RE: pitts style muffler
I've got that aircraft, it'll be maidened today. I have a Super Tigre .90 under the cowl. Completely under the cowl... There is about a 1/2 inch from the top of the head to the side of the cowl. I measured mine and it's about 7 1/2" wide at the widest point. I've installed a Bisson Pitts muffler on it. While the cowl is flexible enough, I cut about 1/2 an inch off the tubes just to give me a bit more room. I installed the dummy radial and that makes slipping the cowl on and off a bit tricky has the prop shaft has to clear opening before you can get it on far enough to get the down tubes to the exhaust opening. So I enlarged the opening in the front. If you don't use the dummy radial, there probably won't be an issue. Also, just for looks, I cutout between each of the dummy cylinders. I think it looks better than just cutting an opening for the engine head.