have decided to write this article because, not only do I believe people do not understand cam shafts (or the lack there of), I believe, more importantly so, that they do not understand what it will mean to them in the real world. Camshafts or not, engines need to breath. Basically, they do it in one of three ways, with overhead valves, an overhead camshaft on each cylinder bank, or two overhead camshafts on each cylinder bank. What does this mean? Stick with me for a moment.
To begin with, dual overhead cam, twin cam, multi-cam, and DOHC engines are all the same thing. Having two overhead cams (as seen above) refers to having an individual camshaft to operate the intake valves and one to operate the exhaust valves. This can allow better placement of the valves in the combustion chamber at a better angle for maximum performance.
Separate camshafts allow the use of a lighter and more compact valve systems and better cylinder breathing which allows for smoother high RPM operation. By allowing for this, peak power can be had through the use of high revolution instead of the more traditional method of using larger cylinders. This helps to keep engines smaller, lighter, and more fuel-efficient without sacrificing power. A marvelous example of DOHC technology at its best would be Honda's line of VTEC engines. Because of multi valves, overhead cams, and better synchronization of valve timing, Honda has managed to pull power out of small engines that was previously thought to be impossible.
Overhead valve engine designs are far more traditional. They simply use one intake and one exhaust valve (instead of two and two) that open and close based on the set revolutions of the engine. They consider less information and provide inferior breathing capacity. Thus, overhead valve designs typically need to be larger in order to produce power and do not function as smoothly in higher revolutions (in other words, they are typically rougher during overdrive acceleration). General Motors has always been a large proponent of overhead valve designs. Their 3800 series of V6 engines that power millions of Pontiacs and Buicks today, actually began life in 1964 as a slightly smaller V6 plant powering midsize Buick Specials. Now, that engine has come a long way, but aside from available supercharging, the technology is horribly antiquated.
Single overhead cam designs (just overhead cam or OHC) split the difference. One camshaft on each bank of cylinders opens the valves under set timing of the engine. The advantage is that more ??????. Unfortunately, valves can be employed and breathing is more precise than OHV designs. Unfortunately, because DOHC engines have twice as many intake and exhaust valves as a SOHC motor, they run cooler and more smoothly, quietly, and efficiently. Chrysler has a lot of SOHC, six cylinder engine designs. Fords widely used modular V8's are also single overhead cam designs.