Before we try to explain the differences between a SOHC (Single Overhead Camshaft) and DOHC (Dual Overhead Camshaft) setup, for the uninitiated’s sake, let’s try to understand what the duties of a Camshaft are. Inside an internal combustion engine (whether it is air cooled, oil cooled or liquid cooled), a Camshaft is a rotating cylindrical rod with a number of oblong lobes which protrude from it. With every rotation, the differently positioned lobes first push the intake valve(s) to allow air inside the cylinder to mix with fuel and combust, and then the exhaust valve(s) for the resultant gases to escape. In modern motorcycle engines, valve springs provide the return force for the valves to pull back to their shut position once the lobe rotates away.
When a single camshaft governs both intake and exhaust valves, the setup is commonly known as SOHC. On the other hand, when there are two dedicated camshafts, one to push the inlet valves and the other to push the ones which let the gases escape from the cylinder after combustion the setup is called DOHC. In both instances, the camshaft is positioned above the valves for more accurate actuation.
There’s also another setup called OHV (Over Head Valve) which is traditional, less efficient than the other two and is rarely adopted these days. In an OHV design, the camshaft is installed inside the engine block and valves are operated by lifters, pushrods and rocker arms. For this very reason, an OHV engine is also known as a “pushrod” engine. The main disadvantage of an OHV design is that it’s difficult to precisely control the valve timing at high rpm because of the higher inertia caused by a larger amount of valve train components. This makes an OHV design better suited for engines with large volume which offer higher torque at lower RPMs. With the basics out of the way, let us now understand what are the advantages and disadvantages of employing a SOHC or a DOHC setup.
Single overhead cam setup
A SOHC setup typically allows a 2 or 3 valves per cylinder configuration, where usually, one valve allows air to enter and the other allows gases to escape. However, a manufacturer’s engineering prowess can also allow 4 valves per cylinder configuration using SOHC. Such a setup allows for more airflow than 2 valves per cylinder, as there is a larger open area for the air to enter the cylinder and gases to escape when the valves are open. Example: The TVS Apache RTR 200 4V.
Because the mass of the valvetrain is lesser in such a configuration, a SOHC 4 valve engine would have better torque towards the low end. In comparison, because of the higher mass of the valvetrain, a DOHC configuration will produce lower amounts of torque initially with as many valves but the top-end performance will be better. The other advantage of two valves per cylinder setup is the reduction of parts, which saves cost and complexity. The design also tends to be better for low and mid-range torque as the airflow is faster at these engine speeds as a result of a more restricted flow. However, a SOHC setup means that the Camshaft is usually placed in the middle of the head which denies the spark plug its ideal location.
Dual overhead cam setup
DOHC was introduced to improve the volumetric efficiency of an internal combustion engine, the result of which is more powerful. With this design, camshafts can be installed further apart from each other. This allows the intake valves to be at a larger angle from the exhaust valves, which results in a more direct airflow through the engine with less obstruction. In other words, a DOHC engine can breathe better and thus produce more horsepower out of smaller engine displacement. Also, it is easier to implement efficiency-enhancing technologies like Variable Valve Timing in a DOHC engine.
A DOHC setup allows easy implementation of a 4 valve per cylinder configuration. Most of the times, it allows the engine to rev higher. It also allows for the valves to be placed in an optimized setup for maximum performance. A DOHC, four valves per cylinder configuration allows better airflow at high engine speeds, resulting in better top end power. DOHC engines also allow the spark plug to be placed right in the middle of the combustion chamber which in turn promotes efficient combustion. The disadvantage of such a setup is more weight, higher costs and also the fact that driving two camshafts requires more components.
To sum it up, a 4 Valve per cylinder SOHC engine with a lighter valvetrain mass would have better performance characteristics at the lower end of the powerband. However, at higher engine speeds, a DOHC setup with its higher valvetrain mass and as many valves per cylinder will develop higher peak torque and horsepower. You can also check out our extensive blog on disc brakes for deeper insights.
06 May 2021