DOHC vs SOHC: What They Mean and How They Differ

After looking at dual-mass flywheels, we’re diving back into the fine detail of car mechanics to clarify what the acronyms DOHC and SOHC actually mean. In full, they stand for Double Over Head Camshaft (double overhead camshaft) and Single Over Head Camshaft (single overhead camshaft), respectively.

You may also come across Over Head Valves (OHV), a layout where the valves are operated via pushrods mounted along the side of the engine block. It was widely used in the early days of the motor industry, but it’s no longer common today - so we won’t be covering it here.

With the abbreviations clear, let’s look at how each system works in practice, including their strengths and weaknesses, starting with the fundamentals.

What is a camshaft?

A camshaft is a cylindrical shaft positioned at the top of the engine. Along it are raised sections known as "lobes". These "lobes" are what allow the shaft to do its job: controlling the opening and closing of the intake and exhaust valves. The camshaft is driven by either a chain or a belt, which takes its drive from the engine’s main shaft: the crankshaft.

This mechanism is responsible for delivering the four phases of a "four-stroke" engine (intake, compression, combustion and exhaust) by synchronising the valves with the pistons’ position and the corresponding firing order. This operating principle applies to both DOHC and SOHC engines.

With that in mind, what’s the difference between the two systems?

SOHC: how it operates

If the valves are opened and closed by just one overhead camshaft, you’re looking at an engine with a SOHC system - a configuration most commonly associated with engines that use two valves per cylinder. Because SOHC engines are simpler in design, they are generally cheaper to manufacture, but tightening emissions requirements and growing performance expectations mean this solution is becoming less and less common.

DOHC: how it operates

Engines with a DOHC system are more intricate and costlier to produce, yet they are also more widespread - in fact, it’s currently the most used solution in the automotive industry. In a DOHC engine, valve timing is handled by two camshafts: one controls the opening and closing of the intake valves, and the other controls the opening and closing of the exhaust valves.

The key advantage of DOHC over SOHC is that it allows better management of gas flow within the combustion chamber - both during the intake phase (helping to create a more uniform mixture in the combustion chamber) and during the exhaust phase (allowing exhaust gases to be expelled more effectively). The result is improved engine efficiency while also lowering fuel consumption and emissions.