Another point of contention is that long rod engines don't really have much of an advantage in dwell time, and lose the advantage of peak angle when it counts.

The short rod engine puts the unit force vector closer to the perpendicular to the crank at an earlier point then a long rod engine. For example, in this simple drawing (MS Paint OWNS!):


The rod angles are the same at TDC, but the rod moves towards the tangent of the rod journal's path radius quicker then the long rod engine. In other words, the piston will push harder on the crank earler then the long rod motor will. At "just after TDC," the short rod engine will create a higher torque moment, based on the force vector. At "way after TDC," neither will have a real advantage, and at "90 degrees past TDC," the long rod motor is optimal, as the rod angle from the piston to the rod journal is a smaller number of degrees. Unfortunately, most of the force exerted on the crank occurs right after combustion. This gives the advantage to the short rod motor. Even at high RPMs, there will not be a high amount of force on the piston near 90 degrees. This means that not only do you lose the force vector advantage, but you lose the rotational mass and inertia advantage. Rotating mass is the enemy of rev speed.

There is one advantage, however, to a long rod engine, and it isn't a huge one, either. The shorter rod engine will suck harder on the intake valves earlier then a long rod engine will on the intake stroke. This means that the short rod engine will be sucking on the intake valves pretty hard before they even open all of the way up. The long rod engine, which creates more vacuum later, has the advantage of sucking more in while the intake valves are actually open. On a naturally aspirated motor, this will create a higher peak VE in the long rod motor, but it will come at the expense of intake gas velocity (since it isn't pulling air in as hard), and consequently, low end torque. This is why this motor would prefer to rev to the moon. The short rod engine, however, will suck the air in harder and faster, and create more low end torque.

How much of a difference will this all make to a 420a? Probably not a great deal. I think that the positives and negatives would weigh themselves out. The motor with less rotational mass loses effeciency earlier then the motor with more rotational mass. It's a cruel irony, but it allows another factor to play a part. In a high revving motor, short rods have the advantage of being more aerodynamic. This proves to be more of an advantage at 10K RPM plus on a 2.0, when the journal can reach speeds of 86.9 M/S (260.5MM/revolution * 10,000RPM / 60 sec/min). This is 97 MPH. On a 2.4, spinning it to 10,000 RPM would be almost certainly fatal for the motor, creating a speed of 119 MPH at the journals. Less mass moving slower is better for reliability and longevity at any given RPM. so, at high RPMs, this becomes a huge factor, since wind resistance goes up by the fourth power with speed.

So, which setup is best? I am not sure, and it would be difficult to build a motor in one thread to try things out. But, based on my experience, my educated guess would be that a short rod engine would be best, especially if more displacement can be gained. So, if I could do it again, I would do a 2.4 with a 101.5 MM stroke and and an 87.5 MM bore. Maybe a bit more bore, and a tad more stroke, but I will be using a 2.4 crank in a 2.4 block.
-=B-=