Cold compression with first oversize

[tcobbett]

Hi everyone, I had the engine on my 1971 280 SL rebuilt about 7000 miles ago including going to first oversize cylinders, pistons, and rings. I'm getting ready to sell the car, and had a cold compression test done where the numbers were 100 98 100 105 108 and 108 PSI. I've seen references to compression being higher on other 280 SLs and I'm concerned potential buyers may think the compression is low. I'm hoping you might be able to help explain the difference.
I found this info in the MBEngineSpecs document:
General specifications say 9.5-1 compression ratio. 3.4 inch bore, 3.1 inch stroke.
Standard piston diameter for the 280 SL/8 engine is 3.4047-3.4055 inches, and first oversize diameter is 3.4440-3.4448.
Can anyone tell me what acceptable compression numbers should be for my car? Should I be trying to calculate a compression ratio, and if so what is the formula? Should the torque or horsepower be different with first oversize parts?
I'd be grateful for any information you can offer to help provide accurate helpful information to potential buyers.
Thanks, Tyler

[ja17]

Re-take the compression test with a different compression gauge. Often times an old gauge will not be accurate. Compression readings are normally taken with a warm engine.

[Benz Dr.]

I take most of my compression readings on a cold engine because that's what you will have when trying to start after sitting overnight. Warm engines usually start up more easily, while cold ones often won't if you have any problems.

Normal compression on a fresh 280SE/SL engine should be in the 165 - 175 PSI range. About 150 - 155 PSI if you had to use stepped pistons. Sounds like your gauge is broken because your engine probably wouldn't even start with such low numbers.

[tcobbett]

Thank you ja17 and Benz Dr. Will have it redone.

[Cees Klumper]

Also make sure to have the throttle wide open when cranking.

[MikeSimon]

The standard compression ratio on the 1971 280SL was 9.0:1, contrary what Mercedes officially said. With a first oversize, your compression ratio goes up a little, so it may be safe to assume, you should be around the given spec. My worries would be the significant variation of 10PSI between 2 and 5/6.

[tcobbett]

Thank you very much, MikeSimon. I just had the valves adjusted to .004 in and .008 out and I'm waiting to hear the new compression numbers. Could you tell me how to calculate the new compression ratio?

[ja17]

Atmospheric pressure (14.7) times compression ratio 9.0 would be the factory new specified compression. Changes in piston size and configuration would change the factory compression. In addition, milling of the cylinder head decreases the volume of the combustion chamber which would increase compression. Lastly any carbon build-up in the cylinders and combustion chambers would also raise compression. Exact compression figures could be calculated by measuring the volume of the combustion chambers compared to the piston displacement.  Not easy to do at home. Speed shops will often calculate and match compression in all cylinders by measuring these volumes with a liquid, when they "blue print" an engine.

[tcobbett]

Thanks, Joe. With the valves adjusted the compression is now 125 psi in all cylinders, cold.

[rwmastel]