5. Figure 6-1 Downward movement of the piston lowers the air pressure inside the combustion chamber. The pressure differential between the atmosphere and the inside of the engine forces air into the engine.

10. Figure 6-2 Dust and dirt in the air are trapped in the air filter so they do not enter the engine.

19. Figure 6-3 Most air filter housings are located on the side of the engine compartment and use flexible rubber hose to direct the airflow into the throttle body of the engine.

20. Figure 6-4 Some port fuel-injected engines use heated air taken from around the exhaust manifold.

21. Figure 6-5 (a) Note the discovery as the air filter housing was opened during service on a Pontiac Bonneville. The nuts were obviously deposited by squirrels or some other animal. A

22. Figure 6-5 (continued) (b) Not only was the housing filled with nuts, but also this air filter was extremely dirty, indicating that this vehicle had not been serviced for a long time. B

23. Figure 6-6 A resonance tube, called a Helmholtz resonator, is used on the intake duct between the air filter and the throttle body to reduce air intake noise during engine acceleration.

28. Figure 6-7 A typical throttle-body injection (TBI) unit. This TBI uses two injectors. Most V-6 and V-8 engines require two throttle-body injectors, whereas 4-cylinder engines use one injector.

29. Figure 6-8 Heavy fuel droplets separate as they flow around an abrupt bend in an intake manifold.

34. Figure 6-9 Heat radiating from the exhaust manifold heats the intake air on engines equipped with a carburetor or throttle-body-type fuel injection.

35. Figure 6-10 The heat crossover passage allows exhaust gases to flow under and heat the manifold to improve cold engine operation.

39. Figure 6-11 The graph shows the effect of sonic tuning of the intake manifold runners. The longer runners increase the torque peak and move it to a lower RPM. The .600-mm-long intake runner is about 24 inches long.

40. Figure 6-12 Airflow through the large diameter upper intake manifold is distributed to smaller diameter individual runners in the lower manifold in this two-piece manifold design.

42. Figure 6-13 This DaimlerChrysler V-6 engine uses an intake manifold tuning valve that is controlled by the engine computer to switch where the air is directed through the passages of the manifold to allow the engine to produce the most torque possible at every engine speed.

47. Figure 6-14 The exhaust gas recirculation system is more efficient at controlling NOX emissions if the exhaust gases are cooled. A long metal tube between the exhaust manifold and the intake manifold allows the exhaust gases to cool before entering the engine.

54. Figure 6-15 The exhaust gases are pushed out of the cylinder by the piston on the exhaust stroke.

55. Figure 6-16 Example of heat deflector shields placed between the exhaust manifold and the spark plugs and plug wires. Even high-temperature-resistant silicone jacket spark plug wires cannot withstand the high exhaust manifold temperatures.

56. Figure 6-17 Original-equipment (OE) type of tubular steel exhaust manifold.

57. Figure 6-18 A crack in an exhaust manifold is often not this visible. A crack in the exhaust manifold upstream of the oxygen sensor can fool the sensor and affect engine operation.

59. Figure 6-19 Typical exhaust manifold gaskets. Note how they are laminated to allow the exhaust manifold to expand and contract due to heating and cooling.

62. Figure 6-20 An exhaust manifold spreader tool is a tool that is absolutely necessary to use when reinstalling exhaust manifolds. When they are removed from the engine, they tend to warp slightly even though the engine is allowed to cool before being removed. The spreader tool allows the technician to line up the bolt holes without doing any harm to the manifold.

65. Figure 6-21 Exhaust gases expand and cool as they travel through the passages in the muffler.

70. Figure 6-22 A hole in the muffler allows condensed water to escape.

71. Figure 6-23 A high-performance aftermarket air filter often can increase the airflow into the engine for more power.