that is what you do to tune the pipe to the engine. just what macs says to do. don't cut it to short about 1/4 in at a time.also the prop can affect rpm so go with a smaller prop. don't load it down with to much pitch. you want (rpm) also tuned pipes are hard on glo plugs. back in the day ,guys used tuned pipes all the time....but glo plugs cost 1.50 then. 8.00 now. so if you went through 2 plugs a week no biggie. now were talking 16.00 for two.i recently took the tuned pipe off a ys .45 sf two stroke that is made for a pipe because of this.i put a os muffler on it instead. the motor ran 15,000 rpm on the pipe and now runs about 13.400. the plane is 95% more reliable this way too. no needle friggin around flight after flight either. also uses less fuel. plane flies about 70 mph now and it flew close to 90mph with the pipe.i like fast planes but not paying that much for the few mph. just my thought on this . but i love the sound of tuned pipes and miss that.. also consider a jet muffler made by dub jet of jet engineering.