It was/is a hypothetical question. An engine with a given bore and stroke, everything else the same except connecting rod length.
Back in the day, Smokey Yunick was a long rod avocate. But he was mostly involved in restricted roundy-round and engines with stock type
cylinder heads.
A shorter con-rod will move the piston away from TDC sooner and dwell it longer ABDC, effectively increacing the duration of the intake stroke.
Yep, ps valvetrains take a beating, but so do tf, but in a different way. PS is with lift and rpm, TF with its instant launch rpm from an idle
and opening the exhaust valves against the high cylinder pressure of a late burning double based fuel. (Think of a double base gun powder. It takes heat to release the O2 to sustain the burn.)
Here are some comparative graphs of a hypothetical pro stock engine with a 4.72" bore x 3.57" stroke. I have done calculations with rod lengths giving a length/stroke of 1.5 and 2.0, basically in the range where race engine builders normally work, and giving a wide range of value to show the major trends. My spreadsheet calculates piston velocity, acceleration, and side thrust. The calculations are all done at 10500 rpm.
Here is piston velocity with the 1.5 rod ratio. Peak velocity is 172 ft/s, and is at 73 degrees ATC.
Here is piston velocity with 2.0 rod ratio. Peak piston velocity is 168 ft/s, occurring at 77 degrees ATC. Considering the large difference in rod length, there is not much effect on piston velocity, only about 2.5%. Consequently there would not be much effect on inlet port Mach number and engine breathing.
Graph of piston acceleration with 1.5 rod ratio. Peak acceleration is -7447 G's (downward) at TDC.
Piston acceleration at 2.0 rod ratio. Peak acceleration is -6982 G's. This is about a 6.2% decrease in rod tension and peak rod bearing load at TDC overlap. But since the longer rod has more mass, it can't be assumed that there will be less bearing load (and friction). Keep in mind that this is a rod that is 1.785" longer.
Graph of rod side thrust with 1.5 rod ratio. Peak side thrust is 1135 G's, with an absolute average of 676 G's.
Rod thrust with 2.0 rod ratio. Maximum rod thrust of 774 G's, with an absolute average of 479G's. I think this is where the true difference to engine power would be; the average piston side thrust on the cylinder wall decreases by 29% with the longer rod. Whether or not the increase of power due to decreased piston friction balances out the increased engine weight on vehicle performance is another matter. That would mostly depend on if the car could still get down to class minimum weight.
Questions? Comments? Or have I stepped on the cake again?