Single Overhead Cam (SOHC) & Double Overhead Cam (DOHC) - Differences

Even though the electric motors and battery set-up slowly starts to replace the Internal Combustion Engines, really it will not catch up with the performance and efficiency of the ICE I'd say. In this Internal Combustion Engines, valves are responsible for letting the fresh fuel air mixture inside the combustion chamber and letting out the burnt fuel air mixture. These valves are operated by the cam lobe, which is integrated on the camshaft and they're operated by the crankshaft with the help of chain or belt assembly. It is to be noted that the camshaft rotates half a speed of the crankshaft always.

OHV setup

This mechanism is not common these days but it is much popular several decades back. In this mechanism, camshaft stays next to the crank which is geared to it. Rocker arm and valve assembly stays on the head assembly. Camshaft and rocker arm is connected with a help of pushrod. Yeah... Older RE models used this pushrod mechanism and that's why those engines are called as pushrod engines. But it had a major disadvantage. It became difficult to control the timing of the valve at higher RPM band due to the higher inertia caused by more valve train components which is mentioned above. That's why Overhead Cam mechanism is introduced and it further splits into two. Single Overhead Camshaft assembly and Double Overhead Camshaft assembly. Let's dive on those two now.

SOHC arrangement

In this mechanism, the camshaft is placed in the engine's head assembly parallel to the crankshaft and the both of them are connected with a help of chain drive or belt pulley. Still now, motorbike like Kawasaki W800 use a bevel drive for operating the camshaft which is rare and unique. In the camshaft of this setup, cam lobes for actuating both the intake and exhaust valves are mounted in a single shaft itself. Those cam lobes lifts the valves with a help of rocker arms and let the fresh air fuel mixture in and out of the combustion chamber. Generally there'll be two valves for each cylinder where one allows the fresh fuel air mixture inside the chamber at the suction stroke and the other one allows the burnt mixture out of the chamber at the exhaust stroke. Some manufacturers use 4 valves instead of 2V setup which improves the overall breathing efficiency of the engine. There's a advantage in this setup which is none other than a mass of the valve train. The overall mass is less, resulting in better torque in lower RPM band. It also reduces the overall cost and makes it easy for mechanics to work on it.

DOHC arrangement

Instead of single overhead camshaft, this setup consists of individual camshafts for both intake and exhaust valves. As both the camshafts are placed apart from each other, intake cam lobe's profile can be bigger, resulting in more airflow to the engine which directly improves the volumetric efficiency of the engine. Simply, engine with this setup breath efficiently resulting in better performance at higher RPM band. But, due to the higher mass of the valve train assembly, engines with this valve setup will produce lower amount of torque initially. This setup also allows to implement the 4V setup easily, resulting the engine to rev high. There's also a another advantage. In this setup, sparkplug can be placed at the center of the combustion chamber promoting the efficient combustion process. Every system in this world have their own pros and cons. Like that, this system results in more weight and more number of parts which directly results in high cost. This also makes hard for the mechanics to work on the head assembly.

I tried to explain the whole system as simple as I could and if you have any suggestions or corrections on the same, feel free to leave it on the comment section below.