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Originally Posted by bondvagabond
I welded up a set of headers for a geo metro optimizing for this scavenging effect, one of the real engineers one here helped me calculate size and length. The runners were TINY! As in, they don't sell exhaust tubing that small. Torque and fe both increased. This custom header was essentially the opposite of porting.
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You're throwing red meat in front of the Metro owners here (I'm not one). What are you prepared to put up? Pictures or dimensions?
I think a lot is being 'left on the table'. Anywhere in nature you have a branch in a tube with a flowing fluid, from blood vessels to Oak tree branches, the primary swells and then transitions into the fork. Someone should pit 3D printed test manifolds designed by a genetic algorithm against each other, steered by real-world testing. Hydro-forming for the finished product.
Insofar as the rough/smooth part, I recall a theory where the inside of a curve is rough and the outside smooth, or
vice versa. A student of Viktor Shauberger would suggest spiral grooves. Apparently rivers fold over as they go around bends.
I remembered: With flathead Fords it was Porting and Relieving. That was reshaping the part of the combustion chamber cast into the block. And porting also includes reshaping the valve guide area on a pushrod engine.