Afraid there's a bit more to this than just building boost.

As I've come to learn "huffing" a front rotary valve 2C engine isn't as easy as it would seem. The issue comes with keeping the charge in the cylinder. The exhaust port is the first thing that opens as the piston is on its way down after firing. This compresses the mixture in the crankcase. After the exhaust piddles out the spent stuff the transfer ports begin to open and the new intake charge gets squirted up into the combustion chamber. In a boosted condition it's very likely the bulk of the charge will run right out the exhaust port.

I have a couple options. One (most complicated) is to design a new head/liner with a poppet valve that's driven off a camshaft for the exhaust. The second (more realistic) is some creative use with a tuned pipe. The pipe works by taking the sound energy generated and reflecting it back towards the exhaust port to literally stuff the piddling intake charge back in the cylinder. Done correctly it's quite powerful. Tuned exhausts on 125cc motorcross engines are capable of making the combustion chamber displace up around 180cc's of intake charge volume. It makes the powerband quite narrow though as the length of the pipe determines the RPM range/broadness. This is why the "revy" 125's are often picked on as being "peddled" around the track by the gear shift lever. (to keep the thing in the optimal RPM range for max power)

What I'm learning is the trick is to get that charge shot up there with enough velocity to adequately fill the cylinder while avoiding an overly "quick" charge that'll tend to run out the exhaust. If I can get the intake charge "just fast enough" then hopefully it'll fill the cylinder and loiter around while the piston changes direction and closes off the exhaust port.

This leads to another challenge. The actual displacement of the engine isn't exactly correct. (Bore x bore)x stroke x .7854 is the old school displacement formula. Unfortunately it's a bit skewered for a piston port engine. The ports are open for the bulk of the rotation cycle. Actual displacement (usable) is significantly less as anything that really matters takes place from TDC (top dead center) on the piston to when the exhaust port starts to open. The trick with high nitro content is to get enough exhaust timing. This means raising the port- sometimes considerably. Doing so reduces the effective "pressure cycle" in crankshaft degrees as the pressure in the cylinder drops off pretty quick once the port opens. Raising the port elevates the effective RPM powerband of the engine. It has to run faster to make the same power as the shorter port did. Not a bad thing so long as components inside (crank, rod, piston) are able to tolerate it.

I know my stock Moki 210 ran a 16x16 APC at 7400 rpm on 15%. Through extensive tinkering I was able to bump that up to 7900 rpm with 58% nitro. (I had to buy a hydrometer and learn how to blend my own fuel) What kicks my butt is my buddy down in AZwas able to run a 16x14 APC at over 9600 on the ground with 60%. I figured I should have been in the low to mid 8's with the 16 pitch. Makes me think I did something wrong but I can't find out what it is. With smaller props I got as high as 10,200 rpm (14.4x13 APC) My performance goal is to be able to spin that 16x16 at 10,000. That should really wake this engine up. If I get it then I'll look at a custom prop with more pitch.

200mph is the speed target for this bugger. A bold undertaking. I doubt I'll get it without having to really tear into the airframe but who knows? Stranger things have happened.

Ok, back to work. This morning's agenda is to finish the rotors. Gear sets and all.

More to follow as time allows.

C