Difference Cam-in-Block or Overhead Cam

 Some I-head engines have the camshaft located in the block. This engine design is called a pushrod engine, or cam-in-block engine. Cam lobes raise valve lifters that act on pushrods to operate rocker arms and open the valves (Figure 1.31). In late-model vehicles, pushrods are found most often on V-type engines.

A more popular type of valve operating arrangement for late-model engines is the overhead cam design, or OHC. This engine has the camshaft mounted on top of the cylinder head just above the valves (Figure 1.32a). It has the advantage of having fewer parts and less weight. An engine running on the freeway at 3000 rpm has to open and close a valve 25 times per second, so valvetrain weight is very important. It is even more important in high-speed engines.

Some OHC engines have a single cam (SOHC). Each cylinder is provided with two separate lobes to operate the intake and exhaust valves. Highperformance OHC engines often have two cams per cylinder head. On this design, known as dual overhead cam (DOHC), one camshaft operates the intake valves and the other operates the exhaust valves (Figure 1.32b). DOHC engines have become more common in recent years as the base engine of many vehicle manufacturers.

The OHC engine uses a long chain or belt from the crankshaft to the cylinder head to drive the cam(s)     (Figure 1.33). When the ignition system uses a distributor, some OHC engines use an auxiliary shaft to drive it, whereas others have a crankshaftdriven distributor.

FIGURE 1.32 In-line four cylinder overhead cam engine.
(a) An OHC engine with the cam positioned over the valve. (b) A dual overhead cam (DOHC)
engine.

FIGURE 1.33 V-type overhead cam engines. (a) Belt-driven

overhead cam V6. (b) Chain-driven overhead cam V8