Well I started thinking about this the other day when posting on the other thread ... and I suppose I'll start with a brief brief on the relationship between a connecting rod and it's crankshaft because I think it's important to engine acceleration.
It seems simple at first to think that when the crankshaft is at 0 (top dead center) ... with the piston is all the way up ... and when the crankshaft is at 180 (bottom dead center) the piston is all the way down. That part's easy.
What happens when the crankshaft is at 90? You'd think that the piston is halfway down the bore, right? Well that's not exactly true. The angularity of the rod at that point brings the piston down a little bit further - the connecting rod length is the hypotenuse of a triangle where the short side is the sideways distance from the center of the cylinder to the center of the crankpin.
What does this mean to an engine tuner? Well ... there are several issues to discuss: ignition timing, combustion chamber growth rate, vacuum production, camshaft events, etc.
The first thing I'd like to address is the chamber growth rate. Because a piston in a 1.5:1 rod ratio motor moves down from TDC a little bit quicker in a given amount of crank rotation, a short rod combo will increase the chamber size quicker, creating more vacuum, bringing greater pull through the intake port, setting up a high velocity charge for later on in the stroke. Because a short rod combo pulls harder through the ports, it will develop torque at a lower RPM than a long rod motor ... all else being kept equal. At higher RPM, it will run out of breath sooner since it is pulling harder through the ports ... so a long rod combo should make more horsepower by making more torque higher up in the RPM range.
God I love this stuff ...
Another issue is that because the 1.5:1 motor's piston moves faster near TDC than does a 2:1 motor ... the intake valve opening should be given consideration. Because the 1.5 motor develops stronger vacuum ... the intake valve could be opened later to take advantage of the greater pull. Or, because the exhaust gasses are being forced out a little harder (piston approaching top of stroke quicker) one of two things could happen: Either there will be a higher residual pressure in the combustion chamber because there is a bottleneck downstream in the exhaust and the intake should be opened later to allow the pressure to equalize; or there will be a greater scavenging effect from the higher velocity exhaust gas in a header primary, allowing the intake to be opened earlier.
Generally ... I believe and could be wrong ... the short rod motor can deal with greater valve overlap than a long rod motor.
Because the long rod motor's piston moves away from BDC faster than does the short rod motor's piston, the intake valve MUST be closed sooner in a long rod motor ... or else you run the risk of pushing the fresh charge right back out the intake port and losing dynamic compression. The exhaust valve should also be opened later.
Chamber growth rate is something that should be very interesting to people looking at their ignition: the 1.5:1 motor's combustion chamber grows quicker leaving TDC than does a 2.0:1 motor, which allows more room for the expanding combustion gasses, which lowers the early cylinder pressure slightly. This means a short rod motor needs slightly more ignition timing to develop peak cylinder pressure earlier than in a long rod motor. Furthermore, because the piston is moving fastest when the rod and crank are at 90* from one another, and because that happens sooner with a short rod motor, higher cylinder pressure at that point will create the greatest leverage on the crankshaft.
A friend of mine Matt and I talk for days about this ... and it is the basis of the engine and it's acceleration IMO ... one thing to remember is that an engine is an air pump it sucks it in and spits it out ... it's all about how easy you make it happen.
My fingers are now tired.