Originally Posted by nitrodude

Whenever you have a cone shape tube through which fluid flows you create a "venturi effect". Depending on which way the flow is you either accelerate or decelerate the flow( increase or decrease pressure, respectively). In the conical headers’ the flow is from the smaller diameter to the bigger diameter, so it decelerates the flow(lowers pressure). It would be cool if an engine designer could explain why it creates more power.

In contrast to the conical header and to give a better idea, we have the venturis in the engine carbs. These venturis work opposite to the header, in the carb the air that is sucked through the venturi goes from the big diameter to the smaller diameter therefore accelerating the flow