On motocross tracks, there has nearly always been little difference in lap times, by same level riders, on various capacity motorcycles. For the most part, motocross tracks don't have much speed to them, in conventional terms. If you put a speedo on an MX bike, plus a timer for how long a 'high' speed was recorded, the average person would be distinctly unimpressed. We as riders know that 45/50mph through a rough straight can be utterly ballistic and pant staining - to the uninitiated , it's slow. I recall reading that , on the Honda test track, both RJ and Bailey recorded wide open throttle usage for only about 3/5 seconds per lap, and spent most of the time below 1/2 throttle on a full lap on 500s.
I don't know the exact Maximum Average speed for a lap of an approved track (mind you it's not rigidly followed from what I gather) that is specified by the FIM, but it is surprisingly low, I think around the 60 / 80 KPH. So we are not doing a sport that is speed / power dominated like so many motorsports.
Equivalency is about having an equal playing field, in each class. The open class, with bikes of either stroke, will have engines that can put out more usable power than even the best can make use of, in a variety of capacities. The open class enables designers/engineers and Riders to try different approaches.
Smaller capacity bikes have always gone fast int terms of lap times. And there are those talented riders ( well, the average rider to - I could always get consistently faster lap times on my 125s/ small capacity 4t/2t hybrids, than I could get on my open class bikes that I raced at the same time - but I had more fun and better results on the open class bikes), whose style / preferences in a bike enable them to go faster on a smaller capacity bike. Especially when on a track with bikes of , generally, equal performance levels. And the very highly talented riders, who are really suited to the smaller capacity bike, can do well against bigger bikes whilst on the same track, at the same time. But they are in the minority. The bigger bikes use/can use different lines, that interfere with the lines (generally about keeping momentum) of the smaller bikes, cutting momentum, and thus beating them. Examples could go on and on.
To me, the premier , Open class has always been about riding the most demanding and difficult to master bikes, simple as that. That is why it was regarded with such respect, for so long.
450s I've regarded as open classers all along - 250 2ts get eaten by them because of the difference in grunt that the 450s have Only because of the capacity handicap. 4 strokes Do Not have more torque than a 2t of equal capacity, with the same relative level of tuning, it's one of the most BS statements that I continually hear / read with regards to 4ts - They Only have the advantage in torque Because of their extra capacity - even 2t advocates seem not to understand this most basic point at times.
I ride a CRE500 - because it's rock axe reliable, Easy to ride, simple and I'm off the 'get the latest model treadmill', for the time being. And it costs little to run. I don't find it scary / intimidating 99.9% of the time, and I certainly can scare myself just as much when I occasionally get on a 450 4t open classer - both examples of MCs that have more power than I , or any of you, I would suspect, could use in my sort of terrain.
We need a return to 3 classes, and equal capacity rules between 2 an 4 strokes. And a return to the premier class being the Open class.
PS, when I checked the AMA pro rules, I saw the maximum capacity written as 450cc - I saw no reference to it being now an open class, as I think someone has stated above.