Introduction

By one simplified definition, a

typical automotive "induction sys-

tem" is a network of passages con-

necting an engine’s cylinders with

a source of inlet air, attached to

which is a metering device that

regulates air or air/fuel mixtures.

Such systems may include fuel

injectors located downstream of

the air inlet point or accommodate

either carburetors or throttle body

injection (TBI) units at a point serv-

icing all passages to the engine’s

cylinders.

That’s a functional definition. In a

dynamic sense, an induction system

includes a range of pressure condi-

tions that can help an engine

achieve torque (or volumetric effi-

ciency) increases beyond that

obtainable without the benefit of

induction system "tuning." Several

of these possibilities will be present-

ed in the following discussion.

We’ll wade through the "basics"

section first, identifying fundamen-

tal components of an intake mani-

fold, types of manifolds, the concept

of cylinder-to-cylinder air (or

air/fuel mixture) distribution,

manifold "tuning" (in principle), car-

buretor systems, multiple carbure-

tion and the benefits of throttle body

and direct port fuel injection or

multi-point FI.

In the "advanced" segment, we’ll get

more into the essentials of manifold

tuning, how volumetric efficiency

(and torque) are influenced by

manifold design, design compo-

nents of intake manifolds, mixture

motion (in the combustion space) as

influenced by intake manifolds, the

"reversion" phenomenon, and the

concept of "variable" intake mani-

folds. Buckle up.

Basic Information

Fundamental components of

an intake manifold

Since an intake manifold is a

network of passages connecting a

throttling device (air valve, throttle

body or carburetor), you can view

such passages as extensions of the

inlet ports. In fact, that is exactly

how they should be considered. If

they are not, it’s possible for each to

become in conflict with the other,

causing improper air (or air/fuel

mixture) flow quantity and quality.

Generally, at or near the inlet

manifold’s point of air entry, there is

a throttling device. These are locat-

ed where the passages of the man-

ifold (inlet port "extensions") meet in

a common volume. That volume is

often called a "plenum" and repre-

sents a region from which all mani-

fold runners extend. Analogous to

an exhaust system in which all tubes

become joined in a single volume

(collector), the plenum of an intake

manifold is comparable to a head-

er collector.

Types of Inlet Manifolds

If all of an engine’s inlet ports are

connected to a single plenum vol-

ume (chamber), the design is said

to be a "single plane" arrangement.

That means a single cavity is used to

connect all inlet port passages (run-

ners). If some of the runners con-

nect to a cavity different from a sec-

ond cavity connected to the remain-

ing set of runners, the design may

be called a "two plane" or "dual

plane" design. Depending upon

basic engine design (in-line, V-type,

etc.), it’s possible to connect cylin-

Jim McFarland

1 The Manifold Parts of Automotive Induction Systems

Join Jim McFarland as he steps into his role

as the "Performance Professor" and shares

a wealth of knowledge and experience that

will help take you to a higher level of per-

formance.

The Manifold Parts of

Automotive Induction Systems

An intake manifold plays a

vital role in how each of an

engine’s cylinders receives

air or air/fuel mixtures.

Whether the design joins all

cylinders to a common

plenum (single plane) or

divides cylinders in some

fashion, there is a measure

of "cross talk" that exists

among connected cylinders,

during a variety of engine

operating conditions.