Thinking of swapping mufflers but got a question pertaining to my 340

needing backpressure is a myth, you maybe will need to fatten the fuel mixture up a bit as you might go a bit on the lean side depending on how much flow you gain, id run em

*edit* not to mention you said it was dynoed on open manifolds... im sure you have plenty of backpressure from them i doubt mufflers would change much other than noise, id definitely go for it if you are fond of the noise that muffler makes