find a chart for the compression values of different engines?

#2 10-03-2007, 05:17 AM

450+ is extremely good.
350-450 is good.
300-350 is worn.
220-300 means it's extremely worn and likely has a very hard time starting.
Lower than that and it won't even compress enough (make enough heat) to ignite Diesel.

MB's official specs are 24-30bar (350-435psi) is normal, 15bar (217psi) minimum. 3bar (43psi) is the maximum variation between high and low results.

Deni · #3 10-03-2007, 05:40 AM

What about other engines? Does the compression ratio have to do anything with the compression ratio? I.e. I know my engine has a compression ration of 22.5:1. Does that mean that a good compression reading is 22.5 bar or 330.75 psi?

#4 10-03-2007, 06:08 AM

Compression ratio does not determine compression PSI directly. Compression ratio is the ratio between the volume of the cylinder when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke.

To determine the engine's actual PSI you must measure it.

Deni · #5 10-03-2007, 07:36 AM

Quote:

Originally Posted by ForcedInduction

Compression ratio does not determine compression PSI directly. Compression ratio is the ratio between the volume of the cylinder when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke.

To determine the engine's actual PSI you must measure it.

The reason I'm asking is because I'm helping a mechanic with some stuff, as he might help me with tunning my car, or rebuild my engine if somehow

, I kill it.

I ordered a compression testing kit from the US for him, but need to find a chart with values for diesel engines. Do you have any idea where I can find a chart where it states the compression different engines should have?

Thanks!

Deni

DZL_Damon · #6 10-03-2007, 02:05 PM

Like Lance said... It should be above 300psi, and within 10% of each other is what I've read somewhere else. My truck got up to 495 psi, lowest 460psi, but thats a bigger engine (7.3L). My Benz was around 350 I think... not GREAT numbers but acceptible, but it runs great so I don't care. There were no really low numbers to alert me to a problem, so I think she's fine.

vstech · #7 10-03-2007, 02:44 PM

with gassers it's even pressures you are looking for, and anything over 80 is fine to run. with older diesels you need over 250 evenly or 300+ as long as none of the cylinders are less than 30psi lower... for a new DI low compression motor, I think you need like 200+ and perfect balance between the pots. look for dry pressure tests then a small bit of oil in the cylinders to see if the rings/cylinders are leaking oil in the pot will slightly raise the pressure if it's in the pot, if the pressure drop is from the valves, oil will make no noticeable difference.

Deni · #8 10-03-2007, 06:10 PM

Thanks a lot! Really appreciated!

t walgamuth · #9 10-03-2007, 07:08 PM

Quote:

Originally Posted by vstech

with gassers it's even pressures you are looking for, and anything over 80 is fine to run. with older diesels you need over 250 evenly or 300+ as long as none of the cylinders are less than 30psi lower... for a new DI low compression motor, I think you need like 200+ and perfect balance between the pots. look for dry pressure tests then a small bit of oil in the cylinders to see if the rings/cylinders are leaking oil in the pot will slightly raise the pressure if it's in the pot, if the pressure drop is from the valves, oil will make no noticeable difference.

cold numbers run lower. down to about 220 is acceptable. ON a warm engine up to over 400 is really good.

Tom W