we discussed something like this in our dynamics class. and a longer rod makes for faster piston travel away from and towards TDC, along with longer dwell time at TDC and BDC.
The shorter rod has less dwell time, and slower piston speeds to and from TDC. Basically, a long rod yanks the piston up and down faster, and lets it dwell longer, while the short rod moves the piston more gently, but doesn't let it stay at TDC as long.
It's all in the math, as the longer the rod is, the less the throw of the crank perpendicular to the motion of the piston effects the motion of the piston. If you somehow had a rod of infinite length, the motion of the piston with respect to time would look like a sine-wave.
If you had a rod equal to the length of the throw of the crank, the piston would travel at a constant speed at all times, and instantaneously change direction at the top and bottom. (which doesn't really matter, since the machine wouldn't work at all in that case, but it was just an illustration to show the extremes.)
