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

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]

312

CONTENTS

• 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

OBJECTIVES

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

INTRODUCTION

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)

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 further study by reading more advanced publications on the topic of your choice

INTAKE EXHAUST

Conventional head-Siamesed valve ports

1

(a)

EXHAUST INTAKE

Alternate head-individual ports

1

(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

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 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

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)

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)

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)

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

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)

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

ã With ẵ" valve lift, the area of the lift area is 6.28 × = 3.14

Figure 10.14 describes how this works.

SHOP TIP

Do not make the mistake of installing larger valves that 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

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 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) Valve head circumference is 6.28" (ΠD) With ẵ" valve lift, the lift area is 6.28 ì = 3.14

Poly locks or positive lock nuts

Roller

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)

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 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)

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 • 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

A

C B

No metal needs to be removed from here

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

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)

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 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)

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 horsepower per cubic inch Adding 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

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

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)

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

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