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just that it was a given. So no matter how smooth you make the surface of your tube, your resistance is still limited by the shear coefficient of the fluid. That's why there is a debate about a law of diminishing returns on polishig intakes, manifolds, etc. After some point you can make it smoother for more $$, but you really won't gain much, even if the surface is mirror shiny. However, whoever it was talking about the increased velocity reducing drag is right also. There is a relatioship between the velocity and the friction coefficient. The friction coefficient is a factor in determining the "drag" the fluid feels against the walls, or more correctly, the pressre drop. There is more of this drag when a fluid is moving slowy, in a laminar flow.(imagine cigarette smoke rising in a still room. It goes straight up a long way in a steady stream before it starts to swirl. That straight flow zone is laminar flow and when it starts to swirl it is entering the transition zone between laminar and turbulent flow.) But, in our exhaust, and after the turbo's, the fluid will be traveling in aything but a nice smooth fashion. As you get more turbulent the decrease in drag drops off more rapidly. i.e. making the flow twice as fast when you are already turbulent doesn't give you the same amount of gain as making the flow twice as fast when it was closer to laminar. You can calculate a number, "q", that is directly related to velocity. So "q" changes(increases) with velocity. The pressure drop for a bend is Delta(P) = k*q "k" is a constant that is derived from bend radius of the bend and the diamter of the tube, and the total angle of the bend. Since k is a constant, increasing q increases the pressure drop, all other things being equal. Since q is related to velocity, increasing q increases velocity. i.e. decreasing pipe diameter increases losses around a bend, all other things remaining equal. That's why I think the increased velocity of the 2.5" pipe shouldn't help you as much(you're already in the very turbulent zone) as the increased losses in the bends and area changes would hurt you. I haven't worked out any numbers on it, but that's my gut feel on it, looking at the trends.
Mike
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