Exhaust is a little tricky in figuring the geometry vs the result. the basic idea is to allow expansion of the gases in either a controlled environment, or let it run wild. I found 35 years ago it is easiest to mock up exhausts from copper plumbing parts. Easy to cut, solder, and bend. Sometimes an idea works, and sometimes it don't. 4t's are much easier tho. It just comes down to an engineering degree, or a lot of patience/time.
