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 s[r]
Trang 1• 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)
10
Trang 2INTAKE 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
(a)
EXHAUST INTAKE
Alternate head-individual ports
1 2 3 4
(b)
FIGURE 10.2 T he top sketch (a) shows “Siamese” valve ports that share a manifold runner The bottom sketch (b) shows individual ports.
VINTAGE ENGINES
By the mid-1980s, most manufacturers had replaced carburetors with fuel injection But aftermarket carburetors and manifolds are still in demand on boats and vintage vehicles Some race cars still use carburetors, too, due in part to the influence of NASCAR
Trang 3Port 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
(b)
Trang 4handy 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)
(b)
Trang 5An 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)
FIGURE 10.7 A vintage carbureted intake manifold side-to-side and
lengthwise cutaways showing the exhaust crossover passage (Bottom:
Courtesy of Tim Gilles)
Trang 6fork 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
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)
(b)
Trang 7At 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
Intake valves
Trang 8valve 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 ( Π R²) (1 × 1 = 1).
• The circumference of the valve head laid out is 6.28" ( Π D).
• With ½" valve lift, the area of the lift area is 6.28 × 5 = 3.14.
Figure 10.14 describes how this works.
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
Valve curtain area
Trang 9of 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 (Π R²) (1 × 1 = 1) Valve head circumference is 6.28" ( Π D) With ½" valve lift, the lift area is 6.28 × 5 = 3.14.
Poly locks
or positive
lock nuts
Roller
FIGURE 10.15 Roller tip rocker arms used with a high lift cam help
reduce the friction required to rotate the engine (Courtesy of Tim Gilles)
Trang 10keepers (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
FIGURE 10.19 Indications of valve float (a) Scuff marks on the valve stem above and below the keeper groove from valve lock scrubbing (b) Multiple rocker arm contact areas on the valve tip.