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