In time-honored fashion, racers maintain a constant vigil on rules interpretation and loopholes. As Junior Johnson once commented, “When they’re looking at the ass end, you work on the front end. And when they’re looking at the front end, you work on the ass end.” Chicken farmers everywhere, beware.

But in the interest of reducing or maintaining costs of racing, certain engine components have come under the rule of “no modification.” Intake manifolds often fall into the “spec” category, placing responsibility on the manufacturer to optimize engine performance by redesign through identification and correction of defects in the stock parts replaced.

This story carves out some of the issues that enable engine performance optimization, confined to “spec” intake manifolds, by examining certain companion parts that can be tailored to “regulated” airflow. It would appear that rules evolve at all levels of racing, including those promulgated by individual tracks. It’s not an objective of this story to draw lines on how such decisions are made. Rather, we’ll just get into ways engine performance can be boosted by attending parts selection. But first, some basics about intake manifolds provided in the interest of building an informational foundation for further discussion.

Basic Intake Manifold Components And Features

Since the vast majority of current circle track engines use carburetors, we’ll confine this material to carbureted or “wet flow” intake manifolds. As such, these comprise a network of passages (runners) connecting inlet ports to a space (plenum). Runners can be connected to the plenum in such a way that separates every other cylinder in the firing order or terminating in a common plenum irrespective of firing order.

In the case of the former, more than one level or “plane” is often used, allowing cylinder separation so intake charges are delivered alternatively from one plane to the other. Typically, these are labeled “two-plane” in design (common to V-type engines), while manifolds with runners connected to a common plenum are designated “single-plane.”

Historically, the range of engine rpm in which the greatest volumetric efficiency (torque production) is achieved characterizes the differences between two-plane and single-plane intake manifolds. Despite efforts by some manufacturers to shrink single plane manifolds to perform similarly to two-plane (and the opposite), two-plane manifolds remain the choice for low- and mid-rpm torque, while single-planes are most efficient at higher engine speeds. Generally, these are the fundamental differences between the two types.

Of the features that affect intake manifold “tuning,” runner cross section and length combined with plenum volume can be dimensionally united to improve torque and throttle response. Generally, runner cross section affects the rpm point at which peak torque is produced, while length influences the torque produced above and below this point. Plenum volume, where sufficient space is available, can be made to behave much like a header collector, boosting torque below peak torque rpm. However, in most instances plenums are configured to assist in the transition of air/fuel mixtures from carburetor to manifold runners.

Are other features of an intake manifold important? Certainly. One is the relationship between the carburetor base and plenum floor. Visualize air and fuel being discharged from beneath the carburetor’s throats. Two issues are immediately critical: the dynamics of “turning” air/fuel mixture into intake manifold runners (without encountering air/fuel separation) and the proximity of the carburetor base and plenum floor. Through the use of carburetor spacers, both of these problems can be addressed successfully, if rules permit their use.

Spacers provide an opportunity for air/fuel mixtures to lose a measure of kinetic energy, leaving a carburetor and entering manifold runners. Should you not feel this issue is critical, consider the possibility of mixtures moving in excess of 200 feet/second and changing direction from vertical to something approaching horizontal ... over a carburetor base-to-plenum floor distance of less than six inches. The effect is akin to sticking a water hose in a bucket. Increasing carburetor height helps alleviate the problem of disrupted mixture quality during high rpm, as carburetor size decreases, or both.

Why Stock And Performance Manifolds Differ

It’s not just about power. Aside from providing designs intended to increase airflow, improving cylinder-to-cylinder and air/fuel mixture delivery is important to increased engine parts durability. In stock intake manifolds, it is not uncommon to discover cylinder-to-cylinder air/fuel differences upward to three or four ratios, from the leanest to richest cylinders, traceable to improper manifold design. For this reason alone, particularly when stock intake manifolds are used for racing, mixture inequality can lead to damaged engine parts (pistons, rings and cylinder heads).

If racers were permitted to make manifold modifications that improved cylinder-to-cylinder air and/or mixture distribution, problems of this nature might be resolvable. However, since improved distribution is among the design criteria of aftermarket manifold designers and the modification of stock manifolds can be expensive, engine durability is often a benefactor from the use of “spec” intake manifolds. The fact they produce additional power is a racer plus.

Relating Manifold Size To Engine Size And Rpm

In previous Circle Track material, the relationship among these variables was discussed at length. By way of quick review, barring any changes to the internals of an intake manifold, the larger the engine or higher the rpm, the lower the engine speed at which peak manifold efficiency is achieved and here’s why.

There is an airflow velocity (sometimes called “mean flow” velocity) observed at or near peak torque, dependent upon piston displacement, rpm and flow-path cross section. Although this dimension is fixed in a “spec” manifold, knowing its influence upon torque production in the engine you’re using is helpful. Of the methods that allow you to pinpoint engine speed range where a specific intake manifold’s runner cross-section area tends to boost torque, the following one should be familiar ... if you’re a regular reader of CT. And if you’re not, you should be. The arithmetic goes as follows:

Measure and calculate the area of each intake runner entry and exit (at the cylinder head flange). Add the two values and divide by two, producing an “average” runner cross-section area. Now determine the volume of one cylinder (displacement divided by the number of cylinders). Armed with this information, use the following equation: Torque peak rpm = (88,200 x average runner cross section)/volume of one cylinder. If we assume and plug in some numbers, the results look like this:

Peak torque rpm = (88,200 x 2.9 sq. in.)/43.75ci = 5,946 rpm

Where 43.75ci is the cylinder volume of a 350 V-8 engine, 2.9 sq. in. is the average cross section of an intake manifold runner, and 88,200 is a constant used for units and related conversion factors.

Why is this exercise of value? You now know the rpm at which this particular “spec” intake manifold is designed to boost volumetric efficiency (torque) and the engine speed around which you can begin optimizing other components to support the manifold’s inherent performance range. This is particularly valuable because parts companion to the intake manifold can now be selected (or modified) to contemplate where the manifold is not working best and integrating their performances accordingly.

For example, if you’re dealing with an intake and it’s boosting torque in an rpm range where you’re not running the engine (but should), some gearing or tire size changes may be in order. You may also want to consider some dimensional changes to the exhaust system. Perhaps investigate a different camshaft or where the one you have should be positioned (advanced or retarded). In short, you need to begin building torque in a range of engine speed where the “fixed” intake is not.

On the other hand, even if the intake manifold is working best in the span of rpm being targeted, changing or modifying companion parts to further boost torque in this range could be of additional help. But in either case, knowing where the manifold is providing the greatest gains in power can help you analyze the overall engine package and its performance.

In particular, this information can lead to more sensible selections of other engine parts as they relate to the rpm range in which you plan to run the engine. In a sense, dealing with “spec” intake manifolds is not much different than the use of cylinder heads for which no modifications are allowed. You need to make decisions about (1) a specific span of engine speed where power is required, (2) performance limitations of the parts for which no alterations can be made (in this case intake manifolds) and (3) characteristics of companion parts that can help compensate for rules restricting modifications.

Some General Guidelines For Companion Parts Selection

Let’s begin this with an example case. Suppose we have an engine of 355ci displacement (44.375 ci/cylinder). The “spec” intake manifold is a single-plane 4V design. By measuring its runner entry area (2.75 sq. in.) and exit area (1.98 sq. in.), we determine an average of 2.37 sq. in. Using the previously supplied equation, the peak torque point (for this intake manifold and assuming all runners of equal cross section) computes to 4,710rpm.

Now, we need to examine the rpm range in which we’d planned to run the engine. Perhaps we’d previously thought something like 4,500-7,000 was acceptable, but now we discover the “spec” manifold will be on the declining side of its torque production on the low end of our projected rpm range. Either we adjust the intended rpm range downward or pick companion pieces that boost torque above the manifold’s range of efficiency. In short, we’d like to make the manifold “look bigger” to the engine.

In fact, we might even consider where this manifold is working (relative to rpm) as a plus. The engine can be fitted with a cam and set of headers that work best in the 5,000-7,000rpm span, leaving it to the intake manifold to assist torque production for off-the-corner acceleration. On the other hand, we’d be fooling ourselves by placing dependence on the manifold to produce much useable torque if the engine was spending most of its time above 5,500rpm. Among the reasons for identifying rpm at which an intake manifold is best suited, this is clearly one.

On the chance you might not have yet selected a set of header dimensions, suppose we examine that next. With the knowledge that primary pipe cross-section area influences an engine’s peak torque rpm point (regarding contributions by headers), we decide a torque boost at 5,500 would probably be a sensible place to start considering on-track passing requirements upon exiting a corner.

By performing a little manipulation of the equation provided earlier, Cross-section area = Peak torque rpm x Cylinder volume/88,200. If we plug in the values for our example engine, primary header pipe o.d. needs to be 1.94 inches, in order to produce a torque boost at 5,500. Since 2.0 inches of primary pipe o.d. is the nearest standard size to 1.94 inches, a set of headers built with pipe of 2.0 inches o.d. should get the job done.

But maybe you already have a set of 1.750-inch pipes and need to determine how they match up to the “spec” manifold we’re discussing. If primary pipe wall thickness is 0.040-inch (not uncommon), i.d. is 1.67 inches and the area 2.19 sq. in. Using the same mathematical approach applied to our “spec” intake manifold, peak torque rpm for our headers is about 4,350rpm. Now we have headers peaking their efficiency in the range of our “spec” manifold.

One solution to this problem is to shorten primary pipe length, thereby adding torque above the peak and removing it from below. However, this could be considered a trade-off to properly addressing the situation. Using a set built with 2.0 inches of pipe o.d. (as outlined above) not only goes directly to the issue but also provides some additional torque-shifting opportunity when shortening or lengthening this size pipe.

In the camshaft department, it’s a general rule of thumb that you specify an rpm range in which you’d like the most significant power gains. Since you’re already in the “rpm determination” mode, sharing with your camshaft grinder the ranges of engine speed you’d like to address is not only valuable but also fairly available information. Plus, once you’ve decided upon a camshaft suited to filling in blanks not provided by the “spec” intake manifold, advancing and retarding its installed position can further aid the cause.

Some Concluding Comments

Conforming companion parts to a “spec” intake manifold involves changing the basic shape of an engine’s torque curve. In particular, header design/dimensions and valve timing patterns can be used favorably. Generally, the first order of business is to determine the rpm range in which the “spec” manifold is most productive.

By comparing this information to the span of rpm in which the engine will be used (or should be used), other parts can be selected to augment areas not supported by the “spec” manifold. While cylinder-to-cylinder air and mixture distribution improvements provided by “spec” manifolds benefit engine parts durability, it’s important to determine where power is needed and the manifold is not supportive.

The conclusion of this two-part story focuses on some specific changes that can be made to camshafts, exhaust headers, and other engine components that will help tailor torque curves to compensate for “spec” intake manifolds and provide power where it’s needed. To view Outtakes On Intakes, Part II, Click Here.