I am currently roadracing a 1953 Matchless G80CS (500c.c. OHV Single) and am happily running with a BSA Goldstar open reverse cone megaphone. Factory SH race cams are fitted but with this setup the bike is a bit loud (as they were back in the day)but more and more tracks are enforcing noise restrictions. I am interested in a Supertrapp megaphone as an alternative but what are my options- inlet dia. is 1.75" with outlet at 4" overall length is 13" . Would one of the universal stainless external disc pipes work and what are the decibel levels with open end caps vs. closed?