Mercedes-Benz Forum banner

Compression test at 5,685ft. altitude?

4K views 44 replies 7 participants last post by  Anovak 
#1 ·
Looking over the compression numbers, I found that the M110 engine (280SL) should produce a 9:1 compression in all cylinders. That would be 132.3 psi. Looking at some tables that show the influence of altitude on compression, I found that at 5,685ft. (altitude where I live) atmospheric pressure drops to about .853 of what it is at sea level. Does this mean that after doing a compression test, one would divide the result by .853? E.g., if the readings were all at or about 120 psi., then the accurate figure would be 120/.853 = 140.68 psi?

Just want to get the info so I can do an accurate compression test. Air temp would also seem to affect things, any other factors? Thanks.
 
#2 ·
No compensation necessary. The atmospheric pressure on the other side of the gauge, is reduced also. Absolute cylinder pressures are not as important as relative readings between cylinders. Also you will always be below atmospheric pressure in the intake manifold. Remember to hold the throttle full open during testing, and I think 120ish, PSI would be a good number. Do the test, and report.
 
#4 ·
Yes the actual cylinder pressure will be reduced at higher altitudes.
So 120 psi at sea level will be 120 x 0.853 = 102 psi at 5700 ft.
But as @hotshoe pointed out, your main interest is consistency among the cylinders, not actual pressure.

Theoretical cylinder pressure is a complicated calculation, not just a matter of multiplying atmospheric pressure by the engine's compression ratio, which is simply the ratio of the maximum to minimum volume in the cylinder as the piston cycles. Valve timing and size are involved too.
Actual cylinder pressure is further affected by losses past the valves and rings, and engine temperature.
 
#5 · (Edited)
That sounds right in principle.

On any particular day, exact correction will depend on the actual barometric pressure and temperature as compared with the normal conditions at sea level (normal varies depending whether USA or International figures are used!) But let's say when barometric pressure is 14.696psi (29.92 inHg), temperature 15C (59F) and relative humidity is 0%. In Denver, high/low barometric pressures of 31.24" & 28.83" are quoted on line (presumably corrected to sea level). The variation is about 8%. That makes a difference as would temperature and humidity on day of testing. CORRECTION: MB say to do compression tests at normal engine operating temperature (80C) so air temperature would not be a factor.

Just do each cylinder and make sure there is not much variation and numbers are somewhere near where you calculated. If there is a variation, do a leak down test.

ADDED: Technical Data Book says for M110 that Normal pressure should be 10-12 kp/cm2 and minimum 8.5 kp/cm2. (old book! - 1kp/cm2=14.22psi) No mention of altitude. Throttle plate open. Crank engine 2 or 3 times to blow out crud, then 8 times per cylinder while measuring pressure using starter to spin.
 
#8 ·
Thanks, MBGraham! I was thinking so as well and it is nice to get some confirmation.

I'm going to do it again soon as I did not follow the procedure precisely as outlined in your previous post, namely I did not "blow out the crud with 2 or 3 cranks," nor was the "throttle plate open" as I did not touch the accelerator, just turned the key and cranked the engine (fuel pump disconnected, coil wire off....). Really appreciate your comments....
 
#12 ·
Grounding the coil wire is the generally accepted thing to do. But interesting that in the MB shop manual they don't include that as a step. (Don't have an M110 manual, but for the M116/M117 no mention of need to ground).

Wondering why grounding, instead of just removing coil wire, is often recommended? Maybe if there is no easy path, high voltage would try and find another way to ground through coil insulation or something?
 
#14 · (Edited)
I see.....we disconnected the coil wire from the distributor and not from the coil itself. Will do that next time. I mentioned previously that the fuel pump was disconnected, where I simply took off the positive wire and pulled the rubber insulator forward to completely cover the wire so it could not touch something and short. Thanks.
 
#16 ·
OK, disconnect the coil wire from the distributor and ground that end to neg. battery terminal or any other metal part of the chassis or engine compartment, is that what you are saying? Sorry, just want to be sure I'm doing this right.
 
#21 · (Edited)


whatever its suppose to be at sea level times 0.853 PSI. will be approximate.

It should be more than 132 PSI at sea level because as you compress you also heat. Look up adiabatic or isotropic compression will give you a closer answer. (Lots of thermodynamics here).

Also you cant use 9.1 for your compression ratio. You need to figure out what the compression ratio is from when the intake valve closes. It will be more like 7 or 8: 1
 
#22 ·
Roncallo, please help me understand what you mean by "You need to figure out what the compression ratio is from when the intake valve closes. It will be more like 7 or 8:1."

How does one know when the intake valve closes and how do you get the reading at that exact point?

Mercedes says it should be 9:1, which is where I got the 132.3 psi (9 X 14.7psi = 132.3). How did you come by your thoughts about 7 or 8:1?

I trust I am not taking everything into account that will affect the reading and that this math is approximate, but I'm hoping it gets me into a ballpark that is at least pretty close.
 
#24 · (Edited)
What your asking about comes as part of a full collage course in Thermodynamics. (one of the top 10 most difficult courses on the planet).

But you just need the equation.

p2/p1 = (v1/v2)^gama

For air gama is a constant 1.4, just trust me. For a 15:1 air fuel mixture a gama of 1.3 is generally accepted. For the compression test best to shut the fuel off to conduct it, at least if your going to do the science project discussed here.

You can't get the effective compression ratio without knowing the geometric compression ratio, intake valve closing time, bore, stroke and connecting rod length and doing some really elaborate math and trigonometry. rod length and intake valve closing time are difficult to get. Therefore if you can get the published compression test requirements at sea level and multiply that by 0.853 would be your best bet.

But let me explain any way.

v1 is the volume of the cylinder + the volume of the combustion chamber - the volume of the cylinder below valve closing.

v2 is simple its just the volume of the combustion chamber and when I say volume of the combustion chamber I mean everything above the piston when its at TDC.

in your case p1 = 14.7 * 0.853 = 12.54

Solve for p2

p2 = ((v1/v2)^gama)*p1

we dont need to know v1 or v2 just need to know the ratios. v1/v2 = the effective compression ratio. So lets just assume its 7 for the moment.

Plug in ((7)^1.4)/12.54 = 191.2 PSIA

As dugald pointed out that is 191.2 PSIA (A for absolute) in your case subtract 12.54 to get PSIG (G for what the gauge should read)

Let me caution you about one thing. The automotive industry uses a different way to state compression ratio and is you do happen to find a publisher value for effective compression ratio I am not sure how it is calculated.

in automotive terminology CR = Volume of cylinder/volume of combustion chamber. The v1/v2 in the thermodynamics equation if it were to be looked at as geometric compression ratio would = (Volume of cylinder + volume of combustion chamber)/volume of combustion chamber))for example a 9:1 CR engine would be solved as v1/v2 = 10. Just another little thing to piss you off.

And finally the standard disclaimer. There is no such thing as a perfect isotropic or adiabatic process. But since the motion is reasonably fast with reasonably low temperatures involved there is little time for the temperature rise to be dissipated to the block or water jacket, and thereby loosing its stored energy. Therefor the adiabatic process closely approximates the situation.

See this for a brief discussion.

https://www.grc.nasa.gov/www/k-12/airplane/compexp.html
 
#23 ·
The nominal compression ratio is simply calculated by dividing the maximum volume of the cylinder (that's when the piston is at the bottom) by the minimum volume of the cylinder (when the piston is at the top). However the air/fuel mixture doesn't actually start to compress until the valves are closed which may be sometime after the piston leaves the bottom of the cylinder, so the effective compression ratio will be less than the nominal.

And if the stated 9:1 was the effective compression ratio, the measured pressure would actually be (9 x 14.7) - 14.7 = 117.6. You have to subtract the 14.7 because your pressure gauge measures the pressure above atmospheric pressure. That's why the gauge reads 0 when the pressure is actually 14.7

Further, as JR notes, the pressure is also affected by the temperature of the mixture, which is raised as the mixture is compressed.

Confused? Blame Boyle and Gay-Lussac and be grateful you didn't have to study thermodynamics.
 
#25 · (Edited)
Anovak,
Something you could do, might be to do a Google search for mercedes, m110 and compression and see what others have measured. There are no doubt many discussions on subject. I saw some from U K that were probably done close to sea level. Only confusion was that there are apparently high and low compression M110s. Once you find a few sea level figures, just x 0.854 and there you have it:)

The test results you got at altitude compare, once converted, with the 10-12 kp/cm2 sea level figures i quoted earlier. So maybe no need to do anything more really. Did read that compression in those engines benefits from an occasional good workout !
 
#26 · (Edited)
best advice yet ^^^^


According to a place I found called PointedThree. (wont let me link it)

"FWIW, the M110 section of the G manuals I have state min compression is ca. 7.5 bar for a low compression engine, 8.5 bar for a normal compression engine. Max allowable spread 1.5 bar between highest and lowest cylinders.

I don't know how to tell whether you have a normal or low compression engine.

Good luck. "
 
#27 ·
After some research, I can suggest that my engine, the M110.986x model year 1981, had no low compression counterpart. It would appear that the low compression version of the M110 (.992) found in the 107 were not used after 1976, which also coincides with the introduction of the K-jetronic injection system that appeared in the R107 sometime in late 1975.

I cannot find the compression ratio stated until the .987x and it is 9:1. It would appear all other specs are the same for the .986 and .987 after April, 1978 so am pretty sure this is the correct ratio. Info gleaned here:

https://en.wikipedia.org/wiki/Mercedes-Benz_M110_engine#M110.986

Thanks, Roncallo, got all that.......:D

As for trying to find some results for compression tests others might have done on the .986x M110, no luck so far.....let me know if someone finds some. Thanks, all.....
 
#29 · (Edited)
Fonzi, if you go to that link I provided in post #27, you can scroll up and and down and find every M110 and how they were either carb, d-jet or k-jet and where the low and high compression engines occurred, even the models are listed.

The M110.92x and .93x engines are carburetor engines, with Solex 4A1 carburetor.

The M110.98x and .99x engines are fuel-injected engines, with Bosch D-Jetronic up to the .983 and K-Jetronic from the .984.
 
#30 · (Edited)
This site confirms that 986 has 9:1 CR.

Looks like all M110s from 984 to 990 installed after April 1978 had same HP and torque, so presumably same basic engine? Also interesting that both 986 and 990 were fitted exclusively on 280SL/SLC. Any idea what difference was? OK the link below says it has 8.7:1 compression ratio on the 990 - that must be one difference.

The site linked above has many Mercedes engines listed (they provide a little info free, but want you to pay for more!)
Engines for MERCEDES - Enginedesk
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top