What Is a Cam in an Engine?

A cam is an egg-shaped lobe on a camshaft that converts the engine’s rotating motion into linear motion to open and close the intake and exhaust valves at precisely timed moments. By controlling when, how far, and how long valves move, cams govern power, efficiency, emissions, and the character of an engine across its rev range.

How a Cam Works

In a four-stroke internal combustion engine, the camshaft carries multiple cams (lobes), each dedicated to a valve. Driven by the crankshaft via a timing belt, chain, or gears, the camshaft rotates at half the crankshaft’s speed. As each lobe turns, its raised profile pushes a follower, tappet, lifter, or rocker arm to open a valve; the valve spring then closes it when the lobe rotates past its peak, returning the follower to the cam’s base circle. This synchronized choreography admits fresh air-fuel mixture and expels exhaust gases at exact crank angles.

The Valve Event Sequence in a Four-Stroke Cycle

The following list outlines the typical valve events across the four strokes of a conventional gasoline engine, illustrating how cam lobes orchestrate airflow and combustion timing.

1. Intake: As the piston descends, the intake cam lobe opens the intake valve, drawing air (and fuel in port-injected or carbureted engines) into the cylinder.
2. Compression: Both valves close; the piston rises, compressing the mixture for efficient combustion.
3. Power: The spark ignites the mixture near top dead center; both valves remain closed while expanding gases drive the piston down.
4. Exhaust: The exhaust cam lobe opens the exhaust valve; the piston pushes spent gases out as it ascends.

Many engines also use valve overlap—briefly opening intake and exhaust valves together around the transition from exhaust to intake—to improve scavenging and high-rpm breathing, all dictated by cam shape and timing.

Camshaft Layouts and Where Cams Live

Engines place camshafts in different locations and use varied mechanisms to actuate valves, each with trade-offs in size, complexity, and performance.

• OHV (Overhead Valve), cam-in-block: The camshaft sits in the engine block and pushes lifters, pushrods, and rockers to actuate overhead valves. Compact and torquey; common in many V8s.
• SOHC (Single Overhead Cam): One cam per bank (inline engines have one; V engines have two total) operating both intake and exhaust valves via rockers or direct followers.
• DOHC (Dual Overhead Cam): Separate intake and exhaust cams per bank, enabling optimal port geometry and easier variable valve timing; standard in many modern performance and efficiency-focused engines.

DOHC layouts generally allow higher rpm and finer control of valve events, while OHV designs offer packaging advantages and strong low-end torque.

Key Cam Specifications and What They Mean

Cam behavior is defined by a few core parameters that shape an engine’s power curve and drivability.

• Lift: Maximum valve opening height. More lift admits more air, aiding high-rpm power, but stresses valvetrain components and may require stronger springs.
• Duration: How long the valve stays open, usually measured in crankshaft degrees. Higher duration favors top-end power but can reduce low-speed torque and idle quality.
• Lobe Separation Angle (LSA): Angle between intake and exhaust lobe centers. Narrow LSA increases overlap (snappier response, rougher idle); wider LSA smooths idle and boosts low-end torque.
• Advance/Retard: Shifting the cam’s phasing relative to the crank. Advancing intake typically improves low-end torque; retarding can add high-rpm power.
• Ramp Rates and Profiles: How quickly the valve opens/closes. Aggressive ramps improve flow but demand robust lifters and springs; roller followers allow steeper ramps than flat tappets.

The right combination balances responsiveness, emissions, fuel economy, and peak output for a given vehicle and use case.

From Fixed Cams to Variable Control

Traditional cams are fixed, but modern engines dynamically adjust valve timing and sometimes lift to broaden the powerband and cut emissions.

Common Variable Valve Technologies

Below are widely used systems that modify camshaft phasing and/or valve lift to optimize performance across engine speeds and loads.

• Cam Phasing (VVT): Hydraulically rotates the camshaft relative to the sprocket to change timing on the fly (e.g., Toyota VVT-i, BMW VANOS, GM VVT, Ford Ti-VCT).
• Switchable or Continuous Valve Lift (VVL): Alters valve lift and sometimes duration via multi-step or continuously variable mechanisms (e.g., Honda VTEC, BMW Valvetronic, Nissan VVEL, Porsche VarioCam Plus, Audi AVS).
• Electro-Hydraulic/Multiair: Controls intake valve events with hydraulic “digital” actuation using a cam-driven pump and solenoids (e.g., Fiat/Alfa MultiAir), decoupling lift and timing from the cam profile.

By tailoring valve events in real time, these systems deliver better torque at low rpm, stronger power at high rpm, smoother idle, and improved fuel efficiency with lower emissions.

Materials, Followers, and Lubrication

Camshafts are typically cast iron or forged steel, often surface-hardened (e.g., nitriding) for wear resistance. Followers come as flat tappets, bucket tappets, or roller lifters—roller designs reduce friction and permit faster ramps. Consistent lubrication is critical: pressurized oil forms a protective film between cam and follower. Timing drives—belts (quiet, periodic replacement), chains (durable, can stretch), or gears (robust, noisier)—keep crank and cam synchronized at a 2:1 speed ratio in four-stroke engines.

Maintenance, Wear, and Failure Signs

Keeping the cam and its drive healthy prevents costly engine damage and performance loss. Watch for the following symptoms that can indicate cam or timing issues.

• Ticking or clattering valvetrain noise, especially at start-up or warm idle.
• Rough idle, misfires, loss of low-end torque or top-end power.
• Check engine light with cam/crank correlation or timing codes (e.g., P0010–P0017), often tied to VVT solenoids or stretched chains.
• Metal shavings in oil from lobe or lifter wear, particularly in flat-tappet systems lacking proper zinc/phosphorus additives.
• Oil leaks or degraded oil quality reducing VVT performance and lobe protection.

Timely oil changes with the correct specification, replacing timing belts at the manufacturer’s interval, and addressing VVT actuator or solenoid faults quickly can avert major repairs.

Performance and Tuning Considerations

Upgrading cams is a common tuning path, but it demands a systems approach to maintain reliability and road manners.

• Match the cam to intended rpm range: aggressive profiles move the powerband up but can sacrifice idle and low-speed drivability.
• Support parts matter: stronger valve springs, appropriate lifters, pushrods/rockers (OHV), and adequate piston-to-valve clearance are essential.
• Induction and exhaust: freer-flowing intake and headers/exhaust help realize gains from increased duration and lift.
• ECU calibration: fueling, ignition timing, and variable cam targets must be retuned to the new profile.
• Emissions and legality: some aftermarket cams can affect compliance and inspection readiness.

A balanced build that aligns cam specs with the engine’s airflow, compression, and control strategy delivers the best real-world results.

Bottom Line

The cam is the mechanical “conductor” of an engine’s valve events. Its shape, placement, and control—fixed or variable—determine how an engine breathes, sounds, and performs. From classic cam-in-block pushrod motors to advanced DOHC engines with continuous variable timing and lift, cams remain central to extracting power, efficiency, and reliability from combustion engines.

Summary

A cam is a shaped lobe on a camshaft that converts rotational motion into the linear motion needed to open and close engine valves with precision. Its key parameters—lift, duration, lobe separation, and phasing—govern torque, power, and efficiency. Modern engines enhance cam function with variable timing and lift systems, while proper lubrication and maintenance keep cams reliable. Whether in compact OHV designs or sophisticated DOHC setups, cams are fundamental to how an engine breathes and performs.