Tanya Wells, Sunnyvale, Calif.

Camshafts are used on piston engines to operate the intake and exhaust valves. A camshaft is about an inch in diameter, arm length, and is made of very durable metal. A camshaft contains two or more egg-shaped lobes per engine cylinder. Each lobe is linked either directly, or through a lever arrangement, to the valves. Camshafts are driven by the crankshaft via a belt, gears or a chain. They rotate at one-half engine speed, allowing a four-stroke engine cycle.

Older design engines and those of a heavy-duty nature generally use one camshaft mounted in the engine block, just above the crankshaft, and a push-rod arrangement to transfer the needed movement to the valves, which are located at the top of the engine.

Newer design engines mount one or more camshafts within each cylinder head, allowing a more direct connection with the valves.

Why do some engines have multiple camshafts? Many engines now have four valves per cylinder, and there might not be enough room on a single camshaft for all those lobes. Also, dedicating one camshaft to the intake valves and the other to the exhaust valves allows greater flexibility for valve placement, and varying valve timing and lift.

Valve timing can be varied by means of an on-the-fly adjustable camshaft drive sprocket and variable valve lift can be accomplished by the rocker arms switching between two differing cam lobes. These recent advances enhance engine torque across a broader speed range.

In the past, an original equipment camshaft was optimized for good low-speed performance, smooth idle and reduced emissions. A hot rodder might replace this camshaft with one containing a differing lobe shape that reduced these qualities in exchange for maximum high-speed performance. Variable valve lift and timing systems can bring all these benefits into one package.

Why might an engine have one, two or four camshafts? An older-design engine has one camshaft in the engine block regardless of the number of cylinders or engine layout. A single bank (row of cylinders) four- or six-cylinder overhead-cam engine might have one or two camshafts. And a dual-bank six- or eight-cylinder overhead-cam engine might have two or four camshafts.

The Mazda RX-8’s rotary engine has no valves, making a camshaft unnecessary.

In general, the greater number of camshafts, the higher the performance. Some single-cam engines, such as that used in the Chevy Corvette, make substantial magic the hard way.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-03-25-05 0614EST