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They create reversion with the spent exhaust creating a little back pressure. Kind of like a vortex. Mainly used with drag pipes.
He is correct! Only time I would even think about running them is if I did not have a two into one or two into two or a cross over pipe.
The reason to run a two into one with a cross over pipe is when an exhaust valve opens and closes there is all ways some exhaust gas left in the pipe and with out a cross over or two into one or two into two then next time the exhaust valve opens it suck some of the old exhaust gasses back in to the head.
Now with a two into one or cross over pipe or where the two are tied together the exhaust left in the pipe when the exhaust valve opens on the opposite cylinder it help suck left over gasses out of the other pipe. This is the reason I would never run true duals.
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