
Intake manifold (PN 2975) • Edelbrock’s “breadandbutter” Victor Jr.

Intake manifold (PN 7101) • Designed to operate best in the 1,5006,500rpm range, the

Intake manifold (PN 2901) • Addressing the 2V Sportsman engine market, Edelbrock’

Intake manifold (PN 2940) • Edelbrock’s Victor Jr. Ford 2V Sportsman manifold is

Intake manifold (PN 300110) • According to information from Holley, this “spec&
In timehonored fashion, racers maintain a constant vigil on rules interpretation and loopholes. As Junior Johnson once commented, When theyre looking at the ass end, you work on the front end. And when theyre 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. Its not an objective of this story to draw lines on how such decisions are made. Rather, well 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, well 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 twoplane in design (common to Vtype engines), while manifolds with runners connected to a common plenum are designated singleplane.
Historically, the range of engine rpm in which the greatest volumetric efficiency (torque production) is achieved characterizes the differences between twoplane and singleplane intake manifolds. Despite efforts by some manufacturers to shrink single plane manifolds to perform similarly to twoplane (and the opposite), twoplane manifolds remain the choice for low and midrpm torque, while singleplanes 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 carburetors 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 basetoplenum 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
Its not just about power. Aside from providing designs intended to increase airflow, improving cylindertocylinder and air/fuel mixture delivery is important to increased engine parts durability. In stock intake manifolds, it is not uncommon to discover cylindertocylinder 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 cylindertocylinder 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 heres why.
There is an airflow velocity (sometimes called mean flow velocity) observed at or near peak torque, dependent upon piston displacement, rpm and flowpath cross section. Although this dimension is fixed in a spec manifold, knowing its influence upon torque production in the engine youre using is helpful. Of the methods that allow you to pinpoint engine speed range where a specific intake manifolds runner crosssection area tends to boost torque, the following one should be familiar ... if youre a regular reader of CT. And if youre 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 crosssection 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 V8 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 manifolds 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 youre dealing with an intake and its boosting torque in an rpm range where youre 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
Lets begin this with an example case. Suppose we have an engine of 355ci displacement (44.375 ci/cylinder). The spec intake manifold is a singleplane 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 wed planned to run the engine. Perhaps wed previously thought something like 4,5007,000 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 manifolds range of efficiency. In short, wed 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 5,0007,000rpm span, leaving it to the intake manifold to assist torque production for offthecorner acceleration. On the other hand, wed 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 crosssection area influences an engines 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 ontrack passing requirements upon exiting a corner.
By performing a little manipulation of the equation provided earlier, Crosssection 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 1.750inch pipes and need to determine how they match up to the spec manifold were discussing. If primary pipe wall thickness is 0.040inch (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 tradeoff 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 torqueshifting opportunity when shortening or lengthening this size pipe.
In the camshaft department, its a general rule of thumb that you specify an rpm range in which youd like the most significant power gains. Since youre already in the rpm determination mode, sharing with your camshaft grinder the ranges of engine speed youd like to address is not only valuable but also fairly available information. Plus, once youve 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 engines 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 cylindertocylinder air and mixture distribution improvements provided by spec manifolds benefit engine parts durability, its important to determine where power is needed and the manifold is not supportive.
The conclusion of this twopart 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 its needed. To view Outtakes On Intakes, Part II, Click Here.