Ebook Automotive engines diagnosis, repair and rebuilding (6th edition): Part 2

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312 C

ONTENTS

• Intake and Exhaust Manifolds

• Engine Modifications to Improve Breathing

• Exhaust Manifolds

• Turbochargers and Superchargers

• Belt-Driven Superchargers/Blowers

• Camshaft and Engine Performance

• Checking Camshaft Timing

• Camshaft Phasing, Lobe Centers, and Lobe
Spread

• Variable Valve Timing

• Active Fuel Management/Displacement on
Demand

• Power and Torque

• Measuring Torque and Horsepower

• Dynamometer Safety Concerns

O

BJECTIVES

Upon completion of this chapter, you should be
able to:

• Describe the effects of the
supercharger/turbo-charger on engine performance.

• Describe how cam lobe profile affects high and
low rpm engine performance.

• Advise a customer on high-performance options
for his or her engine.

I

NTRODUCTION

Building high-performance engines has been a
popular pastime for generations. In the 1930s and
1940s, when flathead engines were popular, hot

rodders changed the compression ratio by milling
the cylinder heads; bored cylinders oversized; and
used special intake manifolds, carburetors, and
headers. In the 1950s, 1960s, and 1970s, when
“mus-cle cars” were popular, overhead valve pushrod
engines were commonly modified to achieve
high-end horsepower (Figure 10.1).

In today’s era of the sport compact car, many

four and six cylinder engines develop as much or
more power as eight cylinder engines of the past.
The smaller engines today use multiple valve
com-bustion chambers, along with other modifications
to increase breathing ability. This chapter deals with
intake and exhaust manifolds, turbochargers and

Engine Power and

Performance

C H A P T E R

FIGURE 10.1 A high-performance pushrod engine. (Courtesy of 
Tim Gilles)

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CHAPTER 10 Engine Power and Performance •

313 INTAKE AND EXHAUST MANIFOLDS

The breathing system includes intake and
exhaust manifolds that are carefully designed to
pro-vide a uniform flow to and from all cylinders.
Mani-fold passages are known as runners. When a single
manifold runner feeds two neighboring cylinders,
these are known as “Siamese” ports (Figure 10.2).

Intake Manifolds

When an engine has throttle body fuel injection
or a carburetor, the intake manifold is called a wet 
manifold because it flows both air and fuel. A wet

manifold is designed to provide optimum flow for
the air-fuel mixture and to reduce the chances of the
vaporized fuel turning back into liquid fuel. Intake
manifold runners on these engines have as few
bends as possible.

superchargers, engine performance, camshaft lobe
designs, and variable valve timing. These items
govern the performance of the engine.

Basically, an engine will produce more power
when more of a correctly proportioned air-fuel
mix-ture enters the cylinder. When an engine does not
have a turbocharger or supercharger, it is referred
to as normally aspirated or naturally aspirated. Engines
equipped with turbochargers or superchargers can
breathe more air and, therefore, produce more
power.

An internal combustion engine is a big,
self-driven air pump. The camshaft is the determining
factor in how efficiently the engine pumps air while
operating at various speeds. The overall
perfor-mance of the engine is determined by the grind, or
profile, of the cam. The size and shape of the intake
and exhaust manifold runners and the valve ports
also play a part in determining the engine's
breath-ing ability.

NOTE

This chapter discusses real world situations that
some-times occur on customer vehicles. Aftermarket and
high-performance issues are also covered, primarily because
most shops have customers who can afford to spend
money on their classic automobiles, and some customers
own several of them. These select customers will expect
you to know and understand this material, and if you are
knowledgeable, the word will quickly spread. The aim of
the material provided in this chapter is to “keep it simple.”
The objective is to put you in a position so you can easily
understand the basics of engine performance. If you
should decide to go further in making refinements on a
manufacturer's design, you will need to do further study
by reading more advanced publications on the topic of
your choice.

INTAKE
EXHAUST

Conventional head-Siamesed valve ports

1 2 3 4

(a)

EXHAUST
INTAKE

Alternate head-individual ports

1 2 3 4

(b)

FIGURE 10.2 The top sketch (a) shows “Siamese” valve ports that
share a manifold runner. The bottom sketch (b) shows individual ports.

VINTAGE ENGINES

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314

• SECTION II The Breathing System

Port fuel injection systems inject fuel directly
above the intake valve. The intake manifold is
designed for airflow only because fuel does not
travel through the manifold. Port fuel injection
man-ifolds can be designed with larger runners than wet
manifolds. The runners can also have sharper bends,
because these manifolds do not have to keep fuel
suspended in air. Figure 10.3 shows an intake
mani-fold from a fuel injected four cylinder OHC engine.

Carbureted Manifolds

Intake manifold design is crucial to engine
oper-ation in much the same way as camshaft design.
Parts are engineered to match and each
combina-tion is a compromise. Breathing parts must be
correctly matched to each other. For instance,
purchasing a high-performance manifold without

buying matching components will probably hurt
engine performance.

NOTE

In general, better performance at high rpm results in worse
performance at low rpm.

Intake manifolds that flow air and fuel are
designed to keep the fuel suspended in the air in
fine droplets like fog. By the time the mixture
reaches the combustion chamber, most of the fuel
should be evaporated so it can burn easily. If the
speed of the mixture drops too low, droplets of raw
fuel can fall out of the mixture.

Manifold runner sizes are a compromise.
Large-diameter runners flow well at high speeds, but the
fuel separates from the air at lower speeds.
Through-out the average rpm range of a passenger car,
small-er-diameter manifolds work well to provide enough
flow and keep the fuel in suspension.

Plenum. The air space in the manifold below a
car-buretor or throttle body is known as the plenum. The 
plenum floor is flat and often has ridges cast into it
to catch fuel that drops out of the mixture. This
makes it easier for the fuel to evaporate or to rejoin
the moving air-fuel mixture as it flows through the
manifold.

Dual- and Single-Plane Manifolds

On an eight cylinder engine with a dual-plane
two-barrel manifold, each “barrel” supplies fuel to
four cylinders (Figure 10.4). Manifold runners are
designed to be nearly the same length so they will
flow an equal amount of air and fuel. One barrel
supplies air and fuel to both of the inner two
cylin-ders on the opposite side of the engine and the outer 
two cylinders on its own side. This knowledge is
(a)

Intake runners

FIGURE 10.3 (a) These intake manifold runners for a four cylinder
fuel injected engine are short, large, and relatively straight. (b) An
intake manifold on a late model. (Courtesy of Tim Gilles)

Fuel injectors Intake manifold runners

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CHAPTER 10 Engine Power and Performance •

315

handy when troubleshooting vacuum leaks or
car-buretor failure if the problem is found to be only in
those cylinders served by one barrel.

Figure 10.5 compares dual-plane and
single-plane intake manifolds. The dual-single-plane manifold
(Figure 10.5a) has smaller runners and is better
suited to lower rpm use. A single-plane manifold,

in which both barrels serve all eight cylinders, is
more suited for high-speed use and is not street
legal (Figure 10.5b).

Intake Manifold Coolant Passage

The intake manifold on a V-type engine has a
coolant passage that connects the heads and provides
the coolant outlet where the thermostat is located.

NOTE

A crack in the coolant passage can cause a leak that can be
difficult to diagnose.

Intake Manifold Tuning

Intake manifolds are designed for either
low-speed or high-low-speed use. Drawing air through the
engine so it moves at sufficient speed is the key to
effective engine breathing. For comparison
pur-poses, imagine trying to suck a drink into your
mouth, first through a very small diameter straw
and then through a very large straw. Sucking softly
through the small straw works very well, but if you
suck too hard no more liquid will flow through the
straw. With the large straw, you must suck harder
to raise the liquid toward your mouth. But if you
suck too hard, you will choke on too much liquid.
Upper plane Lower plane

FIGURE 10.4 A closed-type two-barrel dual plane manifold. The
arrows show that each carburetor barrel supplies fuel to four cylinders,
two on each bank. This pattern is also the same on some four-barrel
intake manifolds.

VINTAGE ENGINES

V-type engine intake manifolds are either “open” or “closed.” Older V8s sometimes
used an open manifold, which was lighter and less costly to manufacture, but it required a valley
cover made of sheet metal to seal off the lifter valley. Today's engines use a closed manifold, which
quiets engine noise.

Dual plane

Single plane

FIGURE 10.5 Comparison of dual-plane and single-plane intake
manifolds. (a) Cutaway of a dual-plane manifold. (b) Cutaway of a
single-plane manifold. (Courtesy of Tim Gilles)

(a)

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316

• SECTION II The Breathing System

An engine needs to be able to maintain velocity
and swirl at low speed, yet still be able to deliver a
large volume of air flow at high speed. This can be
accomplished with a butterfly control valve that
changes airflow through the intake manifold by
selecting a primary runner only or by adding a

sec-ondary runner (Figure 10.8).

Resonance Tuning. Resonance tuning is based on
the Helmholtz Resonance Theory. Imagine a tuning

VINTAGE ENGINES

Older engines with carburetors had a manifold heat control valve located at the
bot-tom of the exhaust manifold (Figure 10.6). This device, commonly known as a heat riser, consisted 
of a butterfly valve that fit between the exhaust manifold and exhaust pipe. When the engine was 
cold, the valve would direct part of the exhaust stream through a passage in the intake manifold,
which was beneath the carburetor, to help vaporize the air-fuel mixture. In V-type engines, the heat
riser restricted exhaust flow on one side of the engine only, diverting exhaust through a passage in
the intake manifold (Figure 10.7) to the exhaust manifold on the other side of the engine.

Some heat risers were built into the manifold, whereas others were replaceable. The heat riser
shown in Figure 10.6 has a large counterweight and a bimetal thermostatic spring that opens in
response to heat. Later model heat risers were controlled by engine vacuum. Heat risers
some-times became stuck, often in the open position. But when they stuck closed the manifold could
overheat, which could cause carbon buildup and sometimes crack the floor of the intake manifold.
It was common practice to free up a stuck heat riser by tapping on its shaft with a hammer.

Thermostatic
spring

Counterweight

FIGURE 10.6 Vintage engines with carburetors often had a manifold
heat control valve, often called a heat riser. This one is in the “heat on”
position.

Exhaust crossover
passage from
cylinder head

Intake manifold

(a)

(b)

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CHAPTER 10 Engine Power and Performance •

317

fork held in front of a stereo speaker. If you use an
audio signal generator to control speaker output,
increasing the signal will cause the tuning fork to
vibrate when it reaches its resonant point. As the
signal is increased past the resonant frequency of
the tuning fork, it will stop vibrating. A musical
wind instrument illustrates a similar example of
resonance. The natural frequency of the instrument
varies when the length of the instrument’s hollow
tube is changed by covering holes, which alters the
pitch of its sound.

The behaviors of sound in the preceding
exam-ples can be compared to the way air flows through
the intake manifold of a running engine. As engine
rpm increases, intake and exhaust valves open and
close faster and the frequency of the pulses in the

intake manifold varies. The resonant frequency of
the air in the intake manifold is determined by the
length and volume of its runners, as well as
mani-fold pressure and temperature. Dense and
low-pressure areas exist in vibrating air. A minor
supercharging condition can be created if the
reso-nance can be manipulated to time the pressure
wave, called a standing wave, so its densest part
reaches the valve just as the valve opens.

Variable Length Intake Manifolds

A variable length intake manifold (VLIM) takes
advantage of resonance tuning, using runners of
dif-ferent lengths to provide a 10 –15% torque gain. An
engine’s rpm constantly changes, but an intake
man-ifold runner of fixed length has only one resonance
point. A long runner has a low resonant frequency
and a short runner has high resonant frequency.

Manufacturers use different designs to provide
vari-ations in runner length. One example is shown in

Figure 10.9. Another design uses butterfly valves to
direct air through either a long runner or a short
runner during differing windows of rpm change
(Figure 10.10). The PCM (computer) looks at engine
speed and load and moves the air valves accordingly.

Low RPM High RPM

FIGURE 10.8 When an engine has computer controlled intake
airflow for secondary runners, at low rpm, velocity and swirl are
maintained. At high rpm, there is high flow.

FIGURE 10.9 This port-injected intake manifold has long runners of
varying length. (Courtesy of BMW of North America, LLC)

FIGURE 10.10 Butterfly valves control airflow between the short
and long manifold runners based on engine requirements. (Courtesy 
of Tim Gilles)

(a)

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318

• SECTION II The Breathing System

At 6000 rpm, each valve opens and closes every 20
milliseconds (0.020 of a second). The cylinder cannot
wait for air; it must be available when the valve opens.
Air waves pulse through the intake and exhaust
mani-folds. During valve overlap, a pulsating pressure wave
returning from the exhaust can go into the intake
man-ifold. Tuned intake runners are designed to trap
stand-ing waves in the intake manifold, timstand-ing them so they
are ready to be breathed when the intake valves open.
Engine designers use several methods to get more
than two resonant frequencies so more standing
waves can be produced at various engine speeds.

NOTE

Some manufacturers recommend replacement of the
intake manifold after a catastrophic engine failure. When
an engine has blown up, exploded parts are sometimes
coughed up into the runners of the intake manifold where
metal parts can remain even after cleaning.

Cross-Flow Head

When intake and exhaust manifolds are on
opposite sides of an in-line engine, the head is called
a cross-flow head (Figure 10.11). This design
improves breathing. Cross-flow heads have a
cool-ant passage that provides the intake manifold with
heat to help vaporize the fuel.

Cylinder Heads with Multiple Valves

Some high-performance late-model engines
use three, four, or even five valves per cylinder

(Figure 10.12). These multiple valve designs have
become popular due to improved higher rpm
breathing. Compared to two valve heads, more flow
area for a given amount of valve lift is possible.
Mul-tivalve combustion chambers can be made smaller
with a more central spark plug location. This reduces
the chances for an engine to knock, allowing higher
compression ratios and, therefore, more power.

Very lean air-fuel mixtures are desirable, but
they will not ignite unless the fuel is mixed well in

the combustion chamber. At high engine rpm there
is plenty of turbulence so this is not a problem. At
low speeds, however, multivalve heads tend to
allow fuel to fall out of the mixture. Some multivalve
heads have controllers that open only one intake
FIGURE 10.11 A cross-flow head.

Intake
port

Exhaust
port

FIGURE 10.12 Four-valve combustion chamber. (Courtesy of Tim 
Gilles)

Exhaust valves

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CHAPTER 10 Engine Power and Performance •

319

valve at low rpm and open another one at higher

rpm. This helps maintain velocity and swirl at low
speed and high flow at high speed (see Figure 10.8).
Other multivalve heads use two intake manifold
runners per cylinder that are variably tuned using a
butterfly valve to control airflow.

ENGINE MODIFICATIONS TO

IMPROVE BREATHING

There are several ways to improve engine
breathing, but all of them have limitations. Opening
an intake or exhaust valve too far, or for too long or
short a time, can have an adverse effect on
breath-ing. Intake or exhaust manifold flow can have a
similar negative effect.

Valve Lift

Valve lift describes the distance a valve is
opened. Increased valve lift allows more air and
fuel flow. Unlike an increase in duration, which
keeps valves open longer, valve lift does not cause a
rough idle or ruin low end performance.

Do not confuse valve lift with lobe lift, which,
depending on engine design, is sometimes a
consid-erably smaller measurement. Measuring valve lift is
discussed later in the chapter.

Limitations on Maximum Valve Lift

For performance purposes, why not lift the
valves as high as possible and leave them open for
as long as possible? Several considerations limit
maximum lift. When valve lift reaches 25% of the
port opening, the valve no longer interferes with air
flow. Therefore, lifting the valve beyond this point
will not increase air flow.

NOTE

A curtain area surrounds an open valve (Figure 10.13). 
When valve lift reaches 25% of the diameter of the valve port
opening, this should approximately equal the curtain area.
Lifting the valve beyond this point will provide no benefit.
Example:

• A 2" diameter valve opening has a radius of 1". Its area
is 3.1416 (R) (1 ì 1 = 1).

ã The circumference of the valve head laid out is 6.28" (ΠD).

• With ½" valve lift, the area of the lift area is 6.28 × .5 = 3.14.

Figure 10.14 describes how this works.

SHOP

TIP

Do not make the mistake of installing larger valves that
do not match the port opening. This will not serve a
use-ful purpose if the port opening is too small. One
machin-ist compared this to “a sewer lid flapping over a knot
hole.”

Engineers always have to make compromises.
For instance:

• More lift can cause wear to valve guides, lifters,
and rocker arms. To prevent excess wear,
bronze guides are recommended with high lift

cams as well as rocker arms with roller tips
(Figure 10.15).

• Lifting a valve means compressing a valve
spring. More lift calls for higher tension valve
springs to prevent valve float. The more a spring
is compressed, the higher pressure it exerts,
resulting in excessive wear and decreased
reliability.

Valve Spring Resonance

A valve spring is similar to a crystal water glass
in that it has a resonant frequency or natural
har-monic. If allowed to run undampened at the speed
FIGURE 10.13 A curtain area surrounds an open valve. When valve
lift reaches 25% of the diameter of the head of the valve, lifting the
valve beyond this amount will not flow more air.

Valve
port
Valve

seat

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320

• SECTION II The Breathing System

of its resonant frequency, the spring can either fail
to control the action of the valve, or it can break. The
valve springs on older vehicles usually had a

resonant frequency that occurred at about 4500 –
5000 rpm, limiting the ultimate rpm when valves
would begin to bounce. Today’s springs are designed
with a resonant frequency beyond the normal
oper-ating range of the engine.

NOTE

In restrictor plate racing, all engines must meet the same
specifications and competition is extremely close. This is
why you do not see “better” cars passing “at will” on the
straightaways. A specially designed machine called a
Spin-tron Laser Valve Tracking System (Figure 10.16) can spin
an engine at up to 20,000 rpm to determine the rpm level
where valves bounce or springs “jelly-roll.” If an engine
builder knows that the engine will not rev above 9000 rpm
and the valve springs will not allow valve float until 10,000
rpm (in case the driver makes a mistake), the tested springs
will allow more engine durability than springs that will not
float until 12,000 rpm. Of course there are many other
fac-tors in winning races. For instance, there is always some
valve bounce, but if that can be minimized by testing the
valve springs very closely, a small difference in acceleration
might result in that car winning the race.

An engine accelerating from idle to high speed
goes through changes in spring dynamics two or
three times. Raising its maximum operating range
by as little as 200 –300 rpm can put a race engine
back into the range of spring resonance and valve

float.

Valve Spring Coil Bind

A valve spring can be compressed only so far
before the coils bind or stack up when the thickness
of the spring results in the coils contacting one
another (Figure 10.17). This is why double or triple
springs with inner and outer coils are often used. At

1/2⬙
6.28⬙

3.14 Area

Valve

Valve port opening

Valve head circumference
2⬙

1⬙

1/2⬙ Lift

FIGURE 10.14 Figuring valve curtain area with a 2" diameter valve. Its area is 3.1416 (ΠR²) (1 × 1 = 1). Valve head circumference is 6.28"
(ΠD). With ½" valve lift, the lift area is 6.28 × .5 = 3.14.

Poly locks

or positive
lock nuts

Roller

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CHAPTER 10 Engine Power and Performance •

321

keepers (valve locks) clamp tightly to the stem
of the valve and there is no contact between the
center root of the keepers and the groove in the
valve stem.

• Valve spring shims that are shiny are another
possible indicator of valve float.

• During valve float, open exhaust valves
some-times contact pistons, leaving “witness marks”
(Figure 10.20).

Most of today’s heads are aluminum. Be sure to
use hardened shims under the springs. At high
speeds, intake valve springs tend to fail. Also, when
valves float, springs tend to overheat and lose height
and tension.

Titanium Valves

Heavy valves require stronger springs. Racing
engines use lightweight titanium valves that are
stronger and require less valve closed seat pressure

from the spring, helping prevent valvetrain
separation. High-end racing engine builders replace
FIGURE 10.16 A Spintron machine, which can rotate an engine up to

20,000 rpm, provides racing engine builders with a way to check for
valve spring float and pushrod flex. (Courtesy of Trend Performance, Inc. 
23444 Schoenherr Road, Warren, MI 48089)

Minimum
0.060´´

FIGURE 10.18 Check for coil spring bind at full valve lift, using a feeler
gauge to check around the circumference of the center two coils.

Stacked
coil

FIGURE 10.17 Too much valve lift can cause coil springs to bind.

very high rpm, if valve springs oscillate they will
need some extra space between the coils. On high
speed engines, at full valve lift there should be at
least 0.060" clearance. Use a feeler gauge to check
around the circumference of the center two coils
(Figure 10.18).

Identifying Valve Float

How can you tell if a valve has been floating?
There are several ways:

• If valve locks leave scuff marks on the valve
stem both above and below the keeper groove,
this indicates valve float.

• Another indicator of valve float is when there is
evidence on the tip of the valve stem of multiple
rocker arm contact areas (Figure 10.19). A
nonrotating valve only rotates if it floats. The

(a) (b)

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322

• SECTION II The Breathing System

engines operate at speeds higher than this; they
often use pneumatic valve closing mechanisms.

Porting and Polishing

Porting and polishing are cylinder head
modifi-cations that are done primarily to improve high rpm
performance. The objective is to allow more air to
flow at high speeds.

• Porting is when the size of a passageway is
altered.

• Polishing smoothes the surfaces of the port.

Figure 10.21 shows a combustion chamber and

valve ports in a head that has been ported and
polished.

When the ports in a head are not aligned to match
the ports in the manifold(s), high-speed airflow can
be obstructed. Ports mismatched by more than 1⁄

16"

can be ground to help high-speed performance

(Figure 10.22). This is called match porting.

Porting is not usually worthwhile for street cars
because most factory ports flow 40% more air than
can flow through the valve opening. Smaller ports
can keep air flowing at a higher velocity; with larger
ports at lower rpm, fuel tends to fall out of the
air-fuel mixture.

Airflow Requirements

The amount of intake and exhaust that flows
through manifold runners is measured in cubic feet
per minute (CFM). Larger engines require bigger
titanium valves after every race because there is

always some valve bounce, which more than
dou-bles the load on the valve, drastically shortening its
life expectancy.

Valve Spring Tension with Valves

Closed and Open

With roller lifters, net horsepower remains the
same with increased spring tension when the valve
is closed. At first glance, it would seem that higher
pressure valve springs would consume power when
the valves are closed. However, as each valve opens
and compresses it valve spring, another valve is
closing, decreasing pressure as its spring reextends.
This is known as the regenerative characteristic.

Valve Spring Open Pressure. Too much spring
pressure when the valve is open reduces cam and
lifter life and accelerates valve guide wear. It can
also cause pushrods and rocker arms to flex; this
can be observed using a Spintron machine.

Racing Springs

The quality of valve springs varies widely.
Racing engine builders are known to order springs
made of Swedish steel or rolled wire from Kobe,
Japan. At speeds above 14,000 rpm, even the best
valve springs do not work well. Formula One racing
FIGURE 10.20 During valve float, open exhaust valves sometimes
contact pistons, leaving “witness marks.” (Courtesy of Tim Gilles)

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CHAPTER 10 Engine Power and Performance •

323 NOTE

One popular saying is “Pressure makes flow. Flow makes
volume.”

Certain shapes flow better than others. An
engi-neer alters ports using a flowbench to improve flow
through certain key areas, including the bend at the
valve guide, the ridge around the valve seat, and
the area of the valve seat where the air exits the seat.
Airflow is restricted ½ " above and below the valve
seat because the air must turn 90° and expand. The
upper part of the port only affects flow by 1– 4%.
The curved part of the port restricts about 12% and
the area below the valve accounts for an average of
17–19% of the total restriction.

A flowbench compares the pressure drop
flow-ing through a port or air cleaner with a pressure drop
across an orifice. Vacuum motors pull air through
the intake or exhaust and exhaust and airflow is
cali-brated based on known flow through an orifice of a
specified size. A cylinder adapter the same size as
the engine’s cylinder bore simulates the shrouding of
the cylinder. A switching valve is used when
chang-ing between intake and exhaust flow testchang-ing.

The flowbench uses two manometers, a vertical
manometer and a horizontal incline manometer. The

vertical manometer tells the base test pressure. It is
important when comparing advertised test numbers
to know at which base test pressure the test was
con-ducted. The vertical manometer uses water as the test
liquid; measurements are in inches of water. Adjusting
the flow knob positions the vertical manometer at a
certain level (28" of water, for instance). The incline
manometer uses costly blue fluid ($80 per ounce) that
has twice the specific gravity of water.

A dial indicator attachment is used to open the
valve as flow is tested at different lift points (Figure 
10.24). Pro-Stock engines can have about 1" of valve
lift, whereas motorcycles have about 0.350". The
percentage of flow is read on the incline manometer
and converted using a scale (Figure 10.25). All valve
lift points are tested at the same pressure. The flow
on the vertical manometer is readjusted as lift points
are changed.

Combustion Chamber Shape

Combustion chamber shape also restricts
air-flow. Wedge combustion chambers restrict flow
ports for adequate airflow. An engine of a certain

size can only flow so much air.

NOTE

When someone wants to determine the CFM requirement
for an engine at 100% volumetric efficiency (perfect
breathing conditions), they use the following formula:
CID/2 ì rpm/1728 ì Volumetric Efficiency = CFM

ã Divide the cubic inch displacement (CID) by 2.
• Multiply this fi gure by the maximum rpm, divided by

1728.

• Multiply this by the volumetric effi ciency.

For instance, a 5-liter (302-cubic inch) engine can only flow
524 CFM of air at 6000 rpm. A 5.7-liter (350-cubic inch)
engine will flow a maximum of 607 CFM of air at 6000 rpm.
Would you want to install a 780 CFM carburetor and
mani-fold on this engine if 6000 rpm was its red line? (“Red line”
is the racer’s term for the highest rpm point on the tach
where the engine is shifted.)

Flowbench Testing

On racing engines, airflow though intake and
exhaust passages is tested on a flowbench (Figure 
10.23). Velocity is important. It is more efficient to
keep air moving; at higher velocity more air can be
pushed into the cylinder. Velocity is maintained by
having the smallest port cross-sectional area that
will deliver near maximum flow.

Mismatched
mounting
surface
Remove

metal here

C
B

No metal needs to be
removed from here.

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324

• SECTION II The Breathing System

Incline Manometer

Exhaust
flow control
Vertical

manometer

Orifice selector

Flow
calibration
chart

Switching
valve
Cylinder

adapter

Intake flow
control

FIGURE 10.23 A flowbench for testing airflow. (Courtesy of Tim Gilles)

FIGURE 10.24 The valve is opened to test airflow at different valve lift
points. (Courtesy of Tim Gilles)

Adjust valve travel here Dial indicator

Valve stem tip

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CHAPTER 10 Engine Power and Performance •

325

metered restriction to limit the amount of air that
can enter the engine; this is called “restrictor plate
racing.” At this level of competition, providing an
equal amount of air and fuel flowing to all cylinders
can make the deciding difference in the outcome of
a race.

Professional racers use heads that have been
ported on computer numerical control (CNC)
machines. Porting is done to one port and

combus-tion chamber at a time with follow-up tests made on
a flowbench to judge the results. When the
combus-tion chamber and ports have been optimized for both
intake and exhaust, their shape is “mapped” to
digi-tize them for the CNC machine. The CNC machine
can duplicate the contours of the intake and exhaust
ports for the remaining seven intake ports and seven
exhaust ports (on a V8) and the combustion
cham-bers. For racers, the primary advantage to CNC
port-ing is consistency.

When a port has been designed for a particular
engine, it remains in the computer's memory and
can be duplicated using the CNC machine on future
sets of heads. Sometimes hand porting is attempted
using a die grinder, but this process is time
consum-ing and not very exact. Also, CNC machinconsum-ing has
lowered the cost of CNC ported cast aluminum
heads to the point where hand porting is no longer
a realistic option.

Valve and Seat Angles

Airflow that is not directed becomes turbulent,
which restricts airflow. Angles must be smooth and
curves gradual. High-velocity air cannot make an
abrupt change in direction. Three-angle valve seats
and back-cut valves help achieve consistent airflow.
An example of a back-cut valve is one with a 30°
angle ground between the neck of the valve and its

finished 45° face angle (Figure 10.26b). The
back-cut on the valve should come to within
approxi-mately 0.015’’ of the edge of the desired 45° face
angle. The 30° angle reduces the width of the 45°
face angle, so be careful not to remove too much.

A flatter angle results in a larger gap between
the valve face and seat. For instance, at 0.100" valve
lift, a 30° angle results in a 0.087" opening, whereas
a 45° angle results in an opening of 0.071", and so
on. Some machinists also vary the top angle of the
valve seat. For example, with a 45° valve face and
30° back-cut, the machinist might use a seat top
angle of from 35° to 38°.

because of shrouding (Figure 10.26a), which can
restrict flow by about 35%. Hemispherical
cham-bers allow air to flow more easily into the chamber
because they lack the shrouded area.

Valve Size

Sometimes aspiring racers make the mistake of
choosing the largest valve possible. This presents a
few problems. For instance, with a larger valve flow
is restricted by the shroud. Also, one side of the port
will be closer to the cylinder wall, which restricts
flow. Installing a larger valve also calls for more
vol-ume in the intake port and more work on the intake
manifold. All of this must be done with a flowbench.

A large valve also weighs more. To close it, a
stronger valve spring will be needed. Titanium
valves are much lighter and can be used with lower
tension valve springs. These springs are expensive,
however, so they are mostly used in professional
racing engines.

Porting

Racing professionally is very expensive. Some
types of racing have certain requirements that all
engines must meet. This sometimes includes a

Restriction
or shrouding
19%
11%
4%
12%
19%
35%
Margin

45° face angle
30° back cut

(a)

(b)

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326

• SECTION II The Breathing System

Longer pipes make greater backpressure, which
tends to improve low rpm performance.

Exhaust systems are not severely affected by
bends in the pipe as long as the cross-sectional area
of the pipe is not diminished (with a dent to clear
the steering box, for instance). The primary header
tube should be bigger than the port opening in the
cylinder head. This creates a vacuum in the void
between the header and valve port, preventing
reversion. Reversion is discussed later in this
chapter.

Small head hex screws are usually required so
they can fit next to the header tubes. Notice the
exhaust flange in Figure 10.28. A thick gasket is
usually used with headers like this.

VINTAGE ENGINES

Older engines usually had cast iron cylinder heads. At the factory, exhaust manifolds
were usually bolted to the heads with no gaskets because the newly machined surfaces were
per-fectly flat. In service, replacement gaskets are used to compensate for surface variations that have
developed. If you disassemble an older engine and it does not have exhaust manifold gaskets,
there is a good possibility it has never been disassembled before.

EXHAUST MANIFOLDS

Exhaust manifolds (Figure 10.27) are often made
of cast iron because of its ability to tolerate fast,
severe temperature changes. Exhaust gas
tempera-ture is related to the amount of engine load; when
the engine works hard, or when it has a lean air-fuel
mixture, the exhaust manifold can run almost
red hot.

Headers

Headers are exhaust manifolds made of steel
tub-ing (Figure 10.28). Ordinary mild steel headers tend
to have a shorter service life than cast iron
mani-folds, because they are thinner and can rust through.
Stainless steel headers are more costly but can last
as long as the vehicle.

Headers designed for maximum high rpm
power are short with a large cross-sectional area.
Headers do not improve performance at low rpm
unless they are specifically tuned for low rpm use.

Exhaust
manifold

Oxygen
sensor

FIGURE 10.27 An exhaust manifold. (Courtesy of Tim Gilles)

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CHAPTER 10 Engine Power and Performance •

327

a clamshell, which surrounds the carburetor. This
works well with a carburetor because when it is
enclosed in a box, it operates like a normally
aspi-rated carburetor.

When a blow-through system is used with a
car-buretor, the secondary jets are re-jetted richer and
the float bowl vent tube must be within the top of
the carburetor opening to allow for equal float bowl
pressure. At high boost pressure, a brass carburetor
float can be crushed, so solid foam floats are used.
In addition, a fuel pressure regulator is used to keep
pace with increases in supercharger pressure.

Turbochargers

Some engines have a turbocharger in the exhaust
system (Figure 10.31), commonly referred to as a
turbo. Turbochargers are available as original
equip-ment on many cars and trucks. Retrofits are easier
with electronic fuel injection than they were on
car-bureted engines of the past. Therefore, aftermarket
turbocharging has become more popular in the
high-performance marketplace with many options
available. Knowledge of their operation and service
will be important for you in dealing with your
customers.

TURBOCHARGERS AND

SUPERCHARGERS

A supercharger is an air pump designed to
increase air density in the cylinder. Each cylinder
of a four cylinder, 2-liter (2000 cc) engine has a
displacement of 500 cubic centimeters. Therefore,
if the piston is at BDC and the intake valve is
open, the cylinder will fill with 500 cc of air. This
is 100% volumetric efficiency, a theoretical value
described later in the chapter. When the engine is
running, atmospheric pressure is not a sufficient
force to fill the cylinder completely with air and
its volumetric efficiency will be less than 100%.
Engine power output is directly related to its
vol-umetric efficiency, and supercharging provides a
means of filling the cylinder more completely.
Racers call supercharging “a replacement for
dis-placement.” Original equipment engines today
produce more than 1 horsepower per cubic inch.
Adding 5 psi of boost to a typical inline six
cylin-der GM light truck engine results in about 70
additional horsepower.

NOTE

Normal atmospheric pressure is approximately 15 psi
(14.7 psi at sea level). If a supercharger provides 15 psi of
boost pressure, this effectively doubles the engine size.

There are two primary categories of automotive
supercharging; the exhaust-driven turbocharger and
the belt-driven supercharger. Electric superchargers
are also available in the aftermarket.

Draw-Through or Blow-Through

Superchargers and turbochargers are either

draw-through or blow-through. On carbureted
engines, a draw-through system pressurizes the
intake manifold after the carburetor and air cleaner
(Figure 10.29). This is the only practical way to
install a Roots-type blower (described later in this
chapter). On fuel injected engines, air is pumped
directly into the intake manifold.

A blow-through system pressurizes the air
cleaner above the carburetor or fuel injection
sys-tem (Figure 10.30a). These make easier aftermarket
installations and will fit more easily under a low
hood line (Figure 10.30b). Carbureted

blow-through systems often use an enclosure box, called FIGURE 10.29 A draw-through supercharger.
Inlet

Carburetor or
throttle body

Supercharger

Intake

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328

• SECTION II The Breathing System

A turbocharger is a gas turbine—a small, radial
fan pump driven by energy from heat and pressure
in the moving exhaust (Figure 10.32). It provides a
smaller engine with approximately 40% more torque
and horsepower over a stock normally aspirated

engine. The engine does not use the turbo unless it is
under load, so a smaller displacement engine can
achieve better fuel economy than a larger,
nonturbo-charged engine of comparable power. One drawback

is decreased engine life because the smaller the
engine, the larger the percentage of time the turbo is
used for accelerating and climbing hills. This results
in a hotter running engine.

NOTE

A normally aspirated engine will lose 3% of its horsepower
with every 1000-foot increase in altitude. But a
turbo-charged vehicle will not lose power when driven from low
to higher altitude. The rpm of the turbine increases about
2% for every 1000-foot increase in altitude. This is
espe-cially advantageous for smaller turbocharged aircraft that
use piston engines at higher altitudes.

FIGURE 10.31 Components of a turbocharger system on a four
cylinder engine.

Exhaust
manifold

Exhaust pipe
Turbocharger

FIGURE 10.32 A turbocharger uses the energy of exhaust gas to
force more air-fuel mixture into the cylinder to increase engine power.

Compressed air/fuel

Exhaust
gas

Exhaust
Intake

manifold
Bearings

Compressor

Turbine

FIGURE 10.30 (a) A blow-through supercharger. (b) An aftermarket centrifugal belt-driven supercharger. It is a blow-through, fuel injected design.

(Courtesy of Tim Gilles)

Inlet
Supercharger
Carburetor or

throttle body

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CHAPTER 10 Engine Power and Performance •

329 Turbocharger Airflow

The diffuser is the area between the parallel walls
of the compressor cover and bearing housing where
the air leaves the wheel. The volute is the curved
fun-nel in the cover that increases in size from small to
large. Air directed into the volute from the diffuser is
slowed, which increases its pressure in the diffuser as
the cover fills with static pressure. Air leaving the
compressor wheel is unstable and erratic. The
diam-eter of the cover is considerably larger than the
com-pressor wheel to allow for a diffuser surface of
sufficient diameter to accommodate this unstable air.

Before entering the engine the pressurized air
moves from the compressor, either to a boost tube
or through an aftercooler (covered later). Although
a turbo takes advantage of the energy of exhaust
gas movement to power its impeller, this is not
sim-ply free energy because the turbo itself restricts the
exhaust stream.

Turbine and Compressor

Size Matching

Engineers who design a turbocharger match
the size of the turbine and the compressor to an
engine’s displacement, rpm, and volumetric
effi-ciency. They call this “trimming” a turbo.
Exhaust system flow is subjected to backpressure.
Turbochargers are also very popular with diesel

engines. Diesel fuel contains more energy than
gas-oline. Because it requires more air to burn
com-pletely, turbocharging is very helpful. Turbochargers
are used on virtually all commercial diesel powered
trucks as well as on most tractors.

Turbocharger Operation

A turbocharger is a centrifugal pump;
centrifu-gal force takes the incoming exhaust and throws it
through a snail-shaped outlet. The pump shaft has
two wheels, a compressor and a turbine. The
exhaust wheel is the turbine and the wheel that
forces air into the intake manifold is the compressor
(Figure 10.33). As exhaust pressure spins one wheel,
the other wheel forces more combustible mixture
past the intake valve and into the cylinder,
increas-ing engine efficiency. Figure 10.34 shows a
blow-through turbocharger on a racing engine.

Two types of compressors are radial and axial.
An axial compressor is like a jet engine. It draws air
into the front and pushes it out the back in the same
direction it is already moving. A turbocharger
com-pressor uses a radial-type wheel, which means that
air enters the leading edge of the wheel, called the
inducer, parallel to the turbine shaft. It is then
redi-rected 90° and exits the compressor housing
per-pendicular to the turbine shaft.

FIGURE 10.33 A turbocharger cutaway. (Courtesy of Tim Gilles)

Compressor

Waste
gate

Turbine
Exhaust
housing
Intake

housing

FIGURE 10.34 This turbocharged Cosworth is a blow-through
design. (Courtesy of Tim Gilles)

Left bank
exhaust

Right bank
exhaust

Pressurized
intake manifold

Air pressure
(boost)

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330

• SECTION II The Breathing System

Turbo Boost

The amount of air density a turbo can provide is
known as boost pressure, or boost. The point where
boost begins is called the boost threshold. This
might be something like 1800 rpm, for instance.

Turbo Lag

When a turbo is spinning at low speed, little or
no boost is produced. The time required to bring
the turbo up to a speed where it can function
effec-tively is called turbo lag—hesitation in throttle
response when coming off idle.

Twin Turbos. Twin turbos are sometimes used with
V-type engines. Exhaust power from half of the
cyl-inders feeds two identical turbos, each matched to
one-half of the engine’s required airflow. This

design is popular for road racing because the
smaller wheels spool faster with less turbo lag

(Figure 10.35).

NOTE

A twin turbo is not the same as a compound turbo; twin
turbos are the same size and provide even pressure at the
same time. Multistaged, compound turbos used in extreme
duty performance applications, like tractor pulls, use one
turbo to feed the next. Two, three, or four stages are used
that sometimes reach boost pressures of 100–200 psi.
These engines frequently blow up.

Turbochargers all have some lag. Smaller units
have less, so they are often used in pairs.

To reduce turbo lag, some drag racers use a
nitrous oxide system at low rpm, controlled to shut
off by a pressure switch when a certain level of
boost is sensed. Electronic controls allow another
option for reducing lag. If timing is retarded while
Compare exhaust flow to a small stream of water

flowing from the end of a garden hose. When
you start to restrict flow by moving your thumb
over the end of the hose, pressure builds and the
water squirts out a greater distance. But if you
continue closing off the opening, you cause too

much backpressure and the distance of the flow
drops off again.

The size of the turbine is matched to the engine’s
exhaust system based on the rpm where boost
pres-sure will be applied. An understanding of prespres-sure
is important not only in the design of camshafts
and intake and exhaust systems, but also in
engi-neering the balance between the turbine and
com-pressor ends of a turbocharger. This is similar to the
way port size affects flow in intake and exhaust
passageways at different engine speeds. Smaller
turbines raise more boost pressure, but too small a
turbine housing will choke off flow and cause
excess backpressure.

NOTE

The maximum flow capacity of a turbo compressor is called
“choke.” When pressure reaches a certain point where flow
stops, this is called surge or stall.

When a compressor is too large, it requires more
power to spin; if it does not rotate fast enough, it
will not compress the air sufficiently. With too small
a compressor, the engine might need more air than
the compressor can compress without overheating
the air.

NOTE

When an engine produces sufficient exhaust flow to spin a
supercharger or turbocharger enough to create boost, this
is referred to as spooling.

VINTAGE ENGINES

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CHAPTER 10 Engine Power and Performance •

331

more fuel is injected (only when the throttle is

open a small amount), the excess fuel burns in
front of the turbo, making more heat to cause it
to spin.

Supercharger Pressure Control

Supercharged systems use different ways to
control excess pressure in the exhaust and intake
sides. A fixed geometry turbocharger, like the type
used in many heavy-duty diesel engines requires
no pressure relief because it runs with predictable
boost within a narrow rpm band. An automotive
engine, however, must run at widely varying speeds
so a wastegate is commonly used to control the
speed of the turbine.

Wastegate

Without a wastegate, a turbo could provide so
much power that the engine could destroy itself.

Smaller turbines are used in automotive
turbo-chargers because they are better at providing
low-end boost and avoiding turbo lag. They work
effectively, but if rpm climbs too high, the
increas-ing exhaust flow can push the turbine too fast,
building too much exhaust system backpressure.
Boost pressure opens the wastegate when it reaches
a specified point, relieving pressure by allowing
exhaust flow to bypass the turbine, limiting its
speed and output. Figure 10.36 shows a wastegate
in the open and closed positions. A redundant relief
valve protects the system in case the wastegate
becomes stuck.

The wastegate actuator is applied by air
pres-sure supplied by a hose, which is tapped into boost
pressure at the compressor discharge outlet. When
boost pressure reaches the point in the actuator
where it is higher than the opposing spring
pres-sure, it opens the wastegate.

Internal and External Wastegates. A wastegate
can be either internal or external. A turbo with an
internal wastegate like the one shown in Figure
10.36 works fine for most street applications and is
easier to install when doing an aftermarket
installa-tion. In high flow turbos, however, if the wastegate
is not big enough to divert excessive boost, pressure
will continue to rise. When an engine will have

FIGURE 10.36 A wastegate in open and closed positions. (Courtesy of Tim Gilles)

Exhaust impeller
(turbine)

Waste gate
closed

Waste gate
open

FIGURE 10.35 Twin turbos are sometimes used with V-type engines.

</div>

Ebook Automotive engines diagnosis, repair and rebuilding (6th edition): Part 2

312

<b>C</b>

<b>ONTENTS</b>

<b>O</b>

<b>BJECTIVES</b>

<b>I</b>

<b>NTRODUCTION</b>

<b>Engine Power and </b>

<b>Performance</b>

C H A P T E R

313

<b>INTAKE AND EXHAUST MANIFOLDS</b>

<b>Intake Manifolds</b>

<b> NOTE</b>

<b>VINTAGE ENGINES</b>

314

<b>Carbureted Manifolds</b>

<b> NOTE</b>

<b>Dual- and Single-Plane Manifolds</b>

315

<b>Intake Manifold Coolant Passage</b>

<b> NOTE</b>

<b>Intake Manifold Tuning</b>

<b>VINTAGE ENGINES</b>

316

<b>VINTAGE ENGINES</b>

317

<b>Variable Length Intake Manifolds</b>

318

<b> NOTE</b>

<b>Cross-Flow Head</b>

<b>Cylinder Heads with Multiple Valves</b>

319

<b>ENGINE MODIFICATIONS TO </b>

<b>IMPROVE BREATHING</b>

<b>Valve Lift</b>

<b>Limitations on Maximum Valve Lift</b>

<b> NOTE</b>

<b> SHOP </b>

<b>TIP</b>

<b>Valve Spring Resonance</b>

320

<b> NOTE</b>

<b>Valve Spring Coil Bind</b>

321

<b>Titanium Valves</b>

<b>Identifying Valve Float</b>

322

<b>Porting and Polishing</b>

<b>Airflow Requirements</b>

<b>Valve Spring Tension with Valves </b>

<b>Closed and Open</b>

<b>Racing Springs</b>

323

<b> NOTE</b>

<b>Combustion Chamber Shape</b>

<b> NOTE</b>

<b>Flowbench Testing</b>

324

325

<b>Valve and Seat Angles</b>

<b>Valve Size</b>

<b>Porting</b>

326

<b>VINTAGE ENGINES</b>

<b>EXHAUST MANIFOLDS</b>

<b>Headers</b>

327

<b>Turbochargers</b>

<b>TURBOCHARGERS AND </b>

<b>SUPERCHARGERS</b>

<b> NOTE</b>

<b>Draw-Through or Blow-Through</b>

328

<b> NOTE</b>

329

<b>Turbocharger Airflow</b>

<b>Turbine and Compressor </b>

<b>Size Matching</b>