Yes, I remember that engine well. One of the reasons, I feel, that they were having issues with reliability was that they used the glow internals. Comparing the crank and rod to the designed specific output of the OS and the 26cc Ryobi, it's evident that the Ryobi is much more durable. One of the things I thought about last night after I logged off was that there are two basic types of 26cc Ryobi engine- the half crank (700 and 800 series) and the full crank (1000 series). I'm not sure how much boost this engine will handle, but I surmise that the full crank would be a much more durable engine. I plan on testing both engines, as well as a few piston ported two strokes, such as an old 21.5cc Echo I have laying around.

As for the vacuum in the blower, I'm not sure what you mean by that. Roots blowers are positive displacement pumps, meaning that they always move the same volume of air per rotation, regardless of speed. I suppose that there would be some vacuum between the carb and the blower if the carb was a restriction? Since they don't compress the charge, the only way they produce boost pressure is by overdriving. In a nutshell, if an engine needs 3 CFM at 7000 RPM, and the blower moves 3 CFM at 7000 blower RPM, with a 1:1 ratio, there will be no boost. However, if you were to spin the blower at 14,000 RPM (1:2 ratio), you'd start to see boost. I can't recall the formula to calculate boost with a postive displacement blower, and I'm at work, so I don't have access to my books and notes, but a rough estimate would put that at about 4 PSI, depending on thermal rise of the charge and the VE of the blower. The downside to a Roots blower is that no matter how you cut it, efficiency is in the 50% range, which is poor and lends itself to high intake charge temps.

The blower type I'm using is, as I said before, a postive displacement pump that compresses the charge, so overdriving may not be nessicary. It entirely depends on the efficiency of the design. I have a few different impeller and housing designs I plan on trying.

Something else that occured to me while typing this was a reduction drive for the prop. We all know that big, slow props are more efficient (not to mention quieter) than small, fast props. Now, what if we had this 3 HP 26cc supercharged four stroke putting out a TON of torque through a 2:1 belt reduction drive? The engine can still breathe in the middle of the powerband, and you can have a nice big scale prop.