SOHC vs. DOHC: What’s the Difference and Which Is Better?

Cars can be confusing—there are a variety of engine configurations, ranging from inline four-cylinders to V8s. What’s more, engines are further classified by the location of their camshafts. Most modern engines have either a single overhead camshaft (SOHC) or dual overhead camshaft (DOHC) design.

If you’re like many drivers, you might not know the difference between an SOHC and DOHC engine. To get you up to speed, we’re going to demystify these seemingly complex terms and provide the pros and cons of both designs.

How Camshafts and Valves Work

Before you can comprehend SOHC and DOHC engine configurations, you need to have some background information on camshafts and valves.

You see, inside of an engine, there’s a crankshaft and one or more camshafts. The crankshaft connects to the pistons and connecting rods, while the valves are opened and closed by the camshaft(s). A timing belt or timing chain connects the crankshaft to the camshaft(s) to keep them in sync.

Each of the engine’s cylinders has one or more intake valves and one or more exhaust valves. The intake valves let air (and fuel in the case of port injection) into the cylinder, whereas the exhaust valves let exhaust gases out. The camshaft has a series of egg-shaped lobes, each of which opens a valve at a certain point in the engine’s operating cycle.

At first, while the piston is moving downward, the camshaft opens the intake valve(s) to allow air to be pulled into the cylinder. As the piston begins to move upward, the camshaft closes the intake valve(s).

Then the spark plug fires, igniting the air-fuel mixture inside of the cylinder to create an explosion that sends the piston back downward. Once the piston starts moving upward again, the camshaft opens the exhaust valve(s) to allow spent exhaust gases to exit. The cycle then begins to repeat itself. 

The linear motion of the pistons going up and down in the cylinder causes the crankshaft to turn. From there, the spinning crankshaft transfers rotational force to the rest of the drivetrain to propel the vehicle down the road.

What are the Different Camshaft Location Classifications?

In the early 1900s, some automobile engines had a flathead design with valves located in the engine block. Back then, the term overhead valve (OHV) was used to distinguish engines that had valves in the cylinder head from those with a flat head design.

But flathead engines are no longer used in automotive applications. As a result, the term OHV is now used to describe any engine that has its camshaft located in the engine block. With an OHV design, the camshaft acts on a set of lifters, pushrods, and rocker arms to open the valves located in the cylinder head(s).

The other two camshaft location designations used in modern engines are single overhead camshaft (SOHC) and dual overhead camshaft (DOHC). Both designs have the camshaft(s) located in the cylinder head, rather than the engine block.

What is an SOHC Engine?

A single overhead camshaft (SOHC) engine has just one camshaft located in the cylinder head. Depending on the engine design, the camshaft may operate either rocker arms or cam followers to open the individual intake and exhaust valves.

SOHC engines typically have either two valves (one exhaust and one intake) or three valves (one intake and two exhaust) per cylinder. But there are some SOHC engines, such as Honda’s 3.5L, that use four valves per cylinder.

It’s also important to note that SOHC engines with a ‘V’ or flat configuration have two cylinder heads. Each of those cylinder heads has a camshaft inside. 

What is a DOHC Engine?

A dual (or double) overhead (DOHC) engine has two camshafts per cylinder head. One camshaft operates the intake valves while the other operates the exhaust valves. The design easily allows for four valves (two intake and two exhaust) per cylinder.

In most DOHC engines, the camshafts act on cam followers or lifter “buckets” to open the individual intake and exhaust valves.

DOHC engines with two cylinder heads have four camshafts (two per cylinder head). 

Which is Better: SOHC or DOHC?

SOHC and DOHC engines both have their advantages. DOHC engines allow for better airflow, resulting in improved performance under most driving conditions. On the other hand, SOHC engines have fewer parts, making them less complicated.

Let’s take a look at the advantages of both designs.

Advantages of SOHC:

• Usually less complicated with fewer parts 
• Typically easier to work on
• Lower manufacturing costs
• Improved low to mid-range torque

Advantages of DOHC:

• Provides better airflow through the engine
• Improved horsepower and high-end torque
• Allows for a higher rev limit
• Easier to fit with efficiency-enhancing technologies, such as variable valve timing

In the end, vehicle manufacturers seem to have chosen the DOHC design as the winner. Most newer engines that locate the camshaft in the cylinder head have a DOHC configuration, though there are a few outliers. For example, Honda still uses the SOHC configuration in many of its engines.

Most automakers have moved to a DOHC layout because the design allows for more precise control of engine operation. It’s also easier to implement variable valve timing (VVT) and other fuel-saving technologies into a DOHC engine.

You can learn more about the differences between SOHC and DOHC engines by watching the video below.

SOHC vs. DOHC Fuel Efficiency

Many drivers wonder whether SOHC or DOHC technology is more fuel-efficient. The answer is that it depends on the vehicle and the engine design.

Because SOHC engines have fewer moving parts, they benefit from a slight reduction in weight, which can provide a bump in fuel economy. Meanwhile, DOHC engines allow for improved airflow and more precise control over the engine, both of which can make the vehicle more efficient.

Honda, for example, offers a variety of fuel-efficient SOHC engines, whereas Toyota produces many efficient DOHC engines. In the end, it comes down to how the rest of the engine is engineered around the camshafts and valves.

VTEC DOHC

DOHC engines have seen their fair share of success as early as 1989 when Honda developed its variable valve timing and lift electronic control (VTEC) system.

This technology produced a compact and fuel-efficient engine, utilizing high-speed rocker arms and cam followers on the intake and exhaust sides as well as efficient cam hill switching according to engine speed.

The VTEC DOHC system debuted in the 1989 Honda Integra XSi, which ran on a B16A engine. The same system arrived in the US a few years later in the form of the Honda Acura NSX.

VTEC SOHC

Honda’s VTEC system was initially designed for DOHC engines, but the Japanese automaker also incorporated the same technology in SOHC engines.

During its early stages, VTEC SOHC was incorporated in D and J-Series engines.

But because SOHC engines only use one camshaft for the intake and exhaust valves, slight configurations had to be made.

Honda’s early SOHC engine couldn’t accommodate the VTEC rocker arm, so this engine type only benefited from the VTEC system when it came to the intake valves.

Eventually, SOHC VTEC engines were manufactured with six cam lobes and six rocker arms per cylinder, so the VTEC system worked on the intake and exhaust valves.

This trend began with Honda’s 2009-2012 Acura RL SH-AWD models.