Obviously 4" would be less restrictive but there is a point in which you would not see any more gain. 3" vs 4" pressure drop and velocities reach a point where they are not restrictive for the given vapor flowrate. As a general rule of thumb pressure drop is equal to the square of the velocity. Which means, say you dropped the velocity of the vapor by 2x, the pressure drop you actually dropped is 4x!
So, the area of a 2" pipe is 3.1 sqin while the area of a 3" pipe is 7.1 sqin. The area of a 4" pipe is 12.6" sqin. Bernoulli's equation could be used to calculate the pressure drop. Exhaust tweaking is all about reducing pressure drop/backpressure is what I figure.
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