What Is a Double Overhead Cam (DOHC)?

A double overhead cam (DOHC) engine uses two camshafts mounted in the cylinder head—typically one to operate the intake valves and one for the exhaust valves—allowing more precise control of valve timing and lift, improved airflow, higher power potential, and better efficiency. In automotive terms, DOHC (often called “twin-cam”) is now the dominant valvetrain layout for modern passenger vehicles, from economy cars to high-performance models, because it supports multi-valve designs and advanced systems like variable valve timing and lift.

How a DOHC Valvetrain Works

At its core, a DOHC setup coordinates the opening and closing of the engine’s intake and exhaust valves to move air-fuel mixture in and combustion gases out. The two camshafts are precisely synchronized to the crankshaft via a timing chain or belt, and they act directly on the valves or through low-mass followers.

1. The crankshaft turns a timing chain or belt connected to two camshaft sprockets atop the cylinder head.
2. One camshaft actuates intake valves; the other actuates exhaust valves, typically via lifters/tappets or finger followers.
3. As cam lobes rotate, they press on followers to open valves at exact moments; valve springs close them as the lobes rotate away.
4. Electronic controls can adjust cam phasing (and, in some engines, valve lift) to optimize performance, efficiency, and emissions across RPM and load.

This direct, separated control of intake and exhaust events helps DOHC engines breathe more effectively and rev more freely than many single-cam or pushrod designs.

Key Advantages

DOHC architecture is favored because it enables designs that improve volumetric efficiency, combustion control, and overall drivability, especially when paired with modern electronics and turbocharging.

• Supports multi-valve heads: Commonly four valves per cylinder (two intake, two exhaust) for better airflow and mixture motion.
• Higher rev capability: Reduced valvetrain inertia versus pushrod setups aids high-RPM stability and power.
• Advanced valve control: Simplifies adding variable valve timing (VVT) on both cams and, in some engines, variable valve lift systems.
• Improved efficiency and emissions: More precise timing enhances combustion, often yielding better fuel economy and lower NOx/HC when tuned accordingly.
• Packaging for modern tech: Plays well with direct injection and turbocharging, allowing strong low-end torque and broad powerbands.

Taken together, these benefits explain why DOHC has become the default layout for most modern gasoline and many diesel passenger engines.

Trade-offs and Considerations

While DOHC offers clear performance and efficiency gains, it also introduces complexity and some design compromises compared with simpler valvetrains.

• Increased complexity and cost: Two cams, more components, and tighter tolerances can raise manufacturing and maintenance costs.
• Larger cylinder head: Can be taller/wider than SOHC or pushrod designs, impacting under-hood packaging.
• Potential service demands: Timing chains/tensioners, cam phasers, and seals may require attention over the vehicle’s life.
• Weight: Although often lighter at the valvetrain level than pushrods, the head assembly can add mass versus simpler layouts.

For most modern vehicles, the trade-offs are outweighed by the gains in drivability, emissions compliance, and performance flexibility.

DOHC vs. SOHC vs. OHV (Pushrod)

Understanding how DOHC compares to other common camshaft layouts helps clarify why manufacturers choose one approach over another.

• DOHC: Two cams per head; typically enables four valves per cylinder and easy dual-cam phasing; excels at high-RPM breathing and precise control.
• SOHC: One cam per head; can operate both intake and exhaust valves but with less flexibility; often simpler and smaller than DOHC.
• OHV (Pushrod): Camshaft in the block with pushrods operating valves; compact and torquey, popular in some V8s, but generally limited for very high RPM and complex valve strategies.

Manufacturers match the architecture to goals: compactness and low-end torque (OHV), simplicity and cost (SOHC), or peak efficiency and performance range (DOHC).

Common Terms and Misconceptions

DOHC terminology can be confusing, especially across different engine configurations and marketing terms.

• “Twin cam” usually means DOHC—two separate camshafts in the head.
• Valve count per cylinder: DOHC commonly pairs with four valves per cylinder, but the cam layout doesn’t dictate valve number; there have been 3-, 4-, and even 5-valve heads.
• V engines: A “DOHC V6” or “DOHC V8” has two cams per bank—so four cams total—yet it’s still called DOHC because it’s double overhead per bank.
• VVT and VVL: Variable valve timing and variable valve lift are not exclusive to DOHC, but DOHC simplifies independent intake/exhaust timing control.
• Efficiency vs. performance: DOHC isn’t just for racing; it’s widely used to meet modern emissions and fuel economy standards.

Keeping these distinctions in mind helps decode spec sheets and marketing claims without confusion.

Maintenance and Reliability Notes

Modern DOHC engines are durable, but some components deserve regular attention to ensure long-term reliability.

• Timing chain/belt: Belts require periodic replacement per manufacturer intervals; chains are “lifetime” parts but still depend on oil quality and tensioner health.
• Oil quality: Clean, correct-spec oil is critical for cam phasers, lifters, and timing components.
• Valve adjustments: Some DOHC engines use mechanical lash adjusters that need periodic checks; many modern designs use hydraulic lifters.
• Seals and gaskets: Cam seals and valve cover gaskets can age and leak; early repair prevents secondary damage.

Following the service schedule and using the recommended oil helps DOHC engines deliver hundreds of thousands of reliable miles.

Real-World Examples

From mainstream models to high-performance icons, DOHC underpins much of today’s automotive landscape.

• Honda DOHC VTEC engines (e.g., Civic and Civic Type R) combining high-RPM capability with variable lift/timing.
• Toyota “Dynamic Force” DOHC engines with Dual VVT-i and high-efficiency combustion strategies.
• BMW TwinPower Turbo DOHC inline-sixes with VANOS (variable timing) and Valvetronic (variable lift).
• Ford EcoBoost DOHC turbo engines balancing torque and efficiency across segments.
• Performance leaders from Porsche, Ferrari, and Mercedes-AMG utilizing DOHC with advanced timing and lift systems.
• Counterexample: GM’s modern small-block V8s remain OHV, illustrating that architecture choice depends on goals and packaging.

These examples reflect how DOHC has become the versatile, go-to platform for meeting modern performance and regulatory demands, while alternatives persist where they fit specific brand and engineering priorities.

Summary

A double overhead cam (DOHC) engine uses two camshafts in the cylinder head—one for intake, one for exhaust—to deliver precise valve control, strong airflow, and compatibility with advanced timing and lift systems. That combination supports efficient, clean, and flexible performance, explaining why DOHC dominates modern passenger cars, even as SOHC and pushrod designs continue in niches where simplicity, packaging, or specific torque characteristics are paramount.

What does a double overhead cam do?

DOHC (Dual Overhead Camshaft) is an engine design with two camshafts per cylinder head, allowing for separate, precise control of intake and exhaust valves, which improves engine breathing, boosts power and high-RPM performance, enables variable valve timing (VVT), and increases volumetric efficiency. This leads to greater horsepower, torque, and fuel efficiency compared to simpler engine designs like SOHC (Single Overhead Camshaft).
 
How DOHC Works

1. 1. Two Camshafts: Instead of one camshaft operating all the valves, a DOHC engine uses two separate camshafts. 
2. 2. Independent Valve Control: One camshaft is dedicated to operating the intake valves, while the other handles the exhaust valves. 
3. 3. Improved Airflow: This dual-cam design typically enables a four-valve-per-cylinder configuration (two intake, two exhaust), allowing for a larger volume of air and fuel to enter the cylinder and spent exhaust gases to exit more efficiently. 
4. 4. Direct Valve Actuation: In most DOHC engines, the camshafts act directly on the valves through cam followers or lifter buckets, eliminating the need for rocker arms and reducing component mass. 
5. 5. Variable Valve Timing: The separate camshafts make it easier to implement VVT technologies, which adjust valve timing based on engine speed and load for optimal performance and efficiency. 

Benefits of DOHC Engines

• Increased Power and Torque: Better airflow and a larger valve area for breathing lead to higher horsepower and torque. 
• Higher Engine RPMs: The improved airflow and more efficient valve control allow the engine to rev at higher speeds. 
• Enhanced Fuel Efficiency: Better volumetric efficiency and the ability to optimize valve timing contribute to improved fuel economy. 
• Better Performance: DOHC engines offer a higher response rate, more responsive acceleration, and stronger overall performance. 
• Precise Valve Timing: The independent control of intake and exhaust valves allows for more precise timing adjustments. 

Which is faster DOHC or SOHC?

A DOHC (Dual Overhead Cam) engine is generally faster and offers higher performance than a SOHC (Single Overhead Cam) engine due to independent control of intake and exhaust valves, leading to better airflow, higher horsepower, and increased RPM capability. However, SOHC engines are simpler, lighter, and less expensive to manufacture and maintain, making them a suitable choice for applications where peak performance isn’t the main priority. 
This video provides a visual comparison of SOHC and DOHC engines: 59sFTS-SimulationYouTube · May 29, 2024
DOHC (Dual Overhead Cam)

• How it works: Two camshafts operate the valves in the engine head, with one dedicated to intake valves and the other to exhaust valves. 
• Advantages:
  • Higher Power Output: Independent control allows for more precise valve timing, better airflow at high speeds, and greater potential for horsepower and torque at higher RPMs. 
  • Higher RPM Capability: The simpler valve train (lacking rocker arms) and optimized airflow contribute to the engine’s ability to reach higher revs. 
  • Better Airflow: More valves per cylinder (often four) and the ability to optimize valve size improve the engine’s breathing. 
• Disadvantages:
  • Complexity and Cost: Requires more components, making it heavier, more complex, and more expensive to manufacture and maintain. 

SOHC (Single Overhead Cam) 

• How it works: A single camshaft in the cylinder head operates both the intake and exhaust valves. 
• Advantages:
  • Simplicity and Cost: Has fewer parts, resulting in a simpler, lighter, and more affordable design. 
  • Lower Maintenance: Its simpler configuration makes it easier and less expensive to work on. 
• Disadvantages:
  • Lower Performance Potential: One camshaft controlling both sets of valves limits precise timing and can restrict airflow and power at higher engine speeds. 
  • Less Efficient at High RPMs: The design is less optimized for high-RPM performance compared to a DOHC engine. 

This video explains the pros and cons of SOHC engines: 50sLive And Lets Ride PHYouTube · Nov 26, 2020
In summary, if you prioritize speed and high-end performance, a DOHC engine is generally the faster choice. If efficiency, cost, and simplicity are more important, an SOHC engine is a practical alternative.

What is the difference between single and double overhead cam?

Single overhead camshaft (SOHC) engines use one camshaft to operate both intake and exhaust valves for simpler design, lower cost, and better fuel efficiency, while dual overhead camshaft (DOHC) engines use two separate camshafts, allowing for more valves per cylinder, better airflow, increased horsepower, higher RPMs, and precise valve control. The choice between SOHC and DOHC depends on priorities, with SOHC favoring cost-effectiveness and efficiency, and DOHC excelling in performance and power.
 
This video explains the differences between SOHC and DOHC engines: 58sThe Engineers PostYouTube · Jul 1, 2023
Single Overhead Camshaft (SOHC)

• Operation: One camshaft in the cylinder head operates both intake and exhaust valves. 
• Advantages:
  • Cost-Effective: Fewer components lead to lower manufacturing costs. 
  • Fuel Efficient: Less weight and friction result in better fuel economy. 
  • Simpler Design: Less complex and easier to maintain. 
  • Good Low-RPM Torque: Often produces better torque at lower engine speeds. 
• Disadvantages:
  • Limited Airflow: Fewer intake and exhaust valves restrict airflow, potentially limiting top-end power. 
  • Less Precise Control: Less control over valve timing compared to DOHC. 

Dual Overhead Camshaft (DOHC) 

• Operation: Two separate camshafts in the cylinder head—one for intake valves and one for exhaust valves—are used. 
• Advantages:
  • Improved Airflow: More valves per cylinder allow for better intake and exhaust gas flow, leading to increased engine power. 
  • Higher Power: Better volumetric efficiency results in higher horsepower and top-end torque. 
  • Higher RPM Limit: The more direct valve operation reduces inertia forces, allowing for higher engine revolutions. 
  • Optimal Spark Plug Placement: The DOHC design allows the spark plug to be placed in the center of the combustion chamber for more efficient combustion. 
  • Advanced Technologies: Facilitates implementation of efficiency-enhancing technologies like Variable Valve Timing (VVT). 
• Disadvantages:
  • Higher Cost: More components and complexity increase manufacturing and production costs. 
  • Increased Weight: More moving parts add weight and increase parasitic power loss. 
  • Potential for Lower Low-End Torque: Without technologies like VVT, DOHC engines might have less torque at low RPMs compared to SOHC engines. 

Which is Better?
There’s no universal “better” choice; it depends on your priorities. 

• Choose SOHC for: Opens in new tabBudget-conscious drivers and everyday driving, where fuel efficiency and lower maintenance costs are more important. 
• Choose DOHC for: Opens in new tabPerformance-oriented driving and higher horsepower, as it provides better power delivery at high RPMs and more precise engine control. 

Are DOHC engines good or bad?

a DOHC layout is better than a SOHC layout in many ways- in DOHCs, the valve timing is more precise, and better valve lift is achieved. also, a DOHC setup eliminates the need for rocker arms, hence creating better timing, and valve contact is more direct too. but of course, these are all in stock form.