To expand on what nkaminar said. This is the exhaust on my '29 speedster build. Eventually being fed by a blown B engine. @40" long primaries for @2800 RPM peak torque paired by firing order (1-4,2-3) for even exhaust pulses into an X pipe that will pull a negative pulse on the other upstream paired pipe and share the exhaust pressure pulses between the downstream pipes as each exhaust pulse goes through the system. As built it's actually just about as quiet at idle as the stock exhaust. There's a good bit of science into making it work and just a little guesswork too.
