If you're talking about an intake manifold, the math and logic is similar to that of an exhaust header with the optimal tubing diameter and length being related to the size of the cylinder being filled, and tuning to a particular frequency to optimize for a particular RPM range. For street cars, they're usually going for best low-mid range torque, so the intake manifold runners are as long as practical. The size of the plenum is also calculated in the same manner.
On the atmosphere side of the throttle body, you primarily just need unrestricted airflow. If you could just open it up right there, with something like a bell to help high-velocity air flow into the throttle body, that would work great! But, the downside of that is that while it would make maximum HP at the high end (great for drag racing... not for much of anything else), it's going to also let a LOT of engine noise OUT through the throttle body. It's pretty obnoxious. Ask me how I know.
So the purpose of the intake is not only just to supply fresh, clean air to the engine with minimal restriction, it's also to baffle that intake noise, sort of like a muffler. In addition to properly sizing the intake tube and length to aid in airflow, you'll find that there's almost always a "Helmholtz resonator" in the intake tube. That little box that hangs off of the intake tube and looks like a little useless piece of plastic that doesn't need to be there. It's there to help eliminate the resonance in the intake. It greatly reduces the noise, AND... resonance is the enemy of airflow. Whatever frequency/rpm that intake resonates at, there's going to be a dip in the torque curve. So, the resonator eliminates the resonance AND improves the torque curve, eliminating that dip in torque.
And this is right where the typical "cold air intake" tube leaves you. It's a straight pipe, probably larger diameter than stock, that has done away with the carefully engineered resonator. It MIGHT give you slightly better throttle response at low RPM, and it MIGHT give you slightly more peak horsepower, but it has almost certainly DECREASED torque somewhere in the mid-range.
It's all about trying to keep that column of air in the intake (and the intake runners, and the exhaust header tubes, and the exhaust) moving forward as efficiently as possible.
https://www.brighthubengineering.com...ic-efficiency/