exhaust pipe diameter for a mildly modified 340

273 said

More than likely the mufflers will kill more hp than the pipes, ideally you want zero back pressure, if you already have an exhaust you could measure the back pressure and guesstimate from there.

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Well.......actually, ideally, you want negative pressure and that's possible with some of the mufflers available today, such as the Ultra Flow welded. Some of them flow better than an open pipe, due to their design that actually causes a vortex action to help evacuate exhaust.