Andy,
There are many interactions that are altered by changing the rod/stroke relationship. In mechanical terms the side loading of the major and minor thrust axis of the cylinder wall has its effects. Other important factors in R/S ratio are the changes in piston acceleration and the point of peak piston velocity. This will require changes in the flow path and valve events to optimize the engine. The most important, in my opinion, is the relation of peak torque to peak HP when the R/S ratio is changed. In apples to apples comparison as the R/S ratio numerically increases (longer rod) the spread between the peaks tends to close. The case could be made, in certain applications like a two-speed bracket car or a marine engine, that by not using the longest rod possible and keeping a larger spread from the peaks could be a benefit.
Below is a link containing interactive java applets (upper left of page) that allow visual and graphical data display changes in rod length. Used extreme changes in rod length to better see the changes in piston motion.
http://www.wfu.edu/%7Erollins/piston/
http://www.stahlheaders.com/Lit_Rod%20Length.htm

Bob

Very informative links RM, thanks!

If you read stahlheaders post it lists short and long rod ratios.

Just FYI todays NHRA prostock engines typically run ratios between(1.65 and 1.80) that's for a high winding engine that is trying to have every parasitic loss accounted for. The average stroke in these engines is 3.600. I guess this would fall into his view of a short rod engine.

With a 9 inch deck,they run a little over 6 inch rods in those motors.

I agree with JCPerf, when NHRA banned ceramic composite rods I got to see what Jager had been shipping some teams. Aluminum rods in the Low 6" range weighing around 415gms.

Chris