Before you begin tuning on...
Before you begin tuning on a dyno, you have to check everything to avoid problems like this. Note the fuel in the float. Photo by Rob Fisher
As a rule of thumb, the wider the range of operational rpm, the farther apart (in rpm) you'll need to "tune" the intake and exhaust systems. Conversely, as in the case of tracks where the rpm spread is narrower, you can bring the intake/exhaust tuning points closer together. So how do you do that? Let's return to the flow velocity vs. flow passage section concept.
Take a look at the illustration (figure 4) describing how flow passage section area relates to an engine's peak torque rpm (volumetric efficiency). Recognizing that we're presenting this material in a very simplistic format, compared to more complex computational methods, it turns out the concept still holds true. You may recall a previous point that a mean flow velocity of about 240 ft/sec occurs at peak torque. Based on that information, note how peak rpm shifts upward as a function of increased flow passage section area. At least in terms of this concept, you can evaluate intake manifold runners and exhaust header primary pipes in the same fashion, for purposes of selecting, designing, or evaluating existing components.
You will also find that intake and exhaust systems can be tuned to different rpm points, within the range of anticipated rpm. This can be useful in a number of ways, based on final drive gearing, length of track, and track conditions. In addition, you may find it useful to select or configure intake manifolds with different passage section areas. This will allow you to broaden (flatten) the contributions made to a total torque curve, thus enabling a wider range of effective torque; e.g., coming off corners and continuing past mid-straight-aways.
On a personal note, I've previously worked with NASCAR teams who not only used this concept to their benefit, but combined different intake and exhaust passage size with the appropriate intake and exhaust lobe designs and timing-e.g., short valve events associated with the longer (lower rpm) intake and exhaust flow passages, and longer events for the shorter (higher rpm) intake and exhaust passages.
The approach amounted to treating the engine as a multiple set of single-cylinder engines by using different intake and exhaust lobes to match the tuning points for the intake and exhaust systems (relative to where in the operational rpm range torque boosts were desired). When combined with the proper gearing to match engine speed and track conditions, this method turned out to be a targeted way to create track-specific engine and gearing packages. Properly done, the results were sometimes spectacular and frequently beneficial.
Overall, as pointed out in previous segments of this story Series, it's both possible and helpful to integrate an engine's torque performance with gearing and track conditions (including length, banking, surface, etc.) By so doing, you will move toward optimizing on-track performance by linking engine performance potential with track requirements and opportunities.