A straight pipe won't damage your engine IF the carb is properly jetted. Expect to lose all power below 5000 rpm or so.
Back pressure is evil. It's only purpose is to reduce blow-through during overlap with long duration cams, effectively shortening the cam timing without actually shortening the cam timing. If some means of releaving the backpressure at increased rpm is not incorporated, peak power will suffer. Even something as simple as a butterfly valve can make a huge difference in driveability with a long-duration cam. Something as simple as a selenoid can open the buterfly valve when backpressure is not needed to allow the engine to enjoy maximum flow.
Note that Yamaha engineers put significant work into using sound and pressure reversions to work to widen the TW's powerband, and some cost of peak horsepower. That is why TWs are as torquey as they are for a 9,500rpm engine. Though aftermarket exhausts might, in some cases, actually add a fraction of a peak horsepower, or maybe a lb/ft or two of torque, virtually none can match the stock exhaust for its broad torque curve at the same peak horsepower on a stock or lightly modified engine. If I'm wrong, post actual dyno results and prove it. Be aware that if you ride offroad where approved spark arrestors are required, an aftermarket exhaust can be cause to have your bike confiscated.
However, there are many valid reasons for aftermarket exhausts. Looks, sound, weight savings, and torque curve manipulation, especially with highly modified engines, are all good reasons to go aftermarket.
Back pressure is evil. It's only purpose is to reduce blow-through during overlap with long duration cams, effectively shortening the cam timing without actually shortening the cam timing. If some means of releaving the backpressure at increased rpm is not incorporated, peak power will suffer. Even something as simple as a butterfly valve can make a huge difference in driveability with a long-duration cam. Something as simple as a selenoid can open the buterfly valve when backpressure is not needed to allow the engine to enjoy maximum flow.
Note that Yamaha engineers put significant work into using sound and pressure reversions to work to widen the TW's powerband, and some cost of peak horsepower. That is why TWs are as torquey as they are for a 9,500rpm engine. Though aftermarket exhausts might, in some cases, actually add a fraction of a peak horsepower, or maybe a lb/ft or two of torque, virtually none can match the stock exhaust for its broad torque curve at the same peak horsepower on a stock or lightly modified engine. If I'm wrong, post actual dyno results and prove it. Be aware that if you ride offroad where approved spark arrestors are required, an aftermarket exhaust can be cause to have your bike confiscated.
However, there are many valid reasons for aftermarket exhausts. Looks, sound, weight savings, and torque curve manipulation, especially with highly modified engines, are all good reasons to go aftermarket.
