Overhead camshafts are installed in the engine head above the valves and cylinders. Normal camshafts are located in the engine block beside the cylinders in in-line engines, and between the cylinders in V-8 engines. Camshafts keep the engine in time by moving the valves up and down as the crankshaft moves the pistons up and down. The camshaft is kept in time with the crankshaft either through a timing chain or a timing belt.
Flat-head Engines
In-line flat-head engines have valves and camshafts in the side of the block next to the cylinders. Lifters and push rods are accessible through an access panel on the side of in-line engines. Flat-head V-8 engines are built with valves and camshafts in the center of the block next to the cylinders. The valve stems in flat-head engines point downward, riding on the top of the valve lifters. Flat-head engines achieve peak horsepower at 3,200 to 3,500 rpm.
Overhead Valve Engines
Overhead valve engines use lifters to move pushrods upward, moving rocker arms that open and close intake and exhaust valves. Valve springs return the valves to their seats, closing the port directly above each cylinder. The placement of intake and exhaust valves directly over each cylinder allows overhead valve engines to provide greater performance than flat-head engines. As an example, a 1968 Pontiac 400-cubic-inch engine achieved 360 horsepower at 5,400 rpm, and a 1970 Mercury 428 engine achieved 335 horsepower at 5,200 rpm.
Overhead Cam Engines
Overhead cam engines have one or two camshafts in each cylinder head. Camshaft lobes ride on cam followers that contact the valve stems, opening the valves. Valve springs seat the valves closing the port. The elimination of cam lifters and pushrods allows engines to operate at higher revolutions, providing greater performance and economy. An example is the 2012 Honda Civic Si equipped with the i-VTEC four-cylinder engine. According to Honda Motor Company, this engine is rated at 200 horsepower, and achieves 31 mpg.
Dual Overhead Cams
Performance characteristics of overhead cam engines increase with multiple cams and multiple valves for each cylinder. An example is the engine in the 2012 MazdaSpeed3. The engine has a displacement of 2.3 liters, with dual overhead cams, four valves per cylinder and variable intake valve timing. The rated horsepower is 263 at 5,500 rpm, and the redline is at 6,700 rpm. The efficiency of this engine technology is represented by a highway fuel economy rating of 25 mpg.
