Just the other night I re-read an old HRM article on hopping up the 216 in Frank McGurk's '51 pickup. Starting with a baseline 64 HP at the rear wheels, they added dual carbs, cam, head work and other stuff. Ended up with 106 HP. Adding a split manifold resulted in a gain of 2-3 hp throughout the RPM range....but this step was only after the dual carbs and cam were added. If only adding the exhaust it's, doubtful any gain would have been made. But as part of an overall performance upgrade, it does help (and sounds great!)
