What a dubious topic for a KX500 forum...muahahaha
The older CR's were faster than the late models. I don't beleive that is true of the KX. As far as the late model's go a CR made 56 hp at 5800 rpm while the KX made the same thing at 6500. (credit to Dirt Bike magazine)
I ride a CR500AF. V-force reeds, the PC exhaust, it's got a 39mm carb, a balanced crank and bla bla bla...My Dad also rides an '89 KX500 that he's had for over ten years. With much the same motor mods and a few more still.
Near as we can tell they're the same. As far as seat of the pants A-B-A tests go they're equal. Even dragging the two repeatedly, swapping riders between runs, the two are neck and neck. My CR does cross the line first every time, but it's not even by a bike length. And we all know if we had another 15 meters to run the KX would nose ahead.
It is just a feeling, but it's our opinion the KX makes more power. Every time it fires it seems to lunge ahead harder then my CR. Like it's generating 10 more foot pounds of torque at the crank. But the thing is my CR just builds power sooner. It revs free'er. Like it's not held back by anything. Twist the throttle and it explodes into orbit.
Easy to understand I think. The CR has a shorter stroke and no power valve. Stands the reason to me it builds power sooner. The two seem so much alike and yet they acheive the same thing in two different ways. Smooth and torquey or quick and revy.