[olddog]

A while back, I built a spread sheet to calculate piston position at a given crank position. At the time I was interested in the the thrust vector angle that put power to the crankshaft with different rod ratios, but I digress.

To my surprise, I found that at 90 deg BTDC or ABDC (same position) the piston is not half way in the bore. It is actually more than half way down. Depending on the rod/stroke ratio, the half way point occurs around 80-83 BTDC which is 97-100 ABDC.

On a 4.0" stroke 6.2" rod at 90 deg ABDC the piston is 2.331" down. With a 4" bore and 10:1 static compression ratio there is 5.585 ci above the piston at TDC. So if the intake valve closes at 90 deg ABDC, you would have 34.883 ci above the piston. So the dynamic compression would be 6.25:1 not the 5:1 that you would expect (thinking the piston is half way).

This all assumes that I did all the geometry correctly, and I think I did, but I have made mistakes in the past. I hope this is useful to the subject.