As also previously discussed in this column, flow path length is a factor in how an intake or exhaust system contributes to overall torque output. The rule of thumb here is length affects how a given torque boost "rocks" about its peak rpm point. It's the section area that relates to the peak point since cross section size (flow rate) links directly to engine speed or piston displacement.
For example, given a fixed section area, the 240 feet/second mean flow velocity will occur sooner as piston displacement is increased. And, of course, the opposite is true if engine size decreases. (We've included a simplified illustration intended to help you visualize these relationships.)
So what about intake and exhaust flow paths of non-uniform cross section? Since we're attempting to stay with the hands-on approach to making these concepts a useful tool to engine builders, tuners, and parts manufacturers, we'll avoid how intake passage taper plays into the issue.
For the sake of simplicity, you can calculate the entry and exit section areas, average the two, and use that number for flow path section area in the equation. While it won't provide the most refined data, you may be surprised at how useful it can be.
On the exhaust side, when using headers with specific "steps" or sudden changes in section area, here's how you can view that subject. When what we'll call an exhaust "pulse" experiences a sudden change (increase or decrease) in section area, there will be a corresponding reaction in a reverse direction.
Again, simply stated, each section of primary pipe that differs from another will generate its own contribution to net torque from the exhaust system. And, as you might expect, the influence of each section's length on the whole is much like the intake side.
Now, where's the value in learning about and understanding this month's topic? If you're trying to evaluate an engine's performance, either on the dyno or track, knowing something about two major factors in overall torque output can help a range of topics, including on-track gearing, chassis set up, and driving technique. Certainly there are other engine components and conditions that affect net torque. But it's also a given that intake and exhaust systems have a major influence in where and how a racing engine operates in its intended speed range.
Can you use this "tool" to identify intake and exhaust system dimensions to optimize their application toward specific operational objectives?
Absolutely. Is it possible to size intake and exhaust systems to broaden a net torque curve by increasing the rpm range between their respective torque peaks? Again, absolutely.
Just remember that when you look at an overall torque curve (or data) that displays only one peak, that doesn't mean each system isn't contributing its separate part. I've been a part of tests that helped verify this by literally tuning an exhaust system well beyond a test engine's rpm range to more clearly define torque contribution by the intake system, dimensioned as described in the column you're reading. We then tuned the intake system beyond the available rpm range after re-dimensioning the exhaust system to evaluate its contribution. The peaks for each were remarkably close to the predicted rpm, based on the engine's piston displacement and intake/exhaust dimensions. So the idea works. And when you stop and think about it, it's a quick way to evaluate and match parts to either minimize mistakes, reduce parts investment, or both.