Ladyflyer flat knows what she is talking about!
By the way, my first dirt bike was a B-50 motocrosser, and I worked for a fellow who used to race a Matchless 600cc Typhoon - this was at a Triumph-BSA- Norton shop in IL.

Exhaust tuning works on 4-stroke engines and we used it to boost the power of the B-50. For an engine that peaks around 6500 rpm, we used a straight pipe with a length of around 31 inches (from memory). You can extend the range of the boost effect by using a tapering megaphone. You can also minimize the "off-tune" phase or megaphonitis by using a small reverse cone on the end of the megaphone.

For model engines turning much higher rpm you would need a shorter pipe - a guess of 15-18 inches should put the boost in the 12,000 rpm range.