Quote:
Originally Posted by Ranchero50
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. 
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The two up two down crank and 1423 FO will make the siamesed ports much more efficient and reduce the amount of reversion between cyls.
Not too sure why limiting to 2800 revs, a decent flathead design will go much higher.
J