You really do not want to run with just the header. It will be eardrum damaging loud and the bike will run like s _ _ t.... you get the idea. The power curve will be in the trash can and the carb is virtually guaranteed to not tune for you.
There are all sorts of exhaust pipes for the V-twins that are pretty much just header pipes, and you can be sure that they are running at about 65% of their potential. The exception would be a pro drag bike.
A stock or even modified street engine NEEDS a muffler to fun properly.
The very least would be something like the Hard Krome drag pipe style mufflers.
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