The core difference between overhead cam (OHC) and overhead valve (OHV) engines lies in their valve actuation mechanism: OHV engines use pushrods and rocker arms to transfer camshaft motion to the valves, while OHC engines have the camshaft directly actuating the valves.
Both OHC and OHV engine designs are common in internal combustion engines, and in both types, the valves are strategically located above the combustion chamber to efficiently control the intake of fuel-air mixture and the exhaust of spent gases. However, the path the camshaft's motion takes to open and close these valves is fundamentally different, impacting engine complexity, performance, and packaging.
Understanding Overhead Valve (OHV) Engines
An Overhead Valve (OHV) engine, also known as a pushrod engine, places the camshaft within the engine block, typically to the side of or below the crankshaft. This design necessitates a series of components to transmit the camshaft's rotational motion up to the cylinder head where the valves are located.
How OHV Works:
- Camshaft: Located in the engine block.
- Lifters/Tappets: Ride on the camshaft lobes, converting rotational motion into linear (up-and-down) motion.
- Pushrods: Long, slender rods that extend from the lifters up to the cylinder head.
- Rocker Arms: Pivoting levers that receive motion from the pushrods and press down on the valve stems, opening the valves.
- Valves: Located in the cylinder head, above the combustion chamber.
This chain of components allows for a more compact cylinder head design and a shorter engine block height.
Advantages of OHV Engines:
- Compact Cylinder Head: Fewer components in the cylinder head lead to a smaller, lighter head.
- Shorter Engine Height: The camshaft's location in the block can result in a lower overall engine profile.
- Simpler Manufacturing: Historically, OHV engines were simpler and less expensive to manufacture.
- Good Low-End Torque: Often associated with robust low-end torque delivery due to valve timing characteristics.
Disadvantages of OHV Engines:
- Higher Reciprocating Mass: The weight of the pushrods and rocker arms means more inertia, which can limit maximum engine RPM.
- Valve Float: At high RPMs, the inertia of the pushrod valvetrain can cause the valves to "float" (not close properly) due to a lag in follower motion.
- Less Precise Valve Timing: The indirect actuation can introduce slight flex or play in the system, potentially affecting precise valve timing, especially at higher speeds.
- Limited Valve Count: Typically restricted to two valves per cylinder.
Understanding Overhead Cam (OHC) Engines
An Overhead Cam (OHC) engine positions the camshaft (or camshafts) directly within the cylinder head, above the valves. This design significantly streamlines the valve actuation process, eliminating the need for pushrods.
How OHC Works:
- Camshaft: Located in the cylinder head, directly above the valves.
- Cam Drive: A timing chain or belt connects the crankshaft to the camshaft, synchronizing their rotation.
- Valve Actuation: The camshaft lobes directly actuate the valves, either by pressing on the valve stem directly (often with a bucket tappet) or via a short rocker arm.
- Valves: Located in the cylinder head, directly actuated by the camshaft.
OHC designs are further categorized into Single Overhead Cam (SOHC) and Double Overhead Cam (DOHC).
- SOHC: Features one camshaft per cylinder bank. In an inline engine, this means one camshaft operates both intake and exhaust valves for all cylinders.
- DOHC: Features two camshafts per cylinder bank. Typically, one camshaft operates the intake valves, and the other operates the exhaust valves. This allows for more independent control over valve timing.
Advantages of OHC Engines:
- Higher RPM Capability: Fewer moving parts and less reciprocating mass in the valvetrain allow for higher engine speeds without valve float.
- More Precise Valve Timing: Direct actuation leads to greater accuracy and responsiveness in valve control.
- Multi-Valve Designs: Easier to implement three, four, or even five valves per cylinder, improving airflow and engine breathing for better performance and efficiency.
- Variable Valve Timing (VVT): More readily compatible with advanced VVT systems, allowing for optimized valve timing across various engine speeds and loads.
- Improved Fuel Economy and Emissions: Better control over airflow contributes to more efficient combustion.
Disadvantages of OHC Engines:
- Increased Complexity: More components in the cylinder head and the need for a timing chain/belt can make the engine more complex and potentially more expensive to manufacture and maintain.
- Larger Cylinder Head: The camshafts and their associated components make the cylinder head physically larger and heavier.
- Taller Engine Design: Generally results in a taller engine profile compared to OHV designs.
- Maintenance: Timing belt/chain replacement can be a significant maintenance item.
Comparison Table: OHC vs. OHV
| Feature | Overhead Valve (OHV) | Overhead Cam (OHC) |
|---|---|---|
| Camshaft Location | In the engine block | In the cylinder head |
| Valve Actuation | Indirect, via pushrods and rocker arms | Direct, or via short rocker arms/tappets |
| Components | Camshaft, lifters, pushrods, rocker arms, valves | Camshaft(s), timing chain/belt, valves, tappets |
| Complexity | Simpler valvetrain, more robust | More complex valvetrain, precise control |
| Engine RPM | Generally lower maximum RPM (due to inertia) | Higher maximum RPM (less inertia) |
| Valve Count | Typically 2 valves per cylinder | Easily supports 3, 4, or more valves per cylinder |
| Engine Height | Often shorter overall engine height | Generally taller engine height |
| Cylinder Head | More compact and lighter | Larger and heavier |
| Performance | Good low-end torque, durable | High-performance, better high-RPM power, efficiency |
| Modern Use | V8 truck engines, some American performance cars | Most modern passenger vehicles (cars, motorcycles) |
Practical Implications and Examples
The choice between OHC and OHV designs significantly influences an engine's characteristics and its suitability for different applications.
- OHV engines are often favored in applications where durability, compact engine height, and strong low-end torque are priorities. Examples include many American V8 engines found in pickup trucks and muscle cars, such as those from General Motors' LS family or Ford's Modular engines, valued for their robustness and ability to move heavy loads. Their simplicity also makes them popular in some industrial and agricultural machinery. You can learn more about engine designs for different vehicles.
- OHC engines dominate the modern automotive landscape, particularly in passenger cars, sports cars, and motorcycles. Their ability to achieve higher RPMs, support multi-valve configurations, and integrate advanced features like variable valve timing makes them ideal for optimizing power, fuel efficiency, and emissions. Almost all four-cylinder and many V6/V8 engines in contemporary sedans, SUVs, and sports cars utilize OHC technology, with DOHC being especially prevalent for maximizing performance. For instance, most Japanese and European car manufacturers exclusively use OHC designs.
In essence, while both engine types successfully actuate valves above the combustion chamber, the OHC system, by placing the camshaft directly in the cylinder head, offers a more direct and precise method of valve control, paving the way for advanced performance and efficiency. The OHV system, with its block-mounted camshaft and pushrod-actuated valves, often provides a simpler, more compact (in the head) and robust solution suitable for specific applications.
