What a Double Overhead Cam (DOHC) Actually Does

A double overhead camshaft system uses two camshafts per cylinder bank to open and close the intake and exhaust valves directly, allowing more precise valve timing, better airflow, higher engine speeds, and typically more power and efficiency. In modern engines, DOHC architecture supports multi-valve heads and advanced variable valve timing, helping deliver stronger performance with lower emissions and improved fuel economy.

How DOHC Works

At its core, DOHC separates the control of intake and exhaust valves onto two camshafts mounted in the cylinder head above the valves. This arrangement reduces valvetrain complexity between cams and valves, enabling accurate control of when and how far valves open.

1. The crankshaft drives the camshafts via a timing chain or belt.
2. One camshaft actuates intake valves; the other actuates exhaust valves.
3. Cam lobes press directly on tappets/buckets or use short rocker arms to open valves.
4. Valve springs (or, in rare cases, pneumatic systems) close the valves.
5. With variable valve timing, hydraulic cam phasers adjust cam angle to optimize timing across RPM and load.

Together, these steps allow the engine to breathe efficiently across a wide operating range, improving drivability and performance while keeping emissions in check.

Why Two Cams Matter

Using two camshafts per bank gives engineers more freedom to design efficient cylinder heads, typically with four valves per cylinder. That increases air throughput and promotes more complete combustion.

• Improved breathing: Four-valve layouts and straight intake ports increase airflow.
• Higher rev capability: Lighter, more direct valvetrain control reduces valve float at high RPM.
• Precision timing: Independent control of intake and exhaust events sharpens torque and power curves.
• Better efficiency and emissions: Optimized valve timing improves combustion and reduces pumping losses.
• Compatibility with technology: DOHC works well with variable timing and lift systems, turbos, and direct injection.

These advantages explain why DOHC has become the default in most modern cars and motorcycles, from small turbocharged engines to high-performance applications.

Trade-offs and Practical Considerations

DOHC is not without compromises, especially in packaging and cost compared with simpler designs.

• Complexity and cost: More parts and tighter tolerances can increase manufacturing and repair costs.
• Size and weight: Dual cams and wider heads can complicate engine bay packaging.
• Maintenance access: Cam-driven components, timing chains, and valve adjustments can be harder to service.
• Potential NVH: Chains, phasers, and thin oil passages require good maintenance to avoid noise and wear.

Automakers mitigate these issues with compact head designs, durable timing chains, and long-life lubricants, making modern DOHC engines generally reliable when maintained.

DOHC vs. Other Valvetrain Layouts

Understanding alternatives helps clarify what DOHC uniquely offers.

• SOHC (Single Overhead Cam): One cam per bank often actuates both intake and exhaust valves. It is simpler and sometimes lighter but usually offers less valve timing flexibility.
• OHV/Pushrod: The camshaft sits in the block and uses pushrods and rockers. Compact and torquey at low RPM, but typically limited in high-RPM breathing and multi-valve layouts.
• Camless concepts: Emerging systems use electromagnetic or hydraulic actuation for ultimate control, but they are not yet mainstream in mass-market vehicles.

While SOHC and OHV can be effective, particularly for cost, packaging, or low-RPM torque, DOHC delivers the best balance of top-end power, efficiency, and adaptability to modern technologies.

Key Design Details and Modern Integrations

Valves per Cylinder

DOHC heads commonly use four valves per cylinder (two intake, two exhaust), improving airflow and mixture motion. Some historic designs used three or five valves, but four is the present norm for performance, efficiency, and manufacturing practicality.

Variable Valve Timing and Lift

DOHC enables independent phasing of intake and exhaust cams and, in some engines, variable valve lift. Systems branded VVT, VTC, VANOS, VTEC, MultiAir-like approaches, and others help broaden torque curves, sharpen throttle response, and reduce fuel use and emissions.

Turbos and Direct Injection

With turbocharging and direct injection, precise valve events help manage boost, reduce knock, and optimize combustion. DOHC control improves scavenging and thermal efficiency, key to today’s downsized, high-output engines.

Engine Configurations

In inline engines (e.g., inline-4), DOHC means two cams total. In V engines (e.g., V6, V8), DOHC typically means two cams per bank—four cams total—allowing independent control for each bank.

Maintenance and Longevity

Modern DOHC engines are designed for long service intervals. Still, good maintenance is crucial.

• Oil quality and change intervals: Clean oil is critical for cam lobes, hydraulic lifters, and cam phasers.
• Timing components: Chains and tensioners generally last long but can wear; belts require scheduled replacement.
• Valve clearance: Some designs need periodic adjustment; others use self-adjusting hydraulic lash.
• Early warning signs: Rattling on cold start, rough idle, or codes for cam timing can indicate phaser or chain issues.

Keeping to the maintenance schedule and using the specified oil grade helps DOHC systems remain quiet, efficient, and durable over high mileages.

Common Misconceptions

DOHC does not inherently guarantee better fuel economy or horsepower; it provides the mechanical headroom to achieve those goals. Engine mapping, turbocharging, compression ratio, and overall design choices still determine final outcomes.

Bottom Line

A double overhead cam setup provides precise, flexible control of intake and exhaust valves, enabling higher airflow, higher RPM capability, and seamless integration with variable valve timing and modern fueling strategies. That combination underpins the power, efficiency, and cleanliness expected from today’s engines.

Summary

DOHC uses two camshafts per cylinder bank to directly operate intake and exhaust valves, supporting multi-valve heads and advanced timing control. The result is stronger performance, better efficiency, and cleaner emissions, with trade-offs in complexity and packaging that modern engineering and maintenance largely address.

Is a double overhead cam good?

Yes, DOHC (Dual Overhead Camshaft) engines are generally considered good because they offer superior performance, power, and fuel efficiency due to their ability to control intake and exhaust valves independently and use more valves per cylinder. This design optimizes airflow, allowing for higher engine speeds, better combustion, and reduced emissions, making them a preferred choice for modern performance vehicles despite being more complex and costly than SOHC (Single Overhead Camshaft) engines.
 
Advantages of DOHC Engines

• Higher Power and Performance: DOHC engines use two camshafts to individually operate intake and exhaust valves, enabling more precise control over valve timing and higher valve lift. This improves airflow into and out of the combustion chamber, leading to increased power output and higher peak RPMs. 
• Improved Fuel Efficiency: The precise valve timing and more efficient combustion achieved in DOHC engines can lead to better fuel economy. 
• Better “Breathing” and Efficiency: With separate intake and exhaust cams, DOHC engines can optimize the “breathing” of the engine, resulting in more efficient combustion and better overall performance. 
• More Valves Per Cylinder: DOHC designs allow for more valves per cylinder (typically four or more), which further improves airflow and power generation compared to the standard two valves in most SOHC engines. 
• High-Revving Capability: The optimized design allows DOHC engines to reach higher engine speeds, providing a more responsive and high-revving engine. 

Potential Disadvantages

• Higher Cost: The added complexity of two camshafts and additional valve components makes DOHC engines more expensive to manufacture and maintain. 
• Increased Mechanical Complexity: More parts, including two camshafts and potentially more valve springs and other components, increase the mechanical complexity of the engine. 

When a DOHC Engine is a Good Choice

• Performance-Oriented Driving: Opens in new tabIf you prioritize high horsepower, quick acceleration, and a high-revving engine, a DOHC engine is an excellent choice. 
• Fuel Efficiency: Opens in new tabThe improved efficiency of DOHC engines also makes them a good choice for drivers looking for better fuel consumption. 
• Modern Vehicles: Opens in new tabDOHC engines are the dominant design in most modern cars because they deliver the best combination of power, efficiency, and performance. 

What are the disadvantages of DOHC?

DOHC engines also allow the spark plug to be placed right in the middle of the combustion chamber which in turn promotes efficient combustion. The disadvantage of such a setup is more weight, higher costs and also the fact that driving two camshafts requires more components.

How does a double overhead cam work?

A Dual Overhead Cam (DOHC) engine works by using two separate camshafts – one for the intake valves and one for the exhaust valves – located in the cylinder head, which provides more precise and independent control over airflow and timing than simpler overhead cam (SOHC) or overhead valve (OHV) designs. This setup typically allows for multiple valves (often four) per cylinder, optimizing the intake of fuel-air mixture and the expulsion of exhaust gases, resulting in increased horsepower, higher peak RPMs, and greater overall engine performance.
 
This video explains the basics of a dual overhead camshaft engine: 59sAutomobile basic ideasYouTube · Nov 1, 2024
How DOHC Works in Practice

1. Two Camshafts: Instead of one camshaft, a DOHC engine has two camshafts mounted in the cylinder head, one dedicated to the intake valves and the other to the exhaust valves. 
2. Direct Control: The camshafts are directly connected to the valves, often using bucket tappets, which allows for a more compact design with fewer moving parts than older pushrod systems. 
3. Synchronization: A timing belt or chain connects the crankshaft to both camshafts, ensuring they rotate in precise sync with the engine’s piston movements. 
4. Optimized Airflow: With two separate camshafts, the design can accommodate multiple valves per cylinder (typically two intake and two exhaust), creating a larger total valve area. This improved airflow allows the engine to breathe more efficiently, especially at higher engine speeds. 
5. Engine Performance: The precise timing and direct control of the valves by the two camshafts allow for a wider valve angle between the intake and exhaust valves. This, combined with the increased number of valves, leads to: 
   • Higher Power Output: More air and fuel can enter the cylinder, and exhaust gases can exit more quickly, resulting in greater power. 
   • Higher RPMs: The increased efficiency and precise valve control enable the engine to operate safely and effectively at higher rotational speeds (RPMs). 
   • Enhanced Efficiency: Optimized valve timing improves the combustion process, contributing to better fuel efficiency. 

What are the benefits of the double overhead camshaft?

DOHC technology can improve engine performance by producing more power. With two separate camshafts to control the intake and exhaust valves, the engine can work more effectively. This engine can produce more power than a SOHC engine at higher engine speeds.