This month, we'll step out of the "theoretical box" and into one that's a bit more practical by discussing and focusing on some fundamental ways of relating inlet and exhaust path sizing to an engine's ability to produce torque. In the process, it's important to understand that what follows can be used as a diagnostic approach to determining why torque boosts occurred where they did, as well as a tool for helping shape a given torque curve to match where an engine needs to perform best.
We'll lace all this with a few examples, just to reinforce the points shared. Also, keep in mind that the approach we'll take is pursued somewhat at the risk of oversimplification because there are far more precise and encompassing methods available. What follows are the type of tools that work well in the dyno room or for basic parts selection, void of any intricate math or computer programs.
We've previously mentioned that both intake and exhaust flow is unsteady and somewhat pulsating, punctuated by interruptions that include the opening and closing of valves and pressure excursions involving the combustion space.
The study of wave motion plays into this, among other analyses. But for purposes of our discussion, let's say there will be a "mean flow velocity" in intake and exhaust paths that occur at or very near peak volumetric efficiency (a torque peak) in both these paths. A commonly-accepted value for this is 240 feet/second. Although many intake manifolds have runners with taper (or slightly varying cross section areas) and some headers incorporate "steps" or sudden changes in flow path cross section, we'll initially assume constant flow path areas and then examine the non-constant areas later in our discussion.
Suppose we begin by evaluating how what we've called "mean flow velocity" plays into intake manifold function. Reliable information has shown an engine's torque peak is directly linked to a mean flow velocity of 240 feet/second. Since flow path section area, engine speed, and piston displacement dictate where in the rpm range this flow rate is reached, there is a mathematical relationship from which any one of these can be determined, if the other values are known or assumed. In a simplified format, the equation is as follows:
Peak Torque rpm = (Flow path area) x (88,200) / Displacement of one cylinder)
As an example case, let's assume a total V-8 engine piston displacement of 350 ci, giving us 43.75 ci/cylinder. If the section area of the intake runner is 3.0 square inches, we can plug these values into our little equation to calculate a corresponding torque peak at 6,048 rpm. At least this is where it should occur. Of course, this only addresses how the intake manifold will contribute to the engine's overall torque curve. If we observe this boost (from the intake manifold) occurring at a lower rpm, we could say the engine is "under-cammed," and if it appears at a higher rpm, the engine could be over-cammed.
So, one use for our equation is to evaluate how a particular intake manifold will influence overall torque, particularly at its mean flow velocity. If we'd like to select/design/modify an intake manifold's runner section area to boost torque at a desired point, the equation can be algebraically rearranged to solve for the required section area to read as follows.
Flow path area = (Peak torque rpm) x (Displacement of one cylinder) / 88,200
In this case, let's assume we'd like an intake manifold torque boost at 5,800 rpm (for whatever reason, like gearing, track length, and so on) and need to know the section area associated with this engine speed. Inserting these values into our little equation tool computes an intake flow path area of 2.88 square inches.