Actually the cross over pipe directs an echo or reverberation of the exhaust pulse at the open exhaust valve of the cylinder 180 degrees opposed which aids in scavaging which in turn increases the intake pulse at mid RPM giving you more horsepower in the mid range.
A 2into1 is best in mid range, because of this reverberation, (dictated by the length of the collector), where a 2into2 is best for WOT. A 2into2 with a crossover is a hybrid, supposedly the best of both worlds.
You see this on any motor that has cylinders firing 180 degrees opposed, I4 I6 V6 V8 V10 V12. On I4 bikes you will see alot 4into2into1 with the cylinders siamesed to the cylinder 180 degrees opposed; ie: 1 and 3, 2 and 4.
One of the tricks I've seen to better exhaust flow in a car with a crossover is replacing an H pipe with an X style crossover. I'm sure there wouldn't be an improvement worth the work that it would take to fabricate the exhaust, but it'd be an interesting experiment to see how it moved the powerband on a parallel twin.
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