What Is a Double Overhead Cam (DOHC)?

A double overhead cam (DOHC) engine uses two camshafts per cylinder bank—one to operate the intake valves and one for the exhaust valves—mounted above the combustion chamber in the cylinder head. This layout allows more precise valve timing, supports more valves per cylinder (commonly four), improves airflow and high‑RPM performance, and underpins many modern features like variable valve timing and lift. Today, DOHC architecture is standard in most contemporary gasoline engines and many diesels because it balances efficiency, power, and emissions control.

How DOHC Works

In a DOHC cylinder head, the camshafts sit directly above the valves. Each camshaft has lobes that push on followers (such as lifters, tappets, or finger rockers) to open the valves; springs close them. One camshaft controls intake valves, the other controls exhaust valves. A timing belt or chain links the camshafts to the crankshaft, ensuring their rotation stays synchronized with piston movement. In V-shaped or flat engines, each cylinder bank typically has its own pair of camshafts. The layout facilitates multivalve designs—usually two intake and two exhaust valves per cylinder—which increase the engine’s ability to “breathe” and burn fuel efficiently.

Key Advantages of DOHC

These benefits help explain why DOHC has become the predominant architecture in modern engines across segments, from economy cars to performance vehicles.

• Improved breathing and power: Separate intake and exhaust cams and multivalve heads enable higher airflow and stronger high‑RPM performance.
• Precision control: Independent cams make it easier to implement advanced valve timing and lift strategies, including dual independent variable valve timing (VVT) and cylinder deactivation.
• Efficiency and emissions: Better combustion control helps boost fuel economy and reduce emissions across a wider operating range.
• High‑rev capability: Reduced valvetrain inertia and direct actuation (often via finger followers) support stable valve control at higher engine speeds.
• Design flexibility: Facilitates four valves per cylinder, pent‑roof combustion chambers, central spark plug placement, and high tumble/swirl for cleaner burn.

Together, these attributes make DOHC a strong platform for modern engine technologies, from turbocharging to hybrid integration.

Trade‑offs and Limitations

Despite its advantages, DOHC introduces complexities that can affect packaging, cost, and maintenance.

• Complexity and cost: More components—two camshafts, more bearings, and often more valves—raise manufacturing and repair costs.
• Packaging and weight: The cylinder head is larger and taller than some alternatives, which can challenge engine bay packaging.
• Frictional losses: Additional moving parts can increase friction, though modern coatings and low‑tension designs mitigate this.
• Service considerations: Timing chains/belts, cam phasers (for VVT), and valve‑train components can require specialized service and precise oil quality.
• Interference risk: Many high‑compression DOHC engines are interference designs—if the timing belt/chain fails, valves can contact pistons.

Manufacturers counter these downsides with improved materials, chain tensioner designs, and extended service intervals, but owners should follow maintenance schedules closely.

DOHC vs. SOHC vs. OHV (Pushrod)

Understanding how DOHC compares with other layouts clarifies why different engines adopt different solutions.

• DOHC: Two cams per bank in the head; excels at high‑RPM breathing, multivalve setups, and precise VVT on intake and exhaust.
• SOHC (Single Overhead Cam): One cam per bank; can operate multiple valves via rocker arms; simpler and often more compact than DOHC but with less independent control.
• OHV/Pushrod: Camshaft in the engine block using lifters, pushrods, and rockers; very compact and torquey at low RPM, common in some American V8s, but typically less conducive to very high‑RPM operation and multivalve complexity.

No layout is universally “best”; the choice reflects design goals like size constraints, torque curve, cost, and manufacturing legacy.

Common Applications and Examples

DOHC appears across a wide range of modern vehicles and powertrains because it adapts well to both efficiency‑focused and performance‑oriented designs.

• Performance gasoline engines: Porsche 911 flat‑six, BMW B58 inline‑six, Honda K20C1 (Civic Type R), Mercedes‑AMG M139 four‑cylinder.
• Mainstream turbo fours and sixes: Ford EcoBoost 2.3L/3.5L, Toyota Dynamic Force series (e.g., M20A), Hyundai/Kia Smartstream engines, GM 2.7L Turbo.
• High‑efficiency platforms: Mazda Skyactiv‑G/X families, Toyota/Lexus hybrid gasoline engines using DOHC with dual VVT.
• Diesels and motorcycles: Many modern diesels (e.g., BMW B47) and sport motorcycles rely on DOHC for precise valve timing at high engine speeds.

Whether chasing peak power, drivability, or fuel economy, manufacturers leverage DOHC for its flexibility and control.

Maintenance and Ownership Notes

Proper care ensures DOHC systems deliver their intended performance and longevity.

• Timing components: Adhere to belt/chain service intervals; inspect tensioners and guides for wear or noise.
• Oil quality: Use manufacturer‑specified oil grades and change intervals to keep cam phasers, lifters, and VVT passages clean and responsive.
• Valve adjustments: Some DOHC engines require periodic valve lash checks/adjustments; others use hydraulic lifters that self‑adjust.
• Noise diagnostics: Rattles at start‑up may indicate chain tensioner or phaser issues; address promptly to prevent timing drift.
• Software updates: ECU/TCU calibrations can affect VVT behavior; dealer updates sometimes improve drivability or emissions.

Sticking to the maintenance schedule and monitoring for early symptoms helps avoid costly repairs and preserves performance.

Misconceptions to Avoid

Several common myths can obscure what DOHC does—and doesn’t—guarantee.

• “More cams always mean more power”: Architecture enables potential; tuning, displacement, and forced induction matter just as much.
• “DOHC equals turbocharged”: Many DOHC engines are naturally aspirated; turbocharging is separate from valvetrain design.
• “Only DOHC can use VVT or multivalve”: SOHC systems can also have VVT and multiple valves, though with fewer degrees of freedom.
• “DOHC engines are always interference‑free”: Many are interference designs; follow timing maintenance to avoid damage.
• “Diesels don’t use DOHC”: Numerous modern diesels do, for emissions control and efficiency.

Recognizing these nuances helps set realistic expectations about performance, reliability, and upkeep.

Summary

DOHC—double overhead camshaft—places two camshafts per cylinder bank in the head to control intake and exhaust valves independently. The layout improves breathing, supports multivalve designs and advanced variable timing, and underlies much of today’s efficiency and performance gains. While it introduces added complexity and service considerations, DOHC remains the dominant modern valvetrain architecture because it offers precise control without sacrificing adaptability across vehicle types and use cases.

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.

What is a double overhead cam?

A double overhead cam (DOHC) engine uses two camshafts in the cylinder head to operate the engine’s valves, with one camshaft for the intake valves and the other for the exhaust valves. This setup improves engine performance, allowing for more power and efficiency by enabling better airflow, the use of multiple valves per cylinder, and more precise valve timing.
 
How it works:

1. Two Camshafts: Each cylinder has its own intake and exhaust valves. In a DOHC engine, one camshaft controls the intake valves, and a separate camshaft controls the exhaust valves. 
2. Overhead Location: Both camshafts are positioned above the engine’s cylinders, in the cylinder head, which is why they are called “overhead cams”. 
3. Valve Actuation: The camshafts have egg-shaped “lobes” that push down on the valves at the correct time in the engine’s cycle. 
4. Improved Airflow: Because there is a dedicated cam for intake and one for exhaust, the engine can more efficiently control the intake and exhaust of the air-fuel mixture. 

Key Advantages:

• Higher Horsepower and Torque: Opens in new tabThe ability to precisely control valve timing and increase airflow allows for greater power output. 
• Increased Efficiency: Opens in new tabMore efficient combustion from improved airflow and valve control can lead to better fuel economy. 
• Better Performance at High RPMs: Opens in new tabThe reduced inertia and direct action of the DOHC system allow the engine to operate smoothly and efficiently at higher engine speeds. 
• More Valves Per Cylinder: Opens in new tabDOHC engines often use four valves per cylinder (two intake, two exhaust), which further enhances airflow and overall engine power. 

Which one is better, SOHC or DOHC?

An engine with a DOHC design can produce more horsepower than a SOHC design. Lower Band Torque: SOHC engines produce more torque at the lower ends because they are lighter than DOHC engines. SOHC bikes offer the best mileage as a result. Weight: SOHC engines typically have two or three valves per cylinder.

Is a DOHC engine better for performance?

DOHC engines generally offer better performance and higher power output compared to SOHC engines. The separate camshafts allow for more precise valve timing and higher revving capabilities, resulting in increased power and torque delivery.