"The all new Chrysler Sebring LXi Coupe is sharp. Very sharp. With a 3.0 liter peak 200 horsepower multivalve V6..."

"The all new 2001 Eclipse Spyder...with available 24 valve V6..."

Take the above, if you will. I copied those two pieces of text verbatim from advertisements in a Motor Trend magazine. They sound pretty good, don't they? I used to think so anyway. Unfortunately, the automobile industry loves to throw complex words at us in an effort to sell cars and they hope we will not think about it too much. I remember a few years back when I bought my SVT Ford Contour. I was explaining to a coworker that it had a "2.5-liter, 24-valve, dual overhead cam, V6 engine." I was terribly proud.

She just looked at me and questioned, "What does that mean exactly?"

I explained the mechanical reasoning but that still did not satisfy her.

Another friend used the same line of reasoning on me when boasting about the dual overhead cam engine in her Ford Taurus. I asked why she chose that engine over the standard, overhead valve design. She replied that she did not know but that this engine had more power.

I 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 mid-size 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.

So, why would I buy an OHV engine from Pontiac when I could have a DOHC engine from Honda? Well, to answer that question, you will have to look deep within yourself and analyze your individual preferences. If you are interested in more high adrenalin performance and silky smooth operation, then a DOHC engine may be right for you. Unfortunately, though, DOHC engines commonly offer poor bottom end torque. True, Honda's high tech S2000 makes 240 horsepower. However, it only makes 153 pound feet of torque. In addition, to tap into even that small amount of torque, you have to make sure that the engine is wound up to a lofty 7500 rpms. This means that powerful launches take a little extra consideration and that can be annoying, especially in heavy traffic. OHV engines commonly produce terrific bottom end torque for powerful take offs. For example, Chevrolet's 350 powered Camaros make their peak torque at only 4000 rpm's meaning that almost the moment you step on the gas, you have all of the power you need and over a broader rev range.

Another example would be minivans. Minivans tow a lot of stuff and haul a lot of kids. It would be a good idea to have torque the moment you step on the gas just in order to get the hefty vehicle under way. This philosophy has worked well for the Chrysler and GM vans. Mazda, on the other hand, decided to use Ford's small 2.5 liter, DOHC V6 in their recently redesigned MPV. This engine was a dazzling powerplant in the lightweight, performance oriented Contour. However, installed in the MPV, its lack of torque is unforgivable and the V6's high rpm talents go unappreciated by the MPV's fuel conscious, four speed automatic. On the street, SOHC power plants continue to split the difference. As stated, Ford uses a line of SOHC V8's. In 4.6-liter form, this engine is a terrific match in their Mustangs because they provide high-end rev flexibility but also the low-end grunt commonly associated with muscle cars. However, the Mustang can feel rather sloppy at the track when paired against the precision of an Acura RSX.

Therefore, in summary, dual overhead cam engines are for the more performance oriented or fuel conscious among us. Overhead valve designs will be appreciated by anyone who likes the feel of an old school, American muscle car, or by those that who value towing and hauling. Single overhead cam engines are a nice compromise, but they cannot do high-end revs or bottom end torque as well as the other two, respectively. Overhead valve and single overhead cam engines in passenger cars are on the way out. I know GM is finally getting ready to phase out theirs. As fossil fuels and the ozone layer become increasingly more precious, large engines for passenger cars will slowly disappear. Fear not, though, as DOHC designs are getting better and better by the year. Who knows, some day we may even see them as common elements in environments that demand high torque but have little use for peaky revs, like pickup trucks. For now folks, choose your engine carefully and do not be too swayed by jazzy advertising verbiage. It is how it performs on the street that really counts.