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Sonic Velocity is why headers are wrapped and Mass Velocity is why tubing is sized, right?
From my weak knowledge, the diameter of the tubing determines the torque range based on the engine configuration.
I can't help with the variable exhaust but I do know of a variable intake design.
Ford uses a second set of throttle bodies in their engines to improve the low end air velocity for better low end torque. I've personally seen them in the 3.8 liter V6 in the Ford Windstar where they were located one in each intake just before the fuel injectors. But I know they also used them in the Ford Ranger at some point. The switch to full open was in the 2k-3k RPM range.
Sorry that I can't be of any help.
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-Allch Chcar
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