[jdmccright]

I'm no expert here...I'm mostly good at theoretical bulls**t...but I'll offer my thoughts. The throttle body is generally just an air regulator and mixer with the fuel. The engine pulls air in at some flow rate (displacement x rpm ÷ 2...during the engine cycle half the cylinders are sucking air in and half are squeezing).

The velocity is created by forcing that mass to flow through a smaller cross-sectional area...through the intake runners, plenum, and throttle body. The point of smallest cross-sectional area would be the max velocity of the air. What is good about velocity is there is better mixing and atomization of the fuel when it is injected into the airflow. What is bad is that the restriction causes a vacuum or lowering of density of the air charge. An engine pulling 18" Hg of vacuum is getting only about 60% of atmospheric pressure, thus 60% of the potential air mass for that volume sucked in by the cylinders.

Therein lies the issue. If you increase the TB diameter, you can reduce the vacuum effect, giving more air mass, but then you also reduce the air mass' velocity. You need more fuel and higher fuel pressures to feed that air and maintain a fine spray. Otherwise you're wasting potential power.

I don't see how you would sacrifice low-end torque by increasing the TB diameter since this would decrease the vacuum and increase air density. But too large and you won't get the mixing that you need for complete combustion because the air velocity at low rpms is relatively low. Once rpms go up, the air gets alot more turbulent for good mixing, thus better high-end performance.

I may be way off, but at least it's a start for this discussion....