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.
The most significant factor that affects cylinder-to-cylinder air/fuel distribution in the intake manifold is the spatial relationship between the runner openings into the plenum and the carburetor flange. The floor of the runner is usually the most sensitive area. Adjusting the runner opening so that it "sees" more of the plenum will generally make that cylinder richer. Often the center cylinders shroud the end cylinders in a V-type engine. Careful unshrouding of the end cylinders can richen a lean-running cylinder.
Manifold Selection Typically, the selection of manifold configuration is limited for circle track applications. The choices are single-plane or dual-plane. Usually, engine operation speed determines the manifold configuration. If the power peak of the engine is below 6,500 rpm, then a dual-plane is most probably the manifold of choice. As the power peak moves significantly above 6,500 rpm, then the choice moves to a single-plane. The choice becomes more complicated when the engine's operation speed is somewhere in between. In this case, there is no clear answer, so testing will tell the tale. If well developed, a dual-plane can be more responsive and may be the driver's choice.
Don't Discount the Multiple Influences of Intake Manifolds
These can make or break a good set of cylinder heads ... or be easily flawed by improper use or modification. But regardless of which design or brand chosen or changes made, there are some essentials to keep in mind.
Intake manifolds don't flow in only one direction. There are times, depending upon engine speed and load, when pulses are directed back toward the carburetor (or air inlet point). Technically, this describes bi-directional, unsteady state flow. Despite how this is labeled, these "reverse flow" pulses are disruptive to airflow and air/fuel mixture quality (homogeneity). Either or both conditions can impair power. Therefore, there are only certain intake manifold features that can be evaluated on an airflow bench, although these include mapping of flow pressure profiles (pressure distributions) and specific velocity patterns.
It's also important to recognize intake manifold pressure conditions that encourage increases in unburnable combustion residue (principally exhaust gas). For example, conditions creating some level of manifold vacuum at wide-open throttle allow more contamination of fresh air/fuel charges than when near-zero vacuum exists when the influence of atmospheric pressure is greatest. This can be found in either restricted engines or those required to run 2V or small 4V carburetors.
In these instances, intake manifolds or carburetor spacers designed (or modified) to thwart reverse flow help minimize diluted mixtures and improve power. Another consideration is camshafts and exhaust ports/valves that address reverse exhaust flow as further dampers on reversion, particularly concerning exhaust opening timing. These determinations can be made on an air bench by flowing ports in the reverse direction, particularly at low valve lifts. But we digress.
Overall, the problem of changing the direction of air and fuel flow from approximately vertical to the entry angles of intake manifold runners is crucial to delivering efficiently combustible mixture. Plus, air tends to respond more quickly to throttle changes than fuel. Air and fuel are also prone to separate. In this sense, post-carburetor air/fuel mixture preparation becomes a function of the intake manifold. Surface finish in a manifold's interior can also play a role, trending toward rough instead of smooth surfaces in order to help create or maintain efficient atomization of post-carburetor fuel.
Although it may be difficult to separate the need for mixture quality from net airflow, each must be considered vital to proper manifold function. If Cup engine builders didn't acknowledge this as a key to optimizing power, the inordinate amount of time and expense devoted to intake manifold and carburetor spacer (or restrictor) applications would not be spent. And while the Saturday-night engine builder may not have the resources to address these problems similarly, skillful use of airflow benches can be a suitable substitute. An important ingredient is to become familiar with various air bench techniques that extend beyond mere mass flow measurements, to include pressure patterns and airflow quality.
As are many engine components, intake manifold selection (as rules permit) should include specific ranges of engine speed most frequently used. While peak power numbers may be impressive, or applicable in certain situations, torque production within an intended span of rpm is important to overall race car performance.
Rod length also plays a role. As piston speed around TDC is decreased (with rod length increased), it's helpful to use intake manifolds and inlet port sizes trending toward smaller section areas that aid flow velocity, independent of large piston displacements and high rpm. The rate of pressure drop across the inlet path (boosted by smaller runners) aids volumetric efficiency in the lower- and mid-rpm. In fact, it is worthwhile to consider an intake manifold's runners as extensions of intake ports, requiring that they are mutually compatible in airflow potential and uniformity of pressure distribution ... the latter is particularly important at the interface between manifold and head surfaces. Manifolds that neither reduce port flow nor (by themselves) exceed port flow can be considered "extensions" of a cylinder head.
Be assured investigations by Cup engine builders include some, all, or more of these areas that influence the overall intake manifold function. By the same token, the Saturday-night engine builder can choose and apply those that seem to fit particular power or on-track requirements.
Smallest Volume Possible In circle track racing, it is generally best to approach manifold development (or modification) by starting with the smallest overall volume possible and increasing volume until no power gain is observed. By having the smallest volume possible, the engine will be more responsive to throttle changes and will generally be easier to drive. By their nature, larger volume manifolds respond less quickly but may perform better at higher engine speeds.
"Tuning" intake manifolds When modifying intake manifolds, the three most common tuning elements are runner length, runner taper, and plenum volume. Using an Engine Simulation Program (ESP), the examples (graphics) provided in this section were created by modeling an ARCA engine. ESPs are one-dimensional models that perform wave calculations and are useful in the design of intake and exhaust systems. These high-end computer programs can predict the volumetric efficiency of an engine to within 2 percent of actual run data.
Runner Length Runner length tunes an intake manifold based upon pressure waves, or sound. The longer the runner, the lower the engine speed range in which the tuning will take place. To illustrate this concept, two computer models were prepared: a baseline model and a model with 1 inch of length added to all the runners (no other changes made).
Figure 3 shows the power and torque curves of the engine with the two manifolds. Clearly, the addition of 1 inch of runner length increases peak torque and moves the power peak down 200 rpm. Peak torque increased 2.0 lb-ft but peak power was down 7.9 bhp (brake horsepower). At 7,600 rpm, the power of the longer manifold is 8.3 bhp less than the baseline. (While these specific quantities may not be significant, their direction validates the theory behind the change.)
Figure 4 shows the pressure vs. crank angle at the outlet of the intake manifold (cylinder head juncture) at 7,600 rpm (power peak for the baseline). An increase in pressure here means that the charge density is higher and a better cylinder fill will occur (higher volumetric efficiency). The baseline manifold is doing a better job from shortly after the valve opens until just before it closes. It is interesting to note that both cases had volumetric efficiencies above 100 percent. The pressure vs. crank angle data shows how this can occur. When the intake valve closes, there is significant pressure above ambient (pressure) in the port that provides a mild supercharging effect. (Note: One bar is 14.7 psi or the equivalent of atmospheric pressure or one atmosphere.)
Overall, filling the cylinder is the goal. More mass flow into the cylinder means more power. Figure 5 shows mass flow vs. crank angle, at the intake valve at 7,600 rpm. The greater the area under the mass flow vs. crank angle curve, the more chemical energy is available for power production. The shorter runner manifold traps more mass in the cylinder at this engine speed than does the longer manifold and, therefore, makes more torque, and power.
Figure 6 shows the pressure vs. crank angle at the outlet of the intake manifold at 6,200 rpm (torque peak). The longer runner is doing a better job from peak lift until the intake valve closes. Again, there is pressure above ambien(atmosphere) at the close of the intake valve, in both cases.
Further to the issue of cylinder filling efficiency and manifold runner length, Figure 7 shows mass flow vs. crank angle at the intake valve at 6,200 rpm. The longer manifold runner traps more mass in the cylinder at this engine speed than does the baseline.
Runner taper Taper is the relationship between the size of the runner's inlet opening and the size of the same runner's exit. The effect of increasing taper (the opening is larger than the exit) is to foreshorten the runner. The greater the taper the greater the foreshortening effect. Taper is especially useful when it is difficult to shorten the runner length.
In the previous intake model, the 1-inch longer runners were modified by adding a significant amount of taper to all of the runners to see if the power could be recovered. The bhp improved by 11.6 over the longer runner alone and was up 3.3 over the baseline. Again, reviewing Figure 4, the improvement is from mid-lift on the opening thru mid-lift on the closing. Figure 5 shows again where pressure change affects the filling of the cylinder: by increasing mass flow.
Plenum Volume The plenum controls the pressure interaction (so-called "cross talk") or communication among the cylinders. A large plenum can decrease communication while reducing the volume can increase the interaction. The geometry of the plenum can influence the reflection of the sound waves. Generally, more plenum volume will make more peak power but hurts throttle response and may negatively impact peak torque.
To show the effect that plenum volume can have, the long runner case (computer model) with taper was modified to have significantly more plenum volume. Power at 7,600 increased by 0.9 bhp, again not representing a significant gain but verifying the concept. Figure 8 shows the pressure vs. crank angle for this large plenum manifold. The pressure at the time of intake valve opening is less than in the other cases, due to the larger plenum volume.
The plenum acts as an accumulator and holds more mass at the time the valve is opened, allowing for a longer blow-down period than the other manifolds. (Note: This effect is also useful at lower engine speeds on "restricted intake" engines.) Figure 9 shows mass flow vs. crank angle for this case. The larger plenum delivers its benefit early in the valve event and just after peak mass flow, when compared to the tapered and longer runner. The mass flow vs. crank angle graph shows the longer period of blow-down as provided by the increased plenum volume.
A Few Thoughts on Carburetor Spacers
Think of them as "tuning" tools. Spacers can be configured to address specific conditions in a given engine package. Spacers can add plenum volume, providing benefits already listed in this story. They are capable of helping diminish reversion pulses, improving combustion efficiency, and aiding stability of fuel delivery.
Sometimes, believe it or not, spacers become diagnostic tools by pointing to other problems in an engine requiring companion parts changes ... like valve timing, spark timing, carburetor calibration, or manifold selection. CT