Benefits: Better performance
Drawbacks: Increased complexity
You've probably heard the term "DOHC" or "dual overhead camshafts" when someone talks about an engine. Most people recognize it as a desirable feature to have, but what does it mean? The term refers to the number of overhead camshafts above each cylinder in the engine.
Camshafts are part of your car's valvetrain, which is a system that controls the flow of fuel and air into the cylinders. For many decades cars primarily had OHV engines, meaning overhead valves, also called "pushrods." Pushrods are driven by camshafts inside the engine block. This setup adds mass to the engine and can limit its overall speed.
On an overhead cam setup, the camshaft is much smaller and is inserted above the cylinder head itself, rather than in the engine block. There's one on a single overhead cam (SOHC) engine, while a DOHC engine has two. The benefit to the overhead cam setup is that it allows for more intake and exhaust valves, meaning fuel, air and exhaust can move more freely through the engine, adding power.
While many car companies have done away with pushrod engines, DOHC and SOHC haven't supplanted them quite yet. Chrysler still uses pushrods to generate lots of power for their Hemi V8 engines; General Motors utilizes pushrods on some of their high-tech, modern V8s as well. But DOHC and SOHC engines have been prominent on engines, especially smaller ones, since the 1980s.
The drawback of having overhead cams is that they increase complexity and cost. Are you noticing a trend here yet?
Next we'll learn more still about how valves affect performance when we talk about variable valve timing.