You may have seen the acronym DOHC, SOHC, or OHV when referring to a vehicles engine. What does this mean and what does my vehicle have? To put it shortly, DOHC refers to Dual Overhead Camshaft, SOHC refers to Single Overhead Camshaft, and OHV refers to Overhead Valve. To better understand what these mean, it is important to know how a typical 4 stroke engine works (this is the type of engine you would find in any modern car). 

In a modern 4 stroke engine, there are 4 steps referred to as strokes: Intake, Compression, Combustion/Power, and Exhaust. These 4 strokes ultimately result in the movement of the vehicle.

Intake Stroke

During the intake stroke, the piston moves down the cylinder and the intake valve opens, creating a vacuum and sucking in the air and fuel mixture. On carbureted engines (older vehicles), the fuel comes in with the air and on direct injected engines (modern vehicles) the fuel is squirted directly into the cylinder. 

Compression Stroke 

During the compression stroke, the intake and exhaust valves close and the piston moves back up the cylinder. The air and fuel mixture is compressed and at the top of the cylinder. 

Combustion/Power Stroke

When the air and fuel is compressed and at the top of the cylinder, the spark plug fires and ignites the mixture. This explosion forces the piston back down the cylinder, moving the crankshaft and ultimately the vehicle.

Exhaust Stroke

The piston moves back up the cylinder due to the firing of other cylinders and the exhaust valve opens up and the burned fuel and air is pushed into the exhaust port. The 4 stroke cycle starts over again and continues so long as the vehicle is running. 

All of these strokes are happening very fast, and the speed is referred to as RPM or rotations per minute. The higher the RPM, the faster the engine is operating and rotating the crankshaft. At high RPMs, the 4 stroke cycle can occur hundreds of times a second. 

The camshaft is what opens and closes the intake and exhaust valves to suck in air and force it out during this process. 

OHV

Overhead valve or pushrod engines are typically found in older vehicles or larger engines. The camshaft is installed inside the engine block and is operated through lifters, pushrods, and rocker arms. It is difficult to precisely control valve timing at higher RPM and is better suited for V8 or larger engines. They are lower cost and offer high torque at lower RPM.

SOHC

Single overhead cam or overhead cam engines have a single camshaft installed in the cylinder head. The valves are operated directly by the camshaft and it is much easier to maintain precise timing at higher RPM and 3 or 4 valves per cylinder. One or Two exhaust valves are operated by the camshaft while two intake valves are operated by oil pressure. On a SOHC engine, it is difficult to implement variable valve timing separately for intake and exhaust valves. 

DOHC

Dual overhead cam engines are found in most modern vehicles today. They allow better airflow with less obstruction and are generally much more efficient engines than OHV or SOHC engines. Two camshafts operate 4 valves per cylinder, one separate camshaft for intake and exhaust valves. Variable valve timing and lift can be easily implemented for better fuel efficiency and power.