Hey, this thread got me to experiment. Why, because this topic must have at least five or six hundred threads about it and I haven't cared to experiment before? Well, this thread got a pretty good idea thrown out by someone with lots of experience, and I've just now got a 4stroke that'll easily compare with 2strokes I've got.

I got an OS70FL fourstroke. Turns out that OSMax worked long and hard to make it the same weight as their OS46AX twostroker. That's with mufflers on 'em both. They did things like make the backplate out of plastic. But both weigh almost the same in fighting trim.

The 70FL flew one of my airplanes great with a 13x6 prop. Unfortunately, the engine didn't run great while doing it. And it was lousy to start. So I'd swapped it out with a 46AX. And I was running that one with a 11.5x6 on that airplane. And about the only performance difference was that the twostroker took a bit more takeoff roll. And once in the air, the fourstroke didn't ever really pick up speed.

(I would suggest a very subtle but to me significant value to 4strokes. They make better engines for learning pilots because they have less range of speeds.)

Anyway.......

I decided to try the 46AX with a 13x6. It didn't really like it right away. It was really more my fault for not realizing that the needle was going to be lots more sensitive, but I didn't give it much of a test. Almost right away, I made up a 12.5x5 prop and gave it a try. It appeared to get the rpm's on step better, and had a bunch of fan area. Darned if the airplane didn't takeoff just about like it had with the 13x6/70FL. Either engine would basically "levitate" the airplane.

I'd say that as near as I got to a close comparison, that the 46AX showed about equal "torque" and better horsepower. It would accelerate the test airplane equal to what the 70FL did. I had to prop the 46AX out of it's normal envelope to get that, but it did it. And once in the air, the 46AX with less pitch and fan area had better vertical. I'm not sure why, but put it down to the extra speed the 46AX showed flying level. The 2stroke had about equal "pull" and faster airspeed.

Rule of thumb says that 1.5 is the comparison ratio. A 46size 2stroke would be equal to a 70size 4stroke. One and a half times 46 is 69.

I would hazard a bet that my 46AX (when propped to have close to the same fan area of the other engine) actually showed more torque/horsepower than the 70fl showed. And it's worth nothing to conjecture, but I'm fairly certain the 4stroke wouldn't have run if propped like a 45size.

One thing that does stand out to me about the differences, is that in the size airplanes I like the most (45size), there are very very few that have landing gear tall enough for me to run that 70fl with the prop I would certainly want to run.

What does any of this really matter? close to zip..............