In my quest to lose weight on the bike and not in my wallet I experimented with making a 2 into one using stock head pipes and stock mufflers.
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this didn't turn out to well...lost 6 mph on my test acceleration strip, so next I ran a long drill bit down the exhaust exit, it made a little more noise but didn't pick up any mph on the test course. Next I cut the muffler apart at the front weld of the muffler (where the muffler and head pipe meet) and used a big punch to curve back the center hole I had drilled out, and removed the "screen" on the inside pipe.
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before
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and after.
this did pick up a couple of mph (and a little more noise) on the test course, but was still 4 mph slower than stock pipes and mufflers. After that I put on another set of stock head pipes and gutted another muffler (so it was dual pipes again) and walla, it was now 2 mph faster than it had been stock with a little more noise again, but easily streetable without getting the neighbors up in arms. So this gained a couple mph but didn't loose more than a few ozs.
My next step was to find a used Muzzy header. It was then 2 mph faster than my gutted mufflers and 4 mph faster than stock. All jetting is stock, I know everybody says you need to rejet, and we did the rejetting thing on two different pregens and they both are fastest with stock jetting. Jetting is still a tiny bit rich at wide open throttle but close enough. BTW we are at 1300 ft. above sea level.
ps: also running stock airbox and stock air filter