Here's another way to find the compression pressure at cranking speed. You would think that 16:1 would be 16 ats or 16x14.7=235psi, but its not that simple.
Compression ratio x percentage of cyl. volume that is being compressed.(you're not compressing the entire volume, because the intake valve is still open at the bottom of the compression stroke and some of the air escapes at cranking speed) x 14.7psi ( atmospheric pressure at sea level) x 1.4 for thermal expansion because air is heated when its compressed.
Here's how it would look for a Major. I will use 90 percent for the volume being compressed, because I don't think that the intake valve is open very far into the compression stroke. It could be less than that. I'll try to figure it out some day.
Whilst I have no idea what the opening and closing valve timing sequence is for the FMD camshaft, I do know that the inlet valve overlap is quite some way less than that for a petrol engine, since air has less mass than a petrol/air mix, with the latter overlap in the region of 50-60 ABDC. The reason for any overlap is to take advantage of the continued momentum of the 'gas' flow into the cylinder even though the vacuum causing it, by the pistons downward motion, has ceased. Any well engineered camshaft profile will ensure that the inlet valve is fully closed just before this momentum is overcome by the rising cylinder pressure. I would therefore suggest that the 90% cylinder fill could be amended to 98%, bearing in mind the piston and head combustion area. With that in mind, and using 16.1 as the compression ratio, the cylinder PSI would be in the region of 325 PSI, which relates closely to the 190 RPM cranking pressure of 300 PSI.
You're probably right about 98 percent, Pavel. When I said that its probably less than that, I meant that the intake valve is probably open for a smaller part of the compression stroke.
Even if you know how far up the piston is when the intake closes, you would still need to know the combustion chamber volume to know how many percent of the volume is being compressed.
It would be pretty hard to figure the exact cranking pressure, but it will give you a fairly close estimate.
