ORIGINAL: ttoks
with current 4 stroke engines, you will struggle to get a good enough power to weight ratio to get good performance, however if enough time was put into developing a good 4 stroke, you will have a very good engine, a great example is dirtbikes, which were, up until ten years ago an all two stroke affair, until the yamaha YZF400 was introduced, not as ft as the two stroke of the time, but it got peoples attention, fast forward to today, and 450cc 4 stroke are absolutely dominating the 250CC class so badly there aren't any competitive 250 two stroke in it anymore, and so starting in 2010, it will be 350cc 4 stroke to 250cc two strokes, it isn't a case of 4 stroke need to be twice the size to make the power, thats far from true.
Actually it's not really, in that case with the dirt bikes. The riders are rarely if ever at full throttle except on the short straight's leading up to a jump or on the starts. There off and on the throttle, where a 4 stroke excells over a 2 stroke. Remember 4 strokes have a ton of torque over a 2 stroke. 2 strokes love to go wideopen and stay up high with there rpm's as thats where they make there power. So this type or racing is better suited towards a 4 stroke then a 2 stroke really. Remember that a 4 stroke excells with on and off the throttle responce since it has way mor torque, if you have them both go wide open and stay that way a 2 stroke will prevail.
We race micro midgets(same thing as a sprint car just in a smaller scale) and we have a 125cc(actually allowed up to 136cc cause we are allowed to either bore them or stroke them) class, 250cc 4 stroke class 250cc 2 stroke class(both allowed up to 270cc), 600cc class(allowed up to 636cc's but has to have a stock stroke) and a 1000cc class. Now we have 2 local track's one of which allows the 125cc cars to run with the 250cc 4 strokes. Most of the time(since it's wide open throttle on a 1/4 mile track) the 125cc 2 stroke wins. Once this year did a 4 stroke win and he ad 7 grand into his motor, 4 of it just being in the head. As of recent they have allowed 450cc engine's to run with the 250cc engine's and the 450cc drivers just keept on getting there butts kicked. They would easily have them on the starts but couldnt touch them with there top end.
Getting back to the 250's they have titanium valves 3 or 4 valves per cylinder and are as good as your going to get right now. And the 125's basically kicked there butt's every time except once. And it was more due to the driver, the driver is awesome in anything that he gets in and if he would have been in his 125 that night he would have one. Ohh yeah and up until the zx6r 636 came out the 250cc 2 strokes where actually just as fast and in some cases faster then that of a 600's.
It more or less really depends on where your at, but for racing with our rc's it's going to take much more then twice the size to make the power of a 2 stroke. Not unless they have made 4 valves per cylinder titanium valves dual rate valve spring's, have the valves polished along with having the valve bowls blended along with much much more. The 4 stroke technology of Rc's is basically decades behind that of there larger brethern that are in dirt bikes and four wheelers.
I'm not trying to start an argument here just stating that in a lot of cases a 4 stroke need's to be at least twice a larger as a 2 stroke need's to be. Now there are certain instance's where that is not true again I.E motorcross, and arena cross where they are constantly off and back on the throttle. Plus to make a 4 stroke that fast they need to be rebuild a lot more then a 2 stroke does, there 4 strokes need to be rebuild every 2 races where as a 2 stroke can go as many as 6 or more races before needing to be rebuilt.