DOHC

DOHC: Important Or Just Automotive Gibberish?

One of the most common abbreviations that you see in car brochures is DOHC. Or if you’re looking at secondhand cars from the early 2000s, you’ll probably notice that many cars—especially Japanese ones—have a DOHC sticker on the side.

So, what does it mean? Does it actually matter? Or is it just automotive lingo to sell more cars? The short answer is yes, it matters and is quite important. The long answer, well, we’re going to discuss everything you need to know about DOHC in this post. Here’s our table of contents:

DOHC Engine

DOHC stands for Dual Overhead Camshaft. As the name suggests, this means the engine has two camshafts on the top of the cylinders, rather than just one. The technology has been around since the 1940s, but it only started to grow in popularity in the mid-90s.

Up until then, most engines were either Single Overhead Camshaft (SOHC) or Overhead Valves (OHV) engines. The latter is also known as pushrod engines and is actually still present today in some American muscle cars.

Before we get into how DOHC works and its benefits, you’ll need to understand how camshafts and valves work. Of course, if you already have a basic understanding of internal combustion engines, then you can skip this next section.

The Basics Of Internal Combustion Engines

An internal combustion engine works by mixing fuel and air inside the engine’s cylinder and then combusting them to drive the pistons up and down. The reciprocating motion that the pistons produce is transferred to the crankshaft. This converts it into a rotational force, which it then transfers to the wheels via a set of gears and shafts and drives the car.

The fuel and air mixture enters the engine’s cylinders through valves, and there are two types: the intake and exhaust valves. In the case of the DOHC engines, there are usually two of each type per cylinder. As the name suggests, the intake valve lets fuel and air enter the engine’s cylinder.

Once it mixes, the piston will compress the mixtures, and the spark plug will create a spark to combust this mixture. This tiny controlled explosion drives the piston forward, which as mentioned, is what powers the car. Quick side note, diesel engines don’t have spark plugs; the mixture combusts thanks to the compression.

The combustion will result in exhaust gases which will need to escape the cylinder. After the combustion process, the exhaust valves will open to allow these gases to escape. Clearing out the cylinder to repeat the combustion process once again.

The job of opening these valves goes to the camshaft. The camshaft is a rotating shaft with lobes, also called cams hence the name camshaft. These lobes will push the valve when they come into contact, opening the valves at the correct time. Afterward, the valves will return to their original position and close through the use of springs.

How Dual Overhead Camshaft Works

Now that you have a better understanding of how camshafts and valves work, we can get back to the DOHC system itself. As mentioned, this system has two camshafts on top of the cylinders. One is for opening the intake valves, while the other is for opening the exhaust valves.

It’s really as straightforward as that: two camshafts on top of the engine. One for the exhaust valves, the other for intake valves. We recommend watching the video above to get a better understanding of the DOHC engine’s construction.

Keep in mind that if you have a V-configuration engine, such as a V6 or a V8, then the engine has a total of four camshafts. Two for each back, since V-engines have two separate engine banks. Now, let’s see how it compares to other systems:

DOHC VS Other Configurations

As mentioned, there are three camshaft configurations: DOHC, SOHC, and OHV or also known as pushrod engines. Here’s how DOHC compares to them:

SOHC VS DOHC

The SOHC system, as the name suggests, only has a single camshaft on top of the engine. The camshaft sits right in the middle of the top of cylinders, with lobes that operate both the intake and exhaust valve on either side.

Unlike in a DOHC engine, the valve sits on a rocker arm. The lobes and the camshaft comes into contact with the rocker arm, which then operates the valve. So, the lobes don’t come into direct contact with the valves. It’ll probably be easier to understand by watching this comparison video below:

Most SOHC engines come with either 2 or 3 valves per cylinder. Although more modern ones can come with 4 valves per cylinder as well thanks to advancements in engineering and manufacturing processes.

The benefits of SOHC are that it’s lighter, cheaper, and can be more fuel-efficient. However, the location of the camshaft means that you can’t put the spark plugs on top of the cylinder, instead, you’ll have to put them on the side of the cylinders which is not the ideal location.

Meanwhile, DOHC engines are heavier and more expensive to produce due to more parts. However, the spark plug can sit on top of the engine. Additionally, it has better volumetric efficiency, which allows better airflow and the engine can produce more power.

Carmakers still produce SOHC engines today, but mostly in affordable small cars in developing markets. The benefits of DOHC engines far outweigh the downsides, hence why most engines—especially in markets with more buying power—are now DOHC rather than SOHC. However, there are surprising examples, which we’ll talk about later on.

DOHC VS OHV (Overhead Valves Or Pushrod Engines)

In the past, many cars had their valves at the bottom of the engine rather than at the top, hence why when they moved it to the top, the term OHV was used. This configuration is also known as pushrod engines and is still quite popular with American muscle cars.

A pushrod engine has the camshaft at the bottom of the engine. Specifically, underneath the combustion chambers (cylinders) inside the engine block.

This camshaft has lobes and rotates just like it does in DOHC or SOHC engine, and it connects to the valve at the top of the engine via a connecting rod. When the lobes come into contact with the rod, it pushes the rod which in turn pushes the valve’s rocker arm and opens the valves, hence the name ‘pushrod’.

The video above should give you a good idea of the pushrod engine’s construction, as well as how it compares to DOHC and SOHC engines. Interesting side note, enthusiasts often refer to pushrod engines as flathead engines.

Anyway, pushrod engines make more low-end torque. It’s also very simple, which means it’s likely to be more reliable, and cheaper to produce. Additionally, since the camshaft sits lower and in the middle of the engine, this gives the car a lower center of gravity which can improve handling. The biggest downside of this engine is that it’s unable to rev very high.

This is because the rod has the tendency to lose contact with the valves at higher RPM, and so it just won’t work at a higher RPM. This is why you see this configuration only in muscle cars nowadays since one of the defining characteristics of a muscle car is to have a high-torque engine that doesn’t rev very high.

Examples Of Each Configuration

Let’s take a look at some examples. As mentioned, DOHC engines are now very popular, and almost every car uses them, some examples include:

  • Honda’s K-Series engines. Available in certain Civic models and Accord.
  • Toyota’s GR engines. Most popularly used in the Toyota Tacoma.
  • BMW’s N55 engine. This is the straight-six engine they use for middle to higher-end trims and M-Sport models.

Perhaps more surprising examples come from the SOHC corner. As mentioned, SOHC is still used in most small affordable cars in developing markets. Such as in the Honda Brio, which is a small hatchback on sale mostly in South East Asia. However, turns out some luxury and performance cars still use them.

DOHC

For example, the Mercedes-Benz M279 V12 engine is a SOHC engine. This engine was introduced in 2012 and was used in many AMG models such as the S65 AMG and G65 AMG up until 2019. Another example is Acura, their J-series engine has both a DOHC and SOHC variant. If anything, only the turbo variant of the engine (J30AC) is DOHC.

Finally, some pushrod engine examples include the Corvette C7 with its LT1 engine, and the current C8 Corvette Stingray also has a pushrod engine. The 6.2L V8 Hemi is present in Jeep Grand Cherokee Trackhawk and Dodge’s Hellcat cars is also a pushrod engine.

Why Do Some Cars Still Use SOHC?

So, why is this relatively old technology still around even in luxury cars? Well, as mentioned SOHC engines can have better fuel efficiency. Additionally, they provide better torque in the low-to-mid RPM range as well. In performance luxury cars such as the Mercedes-AMG S65, this is what you want.

Performance luxury cars typically don’t need to rev very high, that’s really more a pure-bred sportscar characteristic. Instead, the low-to-mid torque that SOHC engines deliver is what you’d want in performance luxury cars.

Of course, the biggest benefit of the SOHC configuration is that it’s cheaper to produce. This is why many small affordable cars still use this. It’s cheaper to produce, so the car can be sold for cheaper with a healthy profit margin for the manufacturer.

In a nutshell, each camshaft configuration has its benefits and drawbacks. They also result in different engine characteristics, especially in power delivery. If a manufacturer is making a car that doesn’t need the benefits or characteristics of DOHC, then there’s nothing wrong with going SOHC.

DOHC VTEC?

Speaking of Honda, you’re probably wondering what does DOHC VTEC mean? Well, this means the engine has both the DOHC configuration and VTEC technology. VTEC is Honda’s brand name for their Variable Valve Timing (VVT) system.

A quick lesson on VVTs, this system allows the engine to open the valves for different amounts of time depending on the need and driving condition. For example, when driving slowly, the intake and exhaust valves will open relatively quickly. And when you’re driving more aggressively, these valves will have a larger opening and remain open for longer.

The larger opening allows for more fuel and air to enter the cylinder, which will result in more power from the combustion process. Opening the exhaust valve for longer will also allow for more exhaust gas to escape. Whereas a shorter opening leaves some gas behind, this effectively reduces the engine’s capacity making it more efficient.

DOHC

Honda’s VTEC—and many other VVT systems—works by using several different cam profiles. The more aggressive cam profile will have a larger lobe (cams), which pushes the valves deeper into the engine and it remains open for longer.

As to how the system engages, there are several different types. But in the case of VTEC, it uses oil pressure to switch between different cam profiles. This device that controls this is known as the VVT control solenoid, and it engages depending on the Engine Control Unit’s (ECU) command.

Note that i-VTEC is a slightly different version of VTEC, and it uses variable camshaft phasing to improve efficiency. Some other notable VVT system branding includes Toyota’s VVT-i, BMW’s VANOS or also known as Valvetronic, and Fiat Group’s MultiAir among others.

DOHC 16V

Another common automotive abbreviation you might see in brochures or car badging is ‘DOHC 16V’. The 16V stands for 16 valves, which means the engine has a total of 16 valves. This badging is common in early-2000s Japanese cars with four-cylinder engines.

This isn’t something to boast about nowadays, but older cars with SOHC engines usually have no more than 12 valves (three valves per cylinder). Meanwhile, DOHC engines easily allow for four valves per cylinder. So, it was a big deal when they started making it as it means better performance. Hence why it was often used in marketing materials.

DOHC

Note that 16 valves only apply to four-cylinder engines (four-cylinder, four valves each, 16 valves in total). If your car has four valves per cylinder and is a six-cylinder engine, then it has a total of 24 valves. A V8 engine then has 32 valves in total, and so on and so forth.

Also note that SOHC engines can also have four valves per cylinder, which means you might ‘SOHC 16V’ from time to time. This wasn’t very common back then, but advancements in engineering have allowed SOHC engines to have four valves per cylinder as well.

DOHC Turbo

DOHC Turbo simply means that the car has a DOHC configuration, and the engine is turbocharged. So, no, it isn’t a “turbo” variant of the standard DOHC system. It’s still the same system as in any other car with a dual overhead camshaft system.

If you didn’t know, a turbocharger is a forced induction device that forces more air into the engine’s cylinder. The device works by using the flow of exhaust gases from the engine. This spins the turbine inside the turbo housing, which forces more air into the engine.

Add a bit more fuel along with that extra air and voila! You get more powerful combustion which increases your car’s power. All that with using a minimal amount of extra fuel. This is why turbo engines are very popular nowadays, as engineers can extract more power without having to use too much fuel. You can learn more about turbochargers here.

DOHC Engine Pros & Cons

So, now you know how DOHC engines work and how they compare to other camshaft and valve configurations. Here’s a summary of their benefits and disadvantages:

  1. Better volumetric efficiency, which allows for more power—and sometimes better fuel efficiency—even with a smaller engine displacement.
  2. Can rev to very high RPMs, sometimes up to 9,000rpm if the carmaker deems it’s necessary for the car.
  3. Provides more horsepower and torque at higher RPM.
  4. Allows for the spark plug to be at the ideal position, which is right at the top of the cylinders.
  5. Easily allows for a four-valve design per cylinder, which helps to improve performance.

Meanwhile, here are the disadvantages of the dual overhead camshaft configuration:

  1. More parts mean it’s more expensive to produce and adds weight. Weight-saving measures will be necessary for sports cars with a lightweight focus.
  2. More parts also mean there are more things to go wrong. It’s not necessarily more unreliable, but there’s more chance of things going wrong.
  3. Tends to produce less torque at low and low to mid RPM. But this can be rectified if the carmaker wants the car to have more torque, such as by increasing the length of the engine’s stroke.

Higher RPM Dual Overhead Camshaft

As you can see, there are more benefits than downsides when it comes to the DOHC configuration. And in today’s automotive world, the benefits it offers are very important hence why this configuration is so popular.

However, SOHC engines come with their own set of benefits and they may be what the carmaker needs for a certain model. If this is the case, then it’s not unlikely for them to choose SOHC over DOHC.

Dual Overhead Camshaft Engines: FAQ

Got any more burning questions about DOHC engines? We try to answer as many questions as possible below:

What Does DOHC Mean

DOHC stands for Dual Overhead Camshaft, which means your car has two camshafts on top of the cylinders. One is for engaging the intake valves, the other is for exhaust valves. In contrast, SOHC (Single Overhead Camshaft) engines only have one camshaft responsible for both the intake and exhaust valves. The DOHC configuration allows the engine to have more power and torque at higher RPMs, and in most cases, a better solution than SOHC.

What Is A DOHC Engine

A DOHC engine is an engine with two camshafts on top of the engine’s cylinders. Hence the name Dual Overhead Camshaft (DOHC). The camshaft is a rotating shaft with lobes (or cams) that will open the engine’s valves when they come into contact. This configuration is generally better than Single Overhead Camshaft (SOHC) and Overhead Valves (OHV) since it allows the engine to rev higher. Additionally, this design allows for more power and torque at higher RPMs.

What Does DOHC Stand For

DOHC stands for Dual Overhead Camshaft, which means there are two camshafts that preside on top of the engine’s cylinders. Note that if you have a V-configuration engine, this means you have a total of four camshafts; two for each cylinder bank. However, it’s still referred to as a DOHC engine.

DOHC Or SOHC How To Tell

The easiest way to tell is by finding out your car’s make, model, and trim level, and then searching whether your car has DOHC or SOHC. This information may also be present in your owner’s or service manual, but if not, then you can easily find it online. If you happen to be working on the car and have the timing belt/chain cover off, then take a look at how many cam gears are present. Two gears mean it’s DOHC, meanwhile, a single cam gear means it’s a SOHC engine.

DOHC: Final Thoughts

So, as we’ve repeated endlessly, DOHC stands for Dual Overhead Camshaft. The engine has two camshafts on top of the cylinders; one for activating the intake valve, and the other for the exhaust valve.

The DOHC system is heavier and more expensive to produce since there are more parts. However, the design allows for the engine to rev higher and can produce more power and torque at higher RPMs. Overall, it’s a better system than SOHC, hence why most cars nowadays come with DOHC.

If you’re wondering whether you should get DOHC or SOHC, then personally I will say DOHC. I’ve had experience with both configurations, and I have to say that DOHC is just all-around nicer. That being said, SOHC engines can be good as well.

Test Drive

There’s a lot that goes into designing an engine, and the camshaft configuration is just one of many engineering choices that make up the quality of the engine. The most important thing is to test-drive the car and see if you like it.

That’s pretty much everything you need to know about DOHC engines and how they work. Hopefully, this has been educational for you.

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