Perhaps this element has already been discussed and I missed that section of the thread.
Rod length, rod angle, piston location in cylinder relative to crankshaft position.
I built an OPS 40 years ago and put a Super Tigre G-40 rod in it. Same crank journal size and wrist pin size only the ST rod was shorter. Seems like it was quite a bit shorter, something on the order of 0.150" shorter center to center. The sleeve was dropped in the crankcase to retain stock exhaust port timing.
It ran really well, considerable improvement in performance with same pipe, intake, head and tuning.
Thinking about rod length -vs- rod angle -vs- piston velocity as the crankshaft rotates. Seems like starting at TDC the shorter rod will begin it's descent faster due to the rod angle increase - maybe not much, but there is a difference. As well, once the exhaust and transfer ports are open, the piston will be traveling slower with a shorter rod length due again to decreasing rod angle.
Measuring timing in degrees of crankshaft rotation simply means the port location will be different with rod length changes. I don't know, is it a factor to consider?
Any of you guys mess with rod length? Piston position at BDC relative to crankcase volume and volumetric efficiency would certainly be a consideration.
Hey Steve, interesting point
As far as i can tell, the general consensus is that a longer rod has better overall performance characteristics.
One of the main points cited for the benefits of a longer rod is a reduced rod angle, this leads to less side forces of the piston against the liner, reducing friction and heat as well as wear.
Additionally, with a longer rod you will have a slightly longer dwell period at TDC. The benefits of a longer dwell period are mainly around combustion speed.
If you consider that at the point of combustion, it takes a given period of time for “flame front” to propagate from the point of combustion. If you like, this is the expansion rate of the explosion in the cylinder.
Also consider that with our engines, at very high RPM’s the piston will actually be moving down before the flame front can fully propagate, this leads to a reduction in torque and power
if you have ever looked at a dyno graph you will see that torque and horse power tail off in the higher RPM range. This is largely why it happens. In effect the combustion process become inefficient due to the design.
having a longer dwell period helps improve this.
Incidentally, this is also why many manufacturers have moved in the direction of a longer stroke/smaller bore. Because in a smaller bore the flame front has a smaller distance to travel before hitting the liner edge before forcing the piston down.
The end result is holding torque higher up the RPM range resulting in more HP.
You can see this in action with Novarossi .21 engines, over the years they moved to a longer and longer stroke, the last variants having a bore of only 15.88mm, previously they were 16.26mm
Even with a bore of 16.26mm they had the longest stroke .21 engines.
Hopefully that makes sense?