Sometimes it will restrict the power, sometimes the power will increase. I built my own exhaust from 1/2" conduit. I ran open pipe at first, noticed a drastic drop in top speed and an obnoxious exhaust note. I then welded on a 3/8" washer to the end of the pipe to bring back some backpressure and the top speed increased by a lot! I welded on the stock muffler to the end (left the washer on) and only lost about 1 or 2 mph. It dropped top speed down to 43 mph on a mostly stock 48cc, so I cant complain. I guess you'll just have to see what it does.