Both SOHC and DOHC refer to the camshaft alignment of your engine. A camshaft is basically the conductor of gases and fuel inside the engine. Consisting of a straight metal shaft covered with oblong lobes, the camshaft automatically opens the fuel intake and exhaust valves as it rotates. There are several different forms of camshafts found in engines. There are SOHC (single overhead cam), DOHC (double overhead cam), and OHV (overhead valve) engine layouts, each with their own benefits and drawbacks.
Overhead Valve Engines (OHV)
While this is neither a SOHC or a DOHC engine, overhead valve engines are the predecessors of both SOHC and DOHC engines and are therefore important to understand. Even though there are more pieces in an OHV engine, and, in some degrees, its operation is more complicated, the overall design is much simpler.
The overhead valve engine uses a single camshaft that is connected to a series of pushrods, hence why it is also called a pushrod engine. Instead of the camshaft directly touching the rocker arms connected to the valves themselves, the camshaft touches a series of pushrods, which activate the valves instead. Part of the appeal of pushrod engines is that they can be much smaller. Since the camshaft is placed right in the middle of the engine, it can be much more compact. This style of engine is now seen as an older style of engine because the design of an OHV is comparatively much more straightforward than either a SOHC or a DOHC. There are some benefits and problems with this relatively simple style of engine. For one, the smaller size of the engine is a big draw. Not only does the internal camshaft placement save some space, but the way in which the camshaft is connected to the crankshaft is compact as well. While some pushrod engines are synced up with the crankshaft via a timing belt, the placement of the camshaft allows for it to easily be coupled via a series of gears, which is an overall sturdier setup. Additionally, the simple camshaft setup saves a fair amount of weight as well, meaning that pushrod engines are both lighter and smaller.
There are, of course, a litany of problems with the OHV engines too. One of the more common problems with pushrod engines is its issues with producing power. Because of the momentum of each of the individual elements of the camshaft setup, the pushrod can “float,” wherein it doesn’t close in between cycles. This can, at the best, decrease engine efficiency, or, at the worst, crack a valve. The pushrods themselves can get bent as well. If there are any problems with the pushrods or the camshaft itself, the entire engine generally has to be removed.
SOHC: Single Overhead Cams
SOHC stands for single overhead cam. As opposed to OHV engines, the camshaft is situated over the cylinders of the engine, placing the lobes of the camshaft directly on the rocker arms for the engine intake and exhaust valves. On an engine like a V6, there would be two camshafts for each row of cylinders, and on a straight engine, only one. On SOHC engines, the camshaft activates both the fuel intake and the exhaust valves, meaning that there can be a maximum of two valves per cylinder, much like a pushrod engine. This control over both valves simultaneously means that, in a lot of ways, SOHC engines function similar to their pushrod counterparts. However, they are a bit more robust because they don’t have the inherent risk of bent pushrods and floating. Alongside the added weight of an additional camshaft on some engine layouts, SOHC engines require the use of a timing belt to keep the camshaft synced with the crankshaft.
DOHC: Dual Overhead Cams
DOHC engines are simply engines with another camshaft per cylinder row. For example, on a V6 engine layout, there would be a total of four camshafts. The additional camshafts are intended to take control of either the fuel intake valve or the exhaust valve individually. The extra camshafts also open up the possibility for more valves on each individual cylinder, meaning that a maximum of four valves per cylinder is possible. As it should seem, DOHC engines are more complicated and, consequently, more difficult to produce than SOHC engines. With the increased difficulty of production come some measurable benefits as well. The additional camshafts per cylinder row allow each of the valves to open and close with greater precision. Not only will their timings be closer to optimal, but the additional valves allow the engine to circulate fuel and exhaust with more ease. This means that DOHC engines can produce more power than SOHC and OHV engines. Not only can DOHC engines put more fuel into the cylinder on account of their increased number of valves, but their design means that they can be used at higher RPM as well. There are some additional benefits to DOHC engines. Not only are they more efficient, but they run quieter and cooler as well.
There's a good reason that DOHC engines are the go-to for performance motors. For all that’s good about DOHC engines, however, they are accordingly difficult to repair. With double the parts, there are twice as many parts that could need replacing.
Is DOHC or SOHC Better?
SOHC and DOHC are two acronyms that are very useful to know, given that they are plastered all over any engine out on the market today. DOHC engines, while they offer the best performance, are accordingly complicated as most modern automotive technologies are. Even though DOHC engines can cost more money, they are certainly worth the investment all around the board. However, SOHC and OHV engines aren't without merits, and won't seriously hinder your enjoyment of your vehicle. If you seriously want to upgrade from a SOHC to a DOHC engine, there are engine swaps, such as a Fox Body Coyote swap, that can be performed.