It will chip your valves and chew up the valve seats.Also it will eat up your piston rings.You need the pipe on there to hold the gas in the motor long enough to burn thoroughly.Otherwise the gas ecapes and the motor runs waaaaaay too lean.Not good.

 

wide open exhaust will toast your exhaust valves. I used to have a valve from my old car that burned off about an 1/8" of material after running a wide open exhaust for a month. You need back pressure, just not too much (obviously)