Single-Overhead Camshaft (SOHC) and Double-Overhead Camshaft (DOHC) are two types of camshafts used in an internal combustion engine (ICE). The key difference between the two is what type of camshaft controls the air intake and exhaust flow in the engine.
Basically, an SOHC engine uses a single camshaft for each cylinder head. Conversely, a DOHC engine uses two camshafts for each cylinder head, which makes it more complex and typically requires more money to produce. However, a DOHC has advantages with valve timing, greater total air volume flow through the engine and greater power output, particularly at high-speed engine applications.
Table of Contents
SOHC vs DOHC: Quick Comparison
| Factor | SOHC Engine | DOHC Engine |
| Full form | Single Overhead Camshaft | Double Overhead Camshaft |
| Camshafts | One camshaft per cylinder head | Two camshafts per cylinder head |
| Valve control | One camshaft controls intake and exhaust valves | Separate camshafts control intake and exhaust valves |
| Performance | Good for regular driving | Better for higher RPM and stronger performance |
| Complexity | Simpler | More complex |
| Maintenance | Usually easier and cheaper | Can be costlier due to more parts |
| Fuel efficiency | Depends on tuning | Depends on tuning and valve timing |
| Best for | Simple, affordable engines | Modern, performance-focused engines |
SOHC is a simpler than DOHC and provides an alternative means of allowing for improved air flow and additional options for adjustment relative to either performance or economy. This is done by using DOHC to create multiple possibilities of designing camshafts and timing for both engines to achieve maximum output.
What is a Camshaft?
A camshaft is a rotating shaft inside the engine that controls the opening and closing of valves. These valves allow air and fuel to enter the combustion chamber and let exhaust gases exit after combustion.
Valves of both types must work at their intended time to allow engines a proper ‘breathing’ of air and thus increase horsepower and decrease fuel consumption of an engine’s operation.
In older pushrod or OHV engines, the camshaft sits lower inside the engine block and operates the valves through pushrods. In overhead camshaft engines, the camshaft sits above the cylinders, closer to the valves. SOHC and DOHC are both types of overhead camshaft designs.
What is an SOHC Engine?
Single Overhead Camshaft (SOHC) engines utilise one camshaft per head, which will have both intake and exhaust valves operated by that cam.
Because of their smaller number of parts, SOHC engines are generally more straightforward, lightweight and less costly than DOHC Engines. As a result, the less complicated design of an SOHC Engine will also be easier to maintain than a more complicated Engine design.
SOHC Engines are generally used where low cost, reliability, and simplicity are more important than high power outputs; they suit most regular applications such as city driving, commuting, or low-cost cars.
Advantages of SOHC Engines
- Simpler engine design
- Fewer moving parts
- Lower manufacturing cost
- Usually easier maintenance
- Compact engine layout
- Good enough for regular driving
Limitations of SOHC Engines
- Limited ability to control the rate of the opening and closing of the valves (lower RPM’s than DOHC).
- Not able to utilise as much air at higher RPM‘s as DOHC.
- Less potential for increased airflow on high-performance engines
- Not as suitable for advanced valve timing setups as DOHC
SOHCs are not necessarily bad engines; they just have been designed for low complexity rather than high output.
What is a DOHC Engine?
A DOHC engine uses two camshafts per cylinder head. One camshaft usually controls the intake valves, while the other controls the exhaust valves.
The use of DOHC gives engineers superior control of valve timing and valve operation, helping to enhance the complete breathing of an engine under all conditions (especially when used at higher engine speeds). As a result, you will find that most modern automobiles have DOHC if you want an optimised level (for performance, fuel economy, and emissions).
DOHC engines also work well with technologies like VVT, VTC and dual variable valve timing. These systems adjust valve timing depending on driving conditions, helping improve performance, fuel efficiency and refinement.
Advantages of DOHC Engines
- Better valve control
- Improved airflow
- Stronger high-RPM performance
- Better suited for 4-valve-per-cylinder layouts
- Works well with variable valve timing
- Common in modern petrol engines
Limitations of DOHC Engines
- More complex design
- More moving parts
- Higher manufacturing cost
- Repairs can be costlier in some cases
- Takes up more space than an SOHC setup
Generally speaking, a new DOHC engine is better than an old-style engine; however, the design of a DOHC engine will not determine if your engine is better or worse than another engine, regardless of the engine configuration. Factors such as engine tuning, the type of transmission used, overall vehicle weight, and expected use of the vehicle all contribute to your engine’s performance.
SOHC vs DOHC: Performance Difference
The biggest difference between SOHC and DOHC is how well the engine can control airflow.
The DOHC design allows for separate control of the timing of the intake and exhaust valves, which gives you more options with respect to your engine’s overall performance. This means that you will be able to achieve better airflow through the engine when it is operating at higher RPMs, and therefore produce more power. Therefore, the DOHC design is being used in many of the high-performance engines being produced today.
An SOHC engine can still offer good low-speed performance and reliable everyday driveability. But when it comes to higher RPMs, fast acceleration and stronger top-end power, DOHC usually has the advantage.
| Performance factor | SOHC | DOHC |
| Low-speed city driving | Good | Good |
| High-RPM performance | Limited | Better |
| Valve control | Basic | More precise |
| Airflow potential | Lower | Higher |
| Performance tuning | Limited | Better |
Most buyers won’t realise much difference for traditional city driving.
SOHC vs DOHC: Mileage Difference
Mileage does not depend only on whether an engine is SOHC or DOHC. It depends on many factors, including engine tuning, car weight, gearbox, fuel type, driving style and maintenance.
A SOHC engine is potentially more fuel-efficient because of its lighter and simpler design and therefore does not require as much power to operate compared to DOHC engines, which may be more fuel efficient because they offer superior combustion control and valve timing.
Even then, you cannot accurately claim that SOHC always gives better mileage or DOHC always uses more fuel.
| Mileage factor | Why it matters |
| Engine tuning | Determines whether the engine prioritises mileage or performance |
| Gearbox | Manual, AMT, CVT, DCT and AT affect mileage differently |
| Vehicle weight | Heavier cars usually need more fuel |
| Valve timing | Modern systems can improve efficiency |
| Driving style | Smooth driving improves mileage |
| Maintenance | Dirty filters, old oil and poor plugs can reduce mileage |
A properly tuned DOHC engine offers considerable performance and efficiency, yet you can also achieve good fuel economy with a basic SOHC by maximising engine design for economy.
SOHC vs DOHC: Maintenance and Cost
SOHC engines are usually simpler because they have fewer parts. This can make them easier and cheaper to repair in some cases. The layout is less complex, and access to components may be easier depending on the engine design.
A DOHC engine has more parts than an engine with a conventional (SOHC) configuration, such as 2 camshafts in each cylinder head and a complicated valve train. If any of these parts need to be replaced or repaired, it is likely to cost more than repairing/ replacing the same part on a single overhead cam (SOHC) engine. Therefore, the service costs associated with maintaining a DOHC engine are primarily influenced by the model of the vehicle you purchase vs. the type of camshaft design that has been used.
| Maintenance factor | SOHC | DOHC |
| Design complexity | Lower | Higher |
| Number of moving parts | Fewer | More |
| Repair cost | Usually lower | Can be higher |
| Routine servicing | Depends on vehicle | Depends on vehicle |
| Long-term reliability | Depends on maintenance | Depends on maintenance |
A DOHC engine is not automatically unreliable. With timely servicing and correct oil, a DOHC engine can be just as dependable as an SOHC engine.
SOHC, DOHC, OHV, VVT and VTC Explained
Engine terms can get confusing, so it helps to understand the basics.
| Term | Meaning |
| OHV | Overhead Valve engine, where the camshaft sits in the engine block |
| SOHC | Single Overhead Camshaft |
| DOHC | Double Overhead Camshaft |
| VVT | Variable Valve Timing |
| VTC | Variable Timing Control |
| Dual VVT / Dual VTVT | Variable timing on both intake and exhaust sides |
| 4 valves per cylinder | Usually improves airflow compared to 2 valves per cylinder |
A big plus for DOHC is that you can adjust how long the valves stay open (change the valve timing) so you can control both the intake camshaft and exhaust camshaft separately. This gives you a better balance of horsepower, fuel efficiency and emissions for today’s engines.
How to Identify SOHC or DOHC in a Car Spec Sheet
Car brochures may mention SOHC, DOHC, VVT, i-VTEC, VTVT, Dual VVT or similar terms. These can give clues about the engine layout and valve timing technology.
| Spec sheet term | What it usually means |
| SOHC | One camshaft per cylinder head |
| DOHC | Two camshafts per cylinder head |
| VVT / VTC / VTVT | Variable valve timing technology |
| Dual VVT / Dual VTVT | Variable timing on intake and exhaust sides |
| 4 valves per cylinder | Better airflow potential |
There are many car manufacturers that do not give clear information about their camshaft layout in their brochures. It is the buyer’s responsibility to confirm the specific technical details and/or the manufacturer.
Which Engine Type is Better for Indian Buyers?
Indian car buyers should not only think about camshaft configurations when buying a car. A single overhead cam (SOHC) engine can be reliable and fuel-efficient; however, a double overhead cam (DOHC) engine can be easy to own, provided it’s well-engineered and maintained.
The better engine depends on your usage.
| Buyer priority | Better fit |
| Lower cost and simple ownership | SOHC |
| Everyday city driving | SOHC or DOHC |
| Better performance | DOHC |
| Modern engine technology | DOHC |
| Higher RPM power | DOHC |
| Easy maintenance | SOHC usually has an advantage |
| Better valve timing flexibility | DOHC |
When buying a car, SOHC vs SOHC engines should be one of the last of your priorities. Instead, you should consider all aspects like mileage, performance, reliability, maintenance cost, safety features, transmission and owner rating. The engine type can be a supporting aspect, which you can determine based on your usage and needs.
FAQs
Q. What is the full form of SOHC?
SOHC stands for Single Overhead Camshaft. It means the engine uses one camshaft per cylinder head to control the valves.
Q. What is the full form of DOHC?
DOHC stands for Double Overhead Camshaft. It means the engine uses two camshafts per cylinder head, usually one for intake valves and one for exhaust valves.
Q. Is DOHC better than SOHC?
DOHC is generally better for performance, airflow and advanced valve timing. SOHC is simpler, lighter and usually cheaper to maintain. The better option depends on the engine and the car’s purpose.
Q. Does DOHC give better mileage than SOHC?
Not always. Mileage depends on engine tuning, gearbox, car weight, driving style and maintenance. A well-tuned DOHC engine can be efficient, and an SOHC engine can also deliver good mileage.
Q. Is an SOHC engine reliable?
Yes, SOHC engines can be very reliable because they are simpler and have fewer moving parts. Reliability depends more on maintenance than camshaft layout alone.
Q. Is a DOHC engine expensive to maintain?
A DOHC engine can be costlier to repair because it has more parts and a more complex valve train. However, routine maintenance depends on the car, brand and engine design.
Q. Why do modern cars use DOHC engines?
Modern cars often use DOHC engines because they allow better valve control, improved airflow and easier use of variable valve timing technologies.
Q. What is the difference between DOHC and VVT?
DOHC is the engine’s camshaft layout. VVT is a technology that changes valve timing. A DOHC engine can use VVT to improve performance, mileage and emissions.
