The complexity of an engine’s

cylinder head(s) is realized when

you consider the multiple functions

that must be satisfied. Among them

are the provision of efficient intake

and exhaust passages, a combus-

tion space that optimizes power, a

cooling "jacket" (except in high-

demand racing cylinder heads) that

enables temperature control, gas-

ket sealing surfaces that accommo-

date high pressure combustion

forces, attachment points for other

major components (intake and

exhaust manifolds) and a bolt pat-

tern into the cylinder block that aids

cylinder bore concentricity under a

wide variety of stressful conditions.

Other than that, the objectives of

cylinder heads are a piece of cake.

There are several perspectives to

be considered when investigating

the subject of cylinder heads. Is the

objective to restore a stock head to

service? Are modifications to a stock

head contemplated? Will a substitu-

tion be made for a stock head,

either in design, function or materi-

als? What are the ultimate goals for

even thinking about the parts?

Regardless of what path you may

choose to take, there are some fun-

damentals, which should be part of

your information store. Let’s exam-

ine the basic functional components

of a head, from a performance per-

spective, by looking into some of the

practical (basic) topics. Then we can

step back and investigate more the-

oretical (advanced) topics.

Basic Information

Many beginners to the perform-

ance scene think about cylinder

heads as a means for (1) increas-

ing compression ratio, (2) improv-

ing air flow (both intake and

exhaust) or (3) saving weight (sub-

stituting aluminum for cast iron).

While all of these are factors worth

considering, there are other

important elements.

For example, is the subject head

new or used? If it’s used, will it be

properly inspected for cracks or

similar damage? Are all its gasket-

sealing surfaces true (or re-sur-

faced) and capable of providing

the necessary insurance against

leaks. Assembling and installing a

"re-conditioned" head that won’t

hold proper seal is not the most

enjoyable way to spend a

Saturday afternoon. Time taken

to make certain all the "non-per-

formance" components of a used

head are proper is time well

spent…and then some. Of course,

if the head is new, these concerns

are covered…or should be.

Port size and shape

These need not be areas of con-

fusion, but to many beginning per-

formance enthusiasts, advertising

claims and print media editorial

materials can create questions. To

understand how ports are intend-

ed to work (and respond to modi-

fication), it’s necessary to consider

how a "working fluid" (air, air and

fuel, exhaust gas or combinations

of all these) is caused to move.

It all begins with a difference in

pressure (differential) between two

points. For example, when an

engine’s piston descends, a low-

pressure condition is created in the

cylinder space above the piston.

Atmospheric pressure (or a super-

charger) represents a higher-pres-

sure source, so the "working fluid"

is forced into the lower pressure

cylinder. How this fluid behaves in

the inlet passage is influenced by

the port’s shape, length and wall

texture. For the most part, the

same conditions exist in exhaust

ports. In these cases, cylinder

pressure becomes the dominant

force for exit flow.

If the working fluid is air only,

then changes in its flow direction

or velocity are less influential on

total flow volume. However, if air

and fuel combine to form the

working fluid, their inherent differ-

Jim McFarland

1 Cylinder Heads and Power

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.

Cylinder Heads and Power

Is the objective to restore

a stock head to service?

Are modifications to a stock

head contemplated? Will a

substitution be made for a

stock head, either in design,

function or materials? What

are the ultimate goals for even

thinking about the parts?