Apart from a slew of different engine configurations like I4, V6, W12, etc., there are also several different valvetrain setups. And two by far the most popular valvetrains in the past couple of decades are DOHC and SOHC. The differences between the two are pretty straightforward and simple but not insignificant.
The SOHC vs. DOHC difference is nothing more than the number of camshafts and valves. Both SOHC and DOHC have overhead camshafts, but SOHC has only one per cylinder head, while DOHC has two. Also, DOHC allows four valves per cylinder instead of only two that the SOHC can support, making DOHC better in a wider range of applications.
SOHC Engine Explained
The SOHC meaning is “Single Over-Head Camshaft.” That means the camshaft in a SOHC engine sits on top of the cylinder head right over the valves. With such a setup, the camshaft is in direct contact with the valves, which reduces the number of moving components and makes the engine more efficient.
However, the SOHC engine is only more efficient than older valvetrain setups, namely, the pushrod valvetrain. The pushrod design is called OHV, so the following part will be SOHC vs. DOHC vs. OHV. But it’s important to compare all three together to get a better picture. In a pushrod engine, the camshaft sits inside the engine block (under the cylinder head) and opens the valves using rods and valve forks.
So, putting the camshaft on top of the valves eliminates the need for rods and forks, making the engine more efficient because there is less parasitic weight. In other words, the engine doesn’t have to move the rods and forks, and so some of its power is freed up for moving the vehicle, which also results in lower fuel consumption and less CO2 emissions.
Moreover, by eliminating the push rods, lifters, and forks, the engine becomes significantly lighter. And while that weight difference doesn’t make much of an impact on a car, it certainly does for a motorcycle where every kilogram counts and is felt by the rider.
Also, SOHC design isn’t a relatively new technology developed in the 80s or 90s; on the contrary, it’s been in production cars and motorcycles since the 1920s. But the design also has some drawbacks, which is why there were still plenty of pushrod engines even in the 2000s, and there are some even today.
First of all, the overhead camshaft design takes up more space in the engine bay, making it more difficult to fit big engines into smaller vehicles. That’s especially important for motorcycle applications. Specifically, SOHC engines are longer and taller than pushrod engines. Furthermore, SOHC engines are more complicated, require more maintenance, and are more expensive to produce.
And although we have mentioned that a SOHC design doesn’t allow four valves per cylinder, that’s not exactly impossible to do. However, it’s much easier and cheaper to do it with a DOHC design, so for a 4V engine, DOHC is much better.
But with everything said and done, the SOHC design is still by far the most popular valvetrain in the world and will likely remain that for some time to come, and for a good reason. The SOHC engine is the best of all worlds as it’s more efficient than pushrod engines but also cheaper and more compact than DOHC engines.
What Is the Advantage of SOHC
As we mentioned, the SOHC engine is the best of both worlds when compared to the pushrod and DOHC designs. So, the DOHC vs. SOHC advantages and disadvantages are the same ones the SOHC has compared to OHV. However, when compared to a DOHC, the advantages and disadvantages are reversed. Meaning the disadvantages it has over a pushrod become advantages when compared to DOHC engines.
- More efficient than pushrod engines.
- They are cheaper to make than DOHC engines.
- More compact than DOHC engines.
- Less moving parts in both DOHC and pushrod engines.
- Simpler design than DOHC.
- More torque at lower RPMs than DOHC engines.
- Lower weight than DOHC and pushrod engines.
SOHC Disadvantages
The disadvantages of SOHC engines compared to DOHC are not exactly universal. It all comes down to what the end goal is, and both engines can be better in certain applications.
- More complicated than a pushrod engine.
- Nearly impossible to fit four valves per cylinder.
- Isn’t that compatible with variable valve and cam timing technologies?
- Lower top-end power output potential than DOHC engines.
- They are less compact than pushrod engines.
- They are more expensive than pushrod engines.
DOHC Engine Explained
After explaining what an SOHC engine is and how it works, understanding the DOHC engine will be a breeze because it’s nothing more than an evolution of the SOHC design. In a DOHC engine, the camshaft sits over the valves, just like in an SOHC design.
But the DOHC meaning is “Double Over-Head Camshafts,” which means it has two camshafts in every cylinder head instead of one. Now, an SOHC camshaft opens both intake and exhaust valves since all of them are positioned in a straight line.
So, the only reason to have double overhead cams is if you have four valves per cylinder. In that case, there are two rows of valves, and the easiest way to control them is to have two camshafts, one for each row. That means in a DOHC engine, one camshaft controls the exhaust valves, and the second one controls the intake valves.
So, without four valves per cylinder, there are absolutely no benefits of having a DOHC design. That’s why all the advantages of a DOHC design are actually the advantages of having four valves per cylinder, and all the disadvantages are down to the double camshafts.
Now, the DOHC design is as old as SOHC, but it wasn’t that common until the past two decades, even though it’s much more efficient because it’s significantly more expensive to produce.
That’s when it comes to cars. But when it comes to motorcycles, manufacturers think twice even today before installing a DOHC cylinder head. And the main reason is the weight and size of the engine, both of which play a major role when dealing with a vehicle that the driver moves around using his own weight. Plus, the available space for an engine in a motorcycle is significantly more limited.
That said, when it comes to SOHC vs. DOHC motorcycles, there is no substitute for DOHC and four valves per cylinder when talking about sports bikes and high performance. The same goes for cars and trucks but for another reason.
The DOHC and four valves per cylinder design increase the airflow into the engine significantly, which increases the maximum power output potential. But even more importantly, it increases turbocharger efficiency, and as we all know, 90% of new cars are turbocharged.
That means the DOHC design, despite being more expensive and complicated, is installed on the vast majority of new cars. And the design will only gain in popularity as time goes by, which means it will be around for as long as we have internal combustion engines.
What Is the Advantage Of DOHC
All the advantages of DOHC engines are most noticeable in newer engines with forced induction and variable valve timing. That applies to virtually all engines made after the year 2000. Before that, DOHC engines were mainly found in performance-oriented engines and motorcycles.
- The highest maximum power output of all valvetrain designs.
- Easy to implement variable valve timing technology.
- Better turbocharger efficiency.
- Allows four valves per cylinder.
- More torque at mid and high RPMs.
DOHC Disadvantages
Other than the size, weight, and cost of production, all other disadvantages of DOHC engines are neglectable. As a matter of fact, the three we just mentioned are also neglectable in modern cars and today are only significant when talking about motorcycles.
- The most expensive valvetrain to produce.
- More complicated than SOHC.
- More moving parts means it’s less efficient than SOHC.
- Heavier than SOHC.
- Less compact than SOHC.
DOHC Vs. SOHC Differences
Ultimately, the main difference when it comes to SOHC vs. DOHC engines in the real world is that DOHC engines have four valves per cylinder, while SOHC engines only have two. Plus, the obvious, DOHC engines have two camshafts per cylinder head, while SOHC engines have one.
By having more smaller valves in a cylinder, the engine has better airflow and exhaust scavenging, both of which increase the maximum power output.
Besides having two camshafts and four valves per cylinder, the DOHC engine often has variable valve timing. We mentioned how two valves per cylinder get you better low RPM torque; well, the variable valve timing gets you even better low-end torque from a DOHC design, plus a higher power output at high RPMs.
So, any DOHC engine made in the last twenty years will outperform an equivalent SOHC engine in all respects. That includes high and low RPM torque, fuel efficiency, CO2 emissions, and power output. The only downside is that a DOHC engine is slightly more expensive, but that’s neglectable for older cars.
On the other hand, you will hardly find any new gasoline car with an SOHC engine. Meaning DOHC is your only option, and it’s the better one of the two.
FAQs:
Q: Which is better, SOHC or DOHC?
In general terms, DOHC is better than SOHC. The DOHC design has a higher power potential, makes turbochargers more efficient, and with variable valve timing, is more fuel efficient. However, SOHC is cheaper and smaller, so in applications where that’s the priority, SOHC is better.
Q: Which is better SOHC or DOHC motorcycle?
When it comes to motorcycles, whether SOHC or DOHC is better depends on the application. An SOHC design is lighter, so it’s better for smaller motorcycles, plus it has better low-end torque making it better for off-road bikes. However, DOHC has a better power output potential and revs higher, so it’s better for sports bikes.
Q: Which is more fuel efficient, SOHC or DOHC?
In the real world and normal driving, SOHC is more fuel efficient because it has better low-end torque. However, most modern DOHC engines have variable valve timing, making them just as fuel efficient as SOHC engines, if not more fuel efficient.
Q: Is DOHC reliable?
Yes, DOHC engines are just as reliable as SOHC and OHV engines. The only difference between SOHC and DOHC engines is that DOHC has one extra camshaft and two more valves per cylinder, but that doesn’t hurt reliability or even maintenance costs.
Q: Is SOHC fuel efficient?
Yes, SOHC engines are fuel efficient. Most diesel engines are SOHC, but they are fuel efficient by design. As for gasoline engines, SOHC has better low-end torque making them more fuel efficient in theory. However, a 3-liter SOHC engine will consume more fuel than a 2-liter DOHC. The point is there are too many variables.
Q: Can you convert a SOHC to DOHC?
In some cases, yes, you can convert a SOHC engine to a DOHC cylinder head, but that’s rarely the case. For that conversion to be possible, there has to exist the same engine block with both 8-valve and 16-valve variations, which doesn’t happen too often. The only example I can think of is the Fiat F.I.R.E. engine. Otherwise, it’s basically impossible, and it’s much easier to replace the whole engine.
Q: Are all modern engines DOHC?
No, not all modern engines are DOHC. There are still a lot of new SOHC engines produced, and even OHV engines. However, it is true that most engines produced today, even smaller and cheaper ones are DOHC.
Final Words
Ultimately, there isn’t much of a difference between DOHC and SOHC, or at least, there is no fundamental difference. The DOHC engine has two overhead camshafts and four valves per cylinder, while the SOHC has one camshaft and two valves, but the working principle is identical.
And while generally, the DOHC is regarded as a universally better design, that’s not always the case. Yes, the DOHC has better power potential, can have variable valve timing, and makes turbochargers more efficient. However, DOHC is also bigger, heavier, and more expensive. That means the SOHC is better in a good number of applications, especially motorcycles.
