Supershaft - Good! Just wondered where you stood with that. No Googling?? C'mon! That's today's library! Not saying we should hang our hats on info we get from Forums, but there are various other reliable sources for tech info out there.
Lornce - I can elaborate a bit. It relates to the science of "kinematics", which is the study of relative motion. In this case, the study would be "Crank and Slider" relationships; the piston being the "slider". The reciprocating components part of a base engine design includes consideration of rod length, along with bore & stroke. In fact, race engine builders will change rod length depending on the track in some top-level series (like Sprint Cup) to change power delivery. Or...errrr...best power!
Basically, it goes like this. If you plotted the accelerations (positive and negative) of the piston as the crankshaft rotates at a constant speed, you know it stops at the top and bottom to change direction (the plot looks like that of a sine wave). The piston speed has accelerated to max velocity at 90º BTDC and ATDC in every case. Varying the rod length changes the periods of dwell at the top and bottom (longer rod, more dwell at the top and bottom), and max velocity at 90º.
We generally set or specify ignition timing in crank angle degrees. We do that because it's an easy dynamic reference using a strobe. What we really think about is time...time to combust percentages of the charge. Once we've established the environmental conditions for the combustion (fuel type, A/F ratio, temperature of the chamber and inlet, compression, etc.), the burn takes a fixed amount of time. That's why you need more advance as RPMs increase. Anyway, using a longer rod means that the piston is closer to the top of its travel as the crank passes from 10º BTDC to 10º ATDC (a fixed time at any given RPM). This means you're wasting less energy compressing the expanding gases BTDC, and building more pressure before the piston starts heading down. So, to get to Supershaft's point, you can set the timing later with respect to CA degrees because the time the piston dwells at TDC (and after) for each CA degree is longer at any given RPM
There are also mechanical impacts to all this. First, leverage. Using a longer rod means the crankshaft has more mechanical advantage over the piston as it rotates toward TDC. This is good since you will lose less crankshaft speed and waste less energy pushing the piston up after the mixture lights and the gases start expanding. (Note: I guess it's time to say that crankshaft speed is never constant, so the time to turn the crank 10º isn't the same at 10º BTDC as it is at 90º ATDC). It also means the piston has more mechanical advantage over the crank immediately ATDC, also a good thing. However...there's the bad, too. The bad is that the piston speed at 90º BTDC and ATDC is higher...and piston speed is a limiting factor in engine design. It also means the piston accelerates at a higher rate to that higher speed, since it starts moving later in CA degrees ATDC. That means it begins to move away from that expanding charge faster (remember, there's inertia in that crankshaft), resulting in a more rapidly declining pressure in the chamber (and less force on the crankpin through those degrees) compared to a design with a shorter rod. That's not a good thing.
So it's all about compromise. When engineers design an engine, they design it for its intended use. These are basic piston engine fundamentals, and they can be adjusted. I hope this was worthwhile to someone...Lornce, did it make any sense???