A Dual Overhead Cam system is usually for use with engines that have more than 2 valves per cylinder -- generally 4, but some 3 value-per-cylinder endgines utilize a DOHC system.

The name referes to the idea of having one cam shaft per X number of valves -- and more valves means more cam shafts, usually two vales per cylinder per cam. It gives a lot more power, but it also means there is more to go wrong, as well, because you have twice the stuff of a SOHC if it is a 'V' block, or 4 times if it is a straight block. And lets not forget that adjusting the timing on a single cam system is much easier than the timing on a Dual cam engine.

Once, a method of engine design. Today, a triumph of marketing.

Auto manufacturers (primarily the Japanese) appear to have convinced many people that DOHC (Dual OverHead Cam) engines are new, high-tech, and cool. In fact, many European manufacturers have used them for decades. I personally owned a 1974 Fiat (not exacly a luxury or exotic car) with a DOHC engine, and there was nothing esoteric about it.

There also seems to be a notion about that DOHC implies > 2 valves/cylinder. Again, nothing could be farther from the truth. The advantage of a DOHC engine is not the number of valves you can drive, but their placement. Even in a "conventional", 2-valve-per-cylinder engine, a DOHC design allows the valves to be placed in such a way that the flow of the gas-air mixture through the combustion chamber can be optimized.

The other advantages of a DOHC design are

  • less friction than an OHV (pushrod) design,
  • fewer moving parts than a pushrod design, and
  • the ability (in some engines) to have different valve timing on the intake and exhaust valves.

It should be noted that there are really two "flavors" of DOHC design; in a four cylinder, 8-valve engine, typically one camshaft drives the intake valves and the other drives the exhaust valves. This scheme can be extended to 16-valve, 4-cylinder designs trivially. In a V-6, -8, or -12 engine with two banks of cylinders, each camshaft will generally drive all the valves in one bank of cylinders. This type of design might more properly be called a "double-SOHC" design, but in practice this distinction is very seldom made.

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