| the upper plenum/manifold, therfore the air is dry and boundry layer roughness to promote turbulence and homogeneous air/fuel mixture doesn't apply in this case. I've ported about 100 sets of heads and intake manifolds with flowbench and dyno testing on the same. These were all carbureted combinations so the air was wet with fuel, and what I found every time is that smoothing the casting flash roughness from the intake runners is consistently worth about 10 to 15 ft-lbs accross the entire test range. Keep in mind that I'm not talking about mirror polishing, that's for bench-racing and a total waste of time.
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