I know on Harleys old school 40 inch long drag pipes was the best ..anyone know what works on slants..I think I read late one night it had the same length 40 inches was best ? but was late and a while ago..any know 4sure..thanks

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slant six's RULE!
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It will really depend on the cubic inches, cam timing and, RPM that you want the ram effect. According to the Mopar performance racing manual: 3 front cyl to 1 collector and 3 rear cyl into the other collector.
Primary pipes: 1 5/8 dia by 40 inch long or 1 3/4 dia by 40 inch long.
Transition from primary pipes to collector should be as smooth as possible and approx 6 inches long.
Collector should be 2 1/2 inch by 12 inches long.

On my old stock eliminator car with a 170 engine, What worked best was a 1 1/2 ID primary 40 inch long, and a 2 1/2 by 26 inch long collector. Jack Clifford made me several different sets, and these worked the best. We were looking for off the line torque.
I have seen several current "stocker" inline sixes, and the header configurations are quite different (and expensive). Present day stock racers, change headers, when changing converters, cams, etc. All to "optimize the package". Time and technology move on.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225