What an Overhead Cam Does in an Engine

An overhead cam (OHC) is a camshaft positioned in the cylinder head that opens and closes the intake and exhaust valves with precise timing, improving high‑RPM capability, power, efficiency, and emissions control. By placing the cam above the valves, the system reduces valvetrain mass and complexity compared with pushrod designs, enabling finer control of valve events and easier integration of modern variable valve timing and lift systems.

How an Overhead Cam Works

In an OHC engine, the camshaft sits atop the cylinder head and is driven by the crankshaft via a timing belt, chain, or gears. As the cam rotates, lobes press on followers (tappets/buckets) or short rocker arms to open valves; springs close them. The cam’s rotation is synchronized to the crankshaft so each valve opens and closes at the exact moment needed for the four-stroke cycle. Modern OHC setups often use cam phasers (hydraulic or electric) for variable valve timing and, in some engines, mechanisms for variable valve lift to optimize torque, power, and fuel economy across the rev range.

Step-by-Step: What the OHC Does Each Cycle

The following steps outline how an overhead cam coordinates valve events through a four-stroke cycle.

1. Intake stroke: The intake cam lobe lifts the valve(s) to admit the air-fuel mixture (or air in direct-injection engines) as the piston descends.
2. Compression stroke: Both valves are closed while the piston rises, sealing the chamber for compression.
3. Power stroke: Combustion forces the piston down; valves remain closed for maximum pressure.
4. Exhaust stroke: The exhaust cam lobe opens the valve(s) to expel spent gases as the piston rises.
5. Valve overlap: Briefly, intake and exhaust valves may be open together to improve scavenging and efficiency, tuned by cam profiles and phasing.
6. Continuous timing control: Cam phasers adjust lobe timing relative to the crank to favor low-end torque, high-RPM power, or emissions as conditions change.

Together, these actions allow the engine to breathe efficiently and predictably, which is fundamental to performance, drivability, and emissions control.

Types of Overhead Cam Arrangements

Manufacturers configure OHC systems to balance packaging, performance, and cost. The two most common variants are below.

• SOHC (Single Overhead Cam): One camshaft per bank operates intake and exhaust valves. It’s simpler, lighter, and often cheaper; some designs use rocker arms to actuate multiple valves per cylinder.
• DOHC (Dual Overhead Cam): Two camshafts per bank—one for intake, one for exhaust—allow precise control and typically enable four valves per cylinder. DOHC is common in modern cars and motorcycles and pairs well with advanced variable valve timing and lift systems.

While DOHC often supports higher RPM and better breathing, well-executed SOHC designs can still deliver strong performance and efficiency with fewer parts.

Why Manufacturers Use OHC

OHC designs dominate modern gasoline engines because they enhance breathing and control with fewer moving parts in the valvetrain compared to pushrod systems.

• Precision and stability at high RPM for stronger top-end power.
• Lower valvetrain inertia, enabling aggressive cam profiles without valve float.
• Easy adoption of multiple valves per cylinder for improved flow and combustion.
• Compatibility with variable valve timing/lift for broader torque curves and better fuel economy.
• Cleaner emissions through tighter, more adaptable valve timing control.
• Refinement: reduced mechanical losses and improved NVH when well executed.

These advantages explain why OHC is standard in most contemporary passenger cars and motorcycles, including turbocharged and hybrid applications.

Trade-Offs and Limitations

Despite its benefits, OHC isn’t always the simplest or cheapest approach, especially in large-displacement applications.

• More complex cylinder heads with added components (cams, phasers, guides).
• Increased engine height compared with some pushrod (OHV) designs, which can be more compact vertically.
• Maintenance needs for timing belts (periodic replacement) or chains (tensioner/guide wear).
• Potential interference designs: a broken belt/chain can allow piston-to-valve contact.
• Cost: precision machining and control systems add expense versus basic OHV layouts.

These trade-offs are often outweighed by performance and efficiency gains, but they inform design choices in trucks and performance V8s where OHV architectures remain popular.

OHC vs. Pushrod (OHV) at a Glance

Pushrod engines locate the camshaft in the engine block and use lifters, pushrods, and rocker arms to actuate valves, contrasting with OHC’s in-head cam placement.

• Cam location: OHV in-block versus OHC in-head.
• Valvetrain mass: OHV has more reciprocating parts; OHC reduces mass for higher RPM capability.
• Packaging: OHV can be shorter (lower hood lines) but often wider; OHC heads are taller.
• Performance: OHC eases multi-valve, high-RPM tuning; OHV can excel in low-end torque and compact V8 packaging.
• Complexity: OHV may be simpler and cheaper; OHC offers finer control via VVT/VVL.

Both designs can be exceptionally capable; the choice reflects target use, cost, packaging, and brand engineering philosophy.

Maintenance and Reliability Notes

Regular service keeps an OHC valvetrain accurate and quiet over high mileages.

• Timing belt replacement at manufacturer intervals (often 60,000–105,000 miles/100,000–170,000 km).
• Timing chain systems: listen for rattle; worn guides/tensioners need timely service.
• Clean oil of the correct grade: vital for cam wear surfaces and hydraulic phasers.
• Valve clearance checks/adjustments where required (shim-under-bucket or screw-type adjusters).

Following the service schedule and using quality oil and filters significantly reduces OHC-related issues.

Signs of OHC Timing or Valve Issues

Watch for the following symptoms that can indicate cam timing or valvetrain problems.

• Rattling on cold start, especially from the timing cover area.
• Check-engine light with cam/crank correlation or cam timing codes (e.g., P0011, P0016).
• Rough idle, power loss, or poor fuel economy.
• Backfiring or misfires, particularly after a belt/chain failure or skipped tooth.
• In severe cases, no-start and internal damage on interference engines.

Early diagnosis and repair can prevent costly failures and restore performance and efficiency.

Summary

An overhead camshaft sits in the cylinder head and directly times and actuates the engine’s valves. This placement reduces valvetrain mass and enables precise control of valve events—especially with modern variable timing and lift—delivering better breathing, higher RPM potential, improved fuel economy, and cleaner emissions. While OHC adds some complexity and maintenance needs, it underpins the performance and efficiency of most modern automotive and motorcycle engines.

How does an overhead cam work?

Overhead camshaft, commonly abbreviated to OHC, is a valvetrain configuration which places the camshaft of an internal combustion engine of the reciprocating type within the cylinder heads (“above” the pistons and combustion chambers) and drives the valves or lifters in a more direct manner compared with overhead …

Are overhead cam engines better?

The other main advantage of OHC engines is that there is greater flexibility to optimise the size, location and shape of the intake and exhaust ports, since there are no pushrods that need to be avoided. This improves the gas flow through the engine, increasing power output and fuel efficiency.

Is an overhead cam better than a pushrod?

There are a few good reasons. A big one would be that it is easier to implement variable valve timing into a overhead camshaft design than a pushrod one. OHC designs also are generally able to rev higher and more freely as there is less moving mass for the valvetrain.

What are the benefits of an overhead cam?

Overhead Camshaft (OHC) Engines
The OHC layout reduces the number of valve train components, which allows them to be lighter and stronger. This makes the engine more compact and lightweight overall.