I just want to add my own experiences with the 4 stroke set up. I had been wanting to try them out just for giggles and grins, but had not seen anything at contests that turned me on about them. Then I had the opportunity to buy a Top Flite Score with a Saito .56 in, and it was equiped with a hand made 4 ounce uniflow clunk tank.
I tried "playing it straight" with the Saito and tried running it like my ST.51s and it would cork screw me into the ground almost. I tried running lower RPM and higher pitch and I didn't have enough fuel to do the pattern. If I went back to higher RPM and lower pitch, I could get a decent run one flight, and then the next all hell would break loose and it would drop 2 seconds on the lap time. I got to a point where I understood why some liked them and some hated them!
Just as I was about to put it up for sale, I read somewhere that 4 strokes don't like uniflow, not enough fuel delivery, and that a standard plumber Sullivan tank on muffler pressure was recommended. I tried that and things improved. I discovered Mr Reeves' set up soon after that and installed a nylon screw in the smallest UHP venturi I had, and tuned the intake as per Bob Reeves' instruction, and things got rock solid and way better. I have the opportunity to talk to Gilbert Berringer each year at Oshkosh, and asked him about his intake set up and he confirmed he ran venturis in the 5 to 6mm range with a straight through needle valve, and that kind of matched up with what I ended up with. I asked him to fly it in an afternoon demo a few years back, and he gave me a thumbs up on it. He noticed one thing on my engine that I have noticed my self when comparing it to ther 4 strokes, and that is that my engine is much cleaner, no baked on oil crust with the exception of the muffler. I take that as an indication that my lower RPM run is much cooler that a high RPM set up, and after almost 5 years of running the engine, I have never felt the need to reset the valves. The engine , to me , runs the same now as it did three years ago, so why mess with it. I run Powermaster YS-20/20 fuel, a Rev-Up or Vess 13-7 prop, take off RPM around 7800, fuel load about 3 and 3/4 ounces, 70 foot lines from handle to fuse center line, and 5.3 lap time. Each run is very consistant and repeatable and I am very reluctant to try anything else. Very little to no whip up in wind, which I think is due to the fact that there is no power delivery on every other stroke, and that in turn helps regulate the speed? A more of a pulsing type of power? I really don't know.
I found the whole learning process of the 4 strokes pretty interesting, and I'm happy with the results. It has surved me well in contests, but I am not going to sell off my inventory of ST.51s and St.60s! I find each type of engine has it's own attributes and I kind of like swapping around every now and then. In this day and age, we really do have a pretty wide variety of different types of power plants, and a person should be able to settle on one that they understand and are comfortable with, and I think that is the important thing, understanding the power plant and being comfortable with operating it, no matter what kind it is.
Type at you later,
Dan McEntee