I understood most of that diagnostic procedure posted above with the air fittings and such, but I'm confused on one point. What does the cranking speed have to do with compression?

Compression is the result of squeezing a fixed volume of air (what the cylinder can hold) into a fixed space (The remaining space when the piston has reached the very top of its stroke), true? The air volume is going to be the same, the cylinder capacity is going to be the same, and the top point of the piston's travel is going to be the same no matter how quickly or slowly it reaches that point. So why is having steady cranking speed so important for a compression test? Theoretically it would make the same compression even if you turned the crank with a wrench instead of the starter -- UNLESS you've got wear letting some of that compression pressure leak down as it's produced.

What am I missing?

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL