Originally Posted by greglewis
How does a longer connecting rod fit into the picture? If the rod is lengthened to move the top of the piston to the necessary height (and proper compression), doesn't that also decrease the displacement (piston higher at BDC)?
It's probably obvious to everyone but me - still, a little more education will help.
Stroke is determined by the diameter of the circle travelled by the bottom end of the connecting rod. The closer the big end is to the center axis of the crank, the lower the stroke. If you reduce the radius of the the circle travelled by the big end you need a longer connecting rod to get the piston back up where it needs to be. Just draw a picture of a crank, connecting rod and piston at TDC. Then imagine moving the lower end of the connecting rod closer to the center axis of the crank while keeping the piston in the same place.