Editor's Note: As inanimate as intake manifolds may appear, they provide a path to both major and minor on-track power gains. Selection is one thing; "tuning" them to specific applications is a blend of skill and technology. This story goes right to the heart of the second.
It is appropriate that the intake manifold is mounted on top of the engine. After valve events have been selected, the intake is the primary tuning device for a four-stroke spark ignition engine, just as the expansion chamber is for a two-stroke. In the case of a carbureted V-8, the intake manifold's function is to separate incoming air/fuel charges and direct them into the cylinder head. Tuning becomes the manifold's second function.
Air/fuel charge distribution While many articles have discussed how intake manifolds tune and how to select the appropriate manifold for your engine, few discuss the variation of air/fuel ratio from cylinder to cylinder. This is a critical factor in engine tuning because the mixture can only be leaned to the point where the leanest cylinder is at its operational limit. With individual runner (IR) induction systems and electronic fuel injection, the variation can be tuned to less than 0.5 of an air/fuel ratio. Carbureted V-8 engines typically have significantly worse variation that can be up to four air/fuel ratios from the worst cylinder to best.
Cornering g-forces can have significant effect on the mixture distribution. This can be seen when comparing dynamometer air/fuel ratio data to on-track data. Figure 1 shows on-track air/fuel ratio data from a GM ARCA engine at a 1.5-mile, high-speed track. All four carburetor main jets were identical. The data shows a variation from 12.0:1 for cylinder No. 2 (at 8,100 rpm) to 15.0:1 for cylinder No. 1 at 7,700 rpm (see circles). For optimum power, three air/fuel ratio variations from cylinder-to-cylinder is not a desirable condition. When tested on the dynamometer, this manifold showed a 2.0-2.5 air/fuel ratio variation, thereby verifying the inherent differences between ratio spread on an engine dynamometer and on the track.
Figure 2 compares air/fuel ratio averages from the left bank (cylinders 1,3,5,7) to the right bank (2,4,6,8) of a V-8 engine with firing order 1-8-4-3-6-5-7-2. As you might expect, the data shows the impact of the g-forces making the right bank richer than the left. Obviously, this effect will be more pronounced on tracks with high cornering loads and can be minimized by stagger-jetting the carburetor. This example was selected because it clearly demonstrates the point. Not all manifolds are affected this severely.
Improving cylinder-to-cylinder distribution If an air/fuel distribution problem exists, first check to make certain the carburetor is placed properly on the intake. Another method of adjusting distribution is to move or bend the carburetor booster relative to the throat in which it's installed. (Exercise extreme care when attempting to "bend" boosters. It is also possible to place small "tabs" or "protrusions" on booster bodies to redirect airflow in the throat and alter post-carburetor flow direction.) Carburetor spacers may also have an impact on distribution (see section discussing spacers). Often a four-hole or combination four-hole and open spacer will improve cylinder-to-cylinder distribution.
At best, working with the manifold itself to improve distribution is difficult and should only be attempted when a dynamometer is available with eight channels of air/fuel ratio sensors. Reading spark plugs may not be accurate enough for this type of development.