I have a friend that wants to run his Brodak 40 on muffler pressure. I can't talk him out of this. Does anyone have any experience doing this?
Can't talk him out of it? It's not a big life decision. Switching to or from muffler pressure is easy and worth the experiment. When I used to run Brodak 40s unpressured I would get a distinct richening when headed upwind (losing power into the wind) and leaning when headed downwind (gaining power with the wind) even in moderate wind, similar to Brian's experience with his Chipmunk. This is just the opposite of what we would want to happen even if we wanted the engine to change speed twice a lap, which we don't, at least I don't.
I agree with Pete that the wall of the stock Brodak muffler tube is too thin to drill/tap. There is a double wall thickness where the front and back halves of the muffler tube join, and I've had good luck locating the pressure fitting there. The flat "header" part of the muffler is probably a better and easier place to drill and tap. I don't think the location makes any performance difference, as the pressure is positive throughout the exhaust system. I have even seen engines without mufflers pressurized successfully by locating the pressure fitting in the exhaust stack of the engine.
I've had really good results flying several Brodak 40s, both the early and late versions. I like the fact that with its light muffler it weighs almost the same as the OS20FP with its heavy muffler, while providing twice the displacement.
Tell your friend good luck with it.