compression specs
 

I have tested for compression on my 1990 350 SDL with 220000 miles. I have a range of 320 psi to 420 psi. What am I to expect on this motor?

300 to 320
 

is correct. the higher readings indicate carbon build up.

tom w

how do you do a compression test? and what is good for a 560SL

ive heard of this test before

Tom, is that right? I thought 400 was normal. Spec is 26-32 bar which is 377-464 psi with 3 bar or 43 psi difference between cylinders. Min is 18 bar or 261 psi.

Chain, maybe a leakdown test to provide more info. That much spread isn't good.

Matt, I don't have V8 specs. For a similar vintage 300E normal is 175 psi, 110 psi is low, difference should be within 20 psi. Specs for a 560 won't be much different.

Sixto
95 S420
87 300SDL

thanks Six,


so how do you check it?

With a compression tester. It's a device that fits into the spark plug hole. You crank the engine and read compression in a gauge.

Typically you cut power to the fuel pump, remove all spark plugs, wedge the throttle full open and fit the tester to each cylinder one-by-one noting the max pressure reading after about 10 full cranks. There's a rhythmic sound to cranking. Count 10 cycles of the rhythm.

The easier gauges to use thread into the spark plug hole and have a peak hold feature. Don't forget to release pressure before removing the gauge. Other, usually cheaper, gauges have to be held against the spark plug hole and read while cranking. When cranking stops the gauge goes to zero.

Sixto
95 S420
87 300SDL

Sorry about the bad link.

Here's a picture of a compression tester.

Sixto
95 S420
87 300SDL

well
 

some folks here say that 400 is normal. but i dont see how. air pressure at sea level is 14.7 psi. compression on a 616/617 21.5 to one, on the 603 it is 22. multiply 14.7 x 22 = 323.4 ? right? and as u go higher in elevation the air pressure drops. i dont have a table handy but here in lafayette we are at about 700' above sea level. i am guessing about 14.5.

so the only way i see to get to 400 is carbon build up in the combustion chamber raising it.

tom w

Physics was a long time ago but something doesn't sound right in multiplying 14.7 psi-absolute by compression ratio to come up with 323.4 psi-gauge. It's possible that the non-linearities of air come into play. Air is not infinitely compressible.

Sixto
95 S420
87 300SDL

as further
 

evidence, my 350sdl was tested recently. the compression was 300 on 1-4 290 on 5 and 280 on six. it runs fine, gets the mileage it is expected to get, has lots of power and starts right up. if it was supposed to have 400 psi compression, i dont think it would be running and starting fine.

tom w

the formula
 

does have exceptions. if you have valve overlap you can have a mechanical compression ratio of perhaps 12 to one and because the valves are held open over into the compression stroke have a resulting, ratio of say, 10 to one. this is to say that while the compression stroke is started instead of haveing both intake and exhausr valves closed, one is held open for the first part of the compresssion stroke. as i understand it the inertia of the charge coming into the cylinder helps to evacuate the exhaust and results in a more volitile combustion event. (more power, at the expense of less mileage).

this type of cam overlap is strictly for gas engines and high performance ones at that. (dont think it would work for a diesel, anyway).

so the conclusion is that you can have a compression pressure reading that is nonconforming to the multiplication and reads lower than the calculation but not higher, unless there is carbon buildup, or the head has been milled too much reducing the volumn of the combustion chamber.

tom w

It's been a while, but the ideal gas law is:

PV=nRT

The ratio you are using assumes the T (absolute temperature of the air) remains constant (i.e., the temperature after it is compressed is still ambient). If the temperature has increased due to compression the the (absolute) pressure will increase by the ratio of the (absolute) temperature change. This may account for some of the difference.

A little more info. For a 617 turbo the compression specs (at operating temperatures) are given on this page:

http://skinnerbox.steaky.org/Service/W123/w123CD2/Program/Engine/617/01-010.pdf

Normal compression pressure: 24-30 (about 348-435 psi)
Minimum compression pressure: approx. 15 (about 217 psi)
Permissible difference between cylinders: max. 3 (about 44 psi)

Hope this helps.

so what
 

is a bar? in psi?

tom w

so you'e
 

saying with the formula that because of the heat generated that the compression actually increases because of the heat? and i suppose the in a gas engine the phenomonan is not applicable because of the much lower compression levels? if so then how much less is it if tested cold?

tom w

A bar is about 14.5 psi, almost one atmosphere.

Yes, the heating of the air due to compression will increase the pressure by more than just the compression ratio. This will also occur in a gas engine, but to a lesser extent. I would guess that the cold test pressure would be less than the published hot values, but I do not know how much compressing the air in a cold engine would heat it. I think we need the specific compression test specs for this engine.

I was just trying to explain how the pressure rise in the cylinder could be greater than the compression ratio (which is just the ratio of volume).

compression specs
 

The reason I checked the compression and started all of this discussion was the number 1 cylinder is not firing. I am getting unburnt fuel out the tailpipe and it is running poorly. I have replaced the head gasket and am thinking it may be a timing issue. I had a small compression leak on # 6 cylinder.

If you're not inclined to do a leakdown test, can you characterize the compression test? How many strokes to reach max psi for each cylinder? If #1 is leaking then presumably it'll take more strokes to get to max psi. But that's a goofy way to do it. Do a leakdown test. Doesn't your compression tester have a fitting you can hook up to a shop compressor?

Before that, swap #1 injector with another cylinder. Does the problem shift to the other cylinder?

You can eyeball IP timing. Read the crank pulley timing mark when the IP timing lug is visible through the IP locking tool port. A mirror and small flashlight will help.

Sixto
95 S420
87 300SDL

compression specs
 

Next step is to do a leak down test. Have checked the IP timing and it is set at 15 degrees ATDC by lining up through site opening.

compression specs
 

Also I did switch injectors and no difference.

compression pressure vs temp
 

THINK for a few minutes guys! A diesel engine is COMPRESSION IGNITION--which means the air in the cylinder HAS to be REAL HOT to ignite the atomized fuel. With a '21:1' compression ratio, I would expect peak temp of around 800F.

What happens: if measured compression pressure is LOWER, then the air is not as HOT and fuel spray may not ignite. MB uses the 'glow-plugs', which run very hot to ensure ignition.

A miss-fire may be cause by a dead glow-plug-along with poor injector spray pattern and/or low compression. Sometimes the injector nozzle gets plugged and it shoots out a solid stream of fuel--which is almost impossible to ignite--or doesn't fire until you get to higher RPM, or fires 'late' and knocks real hard (nailing its called). The injector nozzle pintle has a very tiny hole lengthwise, intersecting with an extremely small cross passage, and this greates the very fine 'fog' that is necessary for good ignition. Rebuilt injectors from Dealer and +/$60 or so. DO NOT remove them and then try and clean them yourself--you will destroy them.

if the timing is off
 

it will affect all cylinders the same. so a miss on #1 could not be because of timeing problems.

tom w

"so what is a BAR "
 

ambient air pressure at sea level (14.5 Pounds Per Square Inch)

so if the comp spec
 

is 24 to 30 bar then that is about 360 to 440 psi, right? yes, i guess that is what several said earlier. so now i need to ask my indy if he checked my compression on the sdl cold or at 80 degrees.

tom w

How do you check compression with the engine at operating temperature uniformly throughout the test? In the time it takes to pull the injectors and take 6 readings the temperature would have dropped significantly. Probably risen some unknown amount then dropped some unknown amount :/

Sixto
95 S420
87 300SDL

you can clean them, I would clean mine every couple of years and never put new ones( I had the car 14 years) in, sold it with 250000 still ran like when I got it. Keep a eye on timing chain as mb sell offset timing keys to keep the timing right and keep the valves adjusted

compression specs
 

I tested the compression on a cold motor.

now
 

i looked in my old chiltons from 1970. curious. the compression for a 200/220 is 21 to one. psi is listed from 284 to 327. so not saying anything, i assume that this is a cold test. and apparently the heat thing increasing the psi may have a critical point where it increases the pressure dramatically since the warm 80 degree test for the 603 with a mechanical ratio of 22 to one is 343 to 400 something.

additional food for thought.

tom w

i just looked
 

in my MOTOR imported repair manual from 1981 for compression specs. it says for any mercedes from 72 to 81 the cranking compression should be 313 to 331 with a minimum of 213 psi. now were up to the years for 21.5 to one compression ratio.

i thought of another reason for higher cranking pressure for the newer motors... better rings and better metal.

more food for thought.

tom w