Whether for the street or rac-

ing, automotive muffler tech-

nology is being taken to

new

levels of performance. The fol-

lowing discussion begins with

“sound basics” and concludes

with some current muffling

exercises that may get your

attention.

It all begins in the combus-

tion space. Let’s go to that

location and begin tracing the

steps that evolve immediately

after the “burning” of air and

fuel...and the exhaust valve

first cracks open.

Basic Information

Beginning

The “Exhaust Cycle”

Using a single-cylinder engine

for purposes of discussion, we

know that air {essentially at atmos-

pheric pressure} occupies the

exhaust pipe, just prior to exhaust

valve opening. Combustion pres-

sure inside the cylinder, at a much

higher level than atmospheric, has

been performing work on the pis-

ton during the “expansion” stroke.

This work, transformed into

crankshaft rotation or torque, has

reached a point where it’s time

to remove combustion residue

{exhaust gas} from the cylinder.

As the exhaust valve opens, any

energy left in the cylinder {in the

form of exhaust gas pressure}

enters the exhaust passage {port,

pipes and muffler} on its way to

the outside world.

In simple terms, this “slug” of

exhaust gas pushes out any exist-

ing air in the passage as it makes

its way to the end of the system.

This volume of gas, acting at a

higher pressure than the pipe’s

contents, is more compressed than

the material it evacuates from the

pipe during the exhaust process.

Unabated on its way out the sys-

tem, this “slug” of exhaust gas will

produce a sound as it reaches the

atmosphere that can be both

unpleasant to hear and, possibly,

damaging to the ear. It can also be

pleasant and not damaging. This is

among the reasons various types

of sound muffling devices are used

to change the energy of exhaust

pressure to sound levels more

acceptable to the world outside the

combustion space.

Activity Inside

The Exhaust System

If we examine the motion of the

previously-described “slug” of

exhaust gas {the result of a single

combustion process}, we find that

once it reaches the end of the

exhaust system there is a reaction

to its meeting with atmospheric

pressure. In fact, a pressure wave

or “disturbance” is caused in a

backward direction, causing activi-

ty toward the engine’s cylinder;

even while exhaust gas is leaving

the engine.

Once this wave reaches the

cylinder, it is reflected back

through the system toward atmos-

pheric pressure. It is this back-

and-forth activity that is similar to

the motion of a spring-suspended

weight {see illustration}. At the risk

of oversimplification, this decreas-

ing {with time} simple harmonic

motion describes the action of

pressure waves in an automotive

exhaust system.

The motion of these waves within

the exhaust gas is similar to the

motion of waves on a stream of

water For example, visualize a

stream of water passing along a

creek bed. There is motion and ener-

gy in the stream. If a rock is thrown

into the stream, waves of disturbed

of water are formed. While these

waves may be acting independently

of the main stream, they are still car-

ried along by the stream. Under cer-

tain conditions, it’s possible for these

smaller waves to move in a direction

against the main stream’s flow, much

like the oscillating pressure waves in

an exhaust system.

Jim McFarland

1 Muffled Performance

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.

Muffled Performance