Relationship of Intake Port to Torque Peak
   For a given intake port design, the cross-sectional area of the runner affects the location of an engine's torque peak in the RPM band. The runner length and shape is also very important, but the cross-sectional area will be the strongest determining factor.
Basically, the smaller the runner diameter, the less air potential there is. As the runner gets longer, inertia in the column of air will increase the flow at lower rpms and will tend to decrease the flow at higher rpms. Once again, inlet port cross-sectional area will be the main determinant of total airflow potential.
   

Calculations for a dynamic running engine are complex as we are dealing with camshafts, exhaust systems, and the larger issue of the cylinder head design. For a more comprehensive look at this from a simulation standpoint a $600.00 program like Dynomation may do more to answer your curiosity.

    The calculators on this page will help you plan your attack on cylinder head selection, valve and port sizes and allow you to coordinate your camshaft and exhaust system selection. This page is not designed to answer the questions for turbo or supercharged applications, which is a topic for another day. Generally, in the case of a Harley Davidson V-Twin an LSR 2-1 collector system will do wonders to increase torque. Both RB Racing LSR 2-1 and RB Racing LSR 2-1 Pro Stock Systems are available for both oem and aftermarket heads as well as 113/132" ORCA motors with S&S Special Application "SA" heads
   

Keep in mind that these calculations must be used in conjunction with header tube diameter and length, valve size, head flow, and camshaft selection. For instance, if your camshaft is designed to peak at 4500 RPM, but your manifold and headers are tuned for 6500 RPM, your actual torque peak will fall somewhere between 4500 and 6500 RPM, and the useable torque band from 2500 to 4500 rpm will be lengthened and flattened. On the other hand, if you match the intake, headers, heads, and camshaft all for 5000 rpm, the torque peak will fall very close to 5000 RPM. Also, keep in mind that peak torque and peak horsepower do not occur at the same rpm and that when you shift it is always better to fall back to a region of maximum torque instead of trying to "climb the mountain" to get back to your next redline shift point. We cannot emphasize enough that you must view the engine as a complete system and not concentrate on only one aspect of it..