I'm surprise that you used such thin brass tubing (1/16"). But maybe that's all that's needed. I used 4mm tube on my "first attempt." Many thanks for your response though; I'm sure I'll end up doing something similar. I wouldn't have thought of running it "forward" towards the the front. Also could you comment a bit more about the need for baffling? I don't see why this would be necessary (particularly on the sort of installation I have where the rear of the cowl isn't fitted to the shape of the fuselage).

What do you mean about the 770 running "better" without muffler pressure? Do you mean running smoother. running better at idle, runner better at high RPMs? Or do you mean running better without an exhaust ring entirely? I'm a little afraid of running any engine without muffler pressure to the tank since that's not something I've ever done.