Without adequate and properly

timed airflow, power from an

internal combustion engine will be

less than maximum. Following are

some keys to making certain you

know how to obtain horsepower

from proper supply of air. Also

included are some basic defini-

tions of related and important

terms.

Engine Airflow

(Basic Information)

Engines operate in a “sea” of

air pressure. This either takes the

form of atmospheric influence or

some artificial means of getting air

inside an engine. If atmospheric

pressure is the driving force that

provides air, the engine is said to

be “normally aspirated.” Should a

mechanical means be added to

atmospheric pressure, “super” air

charging is the method of aspira-

tion. Stated another way, an

engine becomes “supercharged”

with air pressure higher than

atmospheric.

The downward movement of

pistons creates an in-cylinder

pressure that is less than atmos-

pheric, causing air to be forced

into the engine, either by atmos-

pheric or some artificial means of

providing pressure (supercharging

is an example of “artificial” aspi-

ration). The term “suction” is

somewhat a misnomer, suggesting

that air is pulled or “drawn” into

cylinders rather than forced there

by atmospheric or mechanically

derived pressure. This difference

in perspective of how air enters an

engine is important to remember,

particularly when considering the

way carburetors operate. It will be

a concept to which these “lessons”

will refer as discussions unfold

during following months.

Cylinder Filling & Torque

The ability of an engine to

ingest air is a measurement called

“volumetric efficiency.” Although

this term will be discussed in more

technical depth a bit later, suffice

that it is a means by which an

engine’s physical capacity is com-

pared to actual air intake. This is a

dynamic measurement obtained

as a function of engine rpm and

load. If an engine accepts a vol-

ume of air that is equal to its phys-

ical volume (piston displacement),

it is said to have a volumetric effi-

ciency of 100%. It can be less or

more, depending upon a variety of

circumstances.

In reality, an engine’s volumetric

efficiency at each rpm of measure-

ment can be represented on a

graph. Sample v.e. and torque

curves are provided elsewhere in

this lesson (see illustration #5).

Interestingly, a volumetric efficien-

cy curve closely approximates an

engine’s torque curve, or the

amount of torque produced at

each rpm of measurement. From

this, you might suspect that there is

a specific relationship between a

volumetric efficiency curve and a

torque curve…and there is.

Generally speaking, increases

or decreases in volumetric efficien-

cy correspond to increases or

decreases in torque. If an engine

modification is made that increas-

es airflow (or volumetric efficien-

cy), the potential for an increase in

torque is available. In fact, the

majority of changes to a stock

engine intended to increase power

are associated with some form of

airflow change.

Of particular importance is the

fact when a change in airflow is

caused (typically an increase), it is

necessary to make changes in fuel

flow and ignition spark timing.

Historically, these changes have

been made to “mechanical”

devices (carburetors and ignition-

point distributors). Today, on-

Jim McFarland

1 Airflow and The Internal Combustion Engine

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.

Air Flow & The Internal Combustion Engine

“Of particular

importance is the

fact when a change

in airflow is caused

(typically an increase),

it is necessary to make

changes in fuel flow and

ignition spark timing.”