Figure 2 - Changes in collector length tend to affect the "tuning range" (below peak brake
Torque output below peak is materially affected by header collector dimensions. Adding volume by way of length tends to build torque below the peak. Experience has shown that dyno testing a range of collector lengths, noting the gains, and relative rpm can become an at-the-track tuning tool, given certain track length and surface conditions. Plus, this is a very convenient, quick, and comparatively inexpensive modification. Keep in mind that you can also tune torque out of this range by shortening or removing collectors completely (see figure 2).
In a similar fashion, you can manipulate torque above peak torque rpm by lengthening or shorting primary pipes. Such adjustments can easily be made at the junction between primary pipes and collector, even if the "collector" does nothing more than merge these pipes. Of course, as previously stated in this Series, such adjustments tend to "rock" the torque curve around peak torque rpm (see figure 3).
Shifting the torque peak(s) rpm higher or lower in the rpm range is a function of pipe diameter (section area inside the pipe), largely because a change in inside diameter affects mean flow velocity at any given rpm. This consequence can become a tool to targeting the rpm at which you want a torque boost, simply because you can manipulate where the critical mean flow velocity occurs (see figure 4).
On The Intake Side - Here, the flexibility for making flow passage dimensional changes is far less than for the exhaust side. However, there are some guidelines you may want to consider.
Figure 3 - Typically, you will note two fundamental conditions (relative to torque output)
First of all, as previously indicated, regardless to which "tuning theories" you subscribe, significantly similar dynamics exist in both intake and exhaust systems. Even the "wave motion" theory maintained by many tuning proponents finds similarities in how both systems (intake and exhaust) can be treated. Despite differences in piston position when optimum pre-combustion conditions favor the intake or exhaust cycles (roughly mid-stroke for the intake cycle and bottom dead center for the exhaust), flow dynamics are surprisingly similar.
Included in the similarities are passage length, section area, and the fact that the roughly 240 feet/second mean flow velocity effects, for all practical purposes, are the same. What this does is aid decisions in the selection, evaluation, and modification of these systems during dyno testing. Even absent the transient load (resistance of a racecar during acceleration) and the dynamic effects not precisely duplicable on an engine dyno, you can still perform some valuable tests, certainly if the data evaluation procedure is valid. We'll address that issue in a moment.
In terms of "tuning" intake manifolds on a dyno, recall our previous comment about the Smokey Ram. Even in manifolds not of the "box" type that don't allow random fuel migration in a comparatively open volume prior to runner entry, manifold flow dynamics related to the part's design can influence cylinder-to-cylinder air/fuel charge mixture distribution. Today's wet-flow benches have provided ample proof of this. And even if you've done a good job of balancing cylinder-to-cylinder air flow on a bench, the "quality" of such flow can materially affect the distribution of fuel in that air during engine operation.
I've seen instances where sorting out distribution problems on an engine test stand can be a good start, and the use of exhaust temperature probes and skillful plug reading will certainly help. Given the response time of today's HC/CO emissions meters, sampling exhaust byproducts immediately downstream of the header flange is better yet.