What is the difference between OHV, OHC, SOHC and DOHC engines?

Updated: August 08, 2019
DOHC engine diagram
DOHC engine, click to see a larger image.

DOHC, SOHC or OHV? It's been a heated argument for many years: which engine design is better? Muscle car enthusiasts will swear by the old-school pushrod design, while younger racecar freaks will say that nothing beats the twin-cam. Who is right? What is the difference between these engines? Let's consider the pushrod engine first.

OHV or "Pushrod" engine

The pushrod design is also known as OHV or OverHead Valve. In an OHV engine valves are placed above the cylinders in the cylinder head. In a typical OHV engine, the camshaft is installed inside the cylinder block (Cam-in-block). The valves are operated through lifters, pushrods and rocker arms. See the illustrations of OHV engine components. An OHV design has been used successfully for many years.

OHV engine animated diagram
OHV or Pushrod engine animation

The downside of this layout is that the number of valve train components create greater inertia, making it difficult to control the valve timing at higher RPMs. This means, that an OHV design is better suited for bigger engines, where the large engine volume offers enough torque at low RPMs.

Advantages of an OHV engine include lower cost, low-end torque and compact size. For example, the 2018 Chevrolet Corvette Z06 is 4.4 inches shorter than 2018 Honda Civic sedan. Yet, thanks to its compact 6.2L OHV V8, the Corvette Z06 can go from 0 to 60 mph in 2.9 seconds. The Corvette's supercharged aluminum 650-hp OHV LT4 engine pumps out crazy 650 lb-ft of torque at 3,600 rpm. OHV engines are also famous for their durability and longevity. It's not uncommon to see older V8 trucks with an OHV V8 engines with 300K miles or more still running strong.

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Maintenance costs are low too. A typical OHV engine has a small timing chain that is not very difficult to replace.
Today, due to constant pressure on car makers to improve the fuel efficiency, you will mostly find an OHV engine in some trucks and V8-powered sports cars. Examples of an OHV engines:
1. Chrysler Hemi 5.7L OHV V8 engine
2. GM 6.2L LSA V8.

OHC or SOHC engine

SOHC engine animated diagram
SOHC engine animation

OHC simply means OverHead Cam, while SOHC means Single OverHead Cam or Single Cam.
In a SOHC engine the camshaft is installed in the cylinder head, and valves are operated either by the rocker arms or directly through the lifters (as in this animation). See this photo of a Mitsubishi SOHC engine.

The advantage of the OHC design is that valves are operated almost directly by the camshaft, which makes it easier to maintain precise timing at higher rpms. It's also possible to install three or four valves per cylinder. Honda successfully uses the SOHC design in its late V6 engines where four valves per cylinder are operated by a single camshaft. Fiat uses a single camshaft and four valves per cylinder in its MultiAir engine, see this illustration of the Fiat MultiAir SOHC engine components. In this engine, two exhaust valve are operated by the camshaft through the tappets, while the two intake valves are operated by oil pressure.

The downside of an OHC engine is that it requires a timing belt or chain with a tensioner, guides, or idlers and other related components. A timing belt must be replaced in regular intervals. A timing chain lasts much longer, but it too might need to be replaced if stretched. Another downside is that it's difficult to implement Variable Valve Timing separately for exhaust and intake valves; something that can be easily done in a DOHC engine.

DOHC or Twin Cam engine

DOHC engine animation
DOHC engine animation

DOHC means Double OverHead Cam. A DOHC engine design is often called Twin Cam or Dual Cam. The majority of modern cars have a DOHC engine. A typical DOHC engine has two camshafts and four valves per cylinder, like the one in this animation.

One camshaft operates intake valves, while another camshaft controls exhaust valves on the opposite side. See the illustration of DOHC engine components.

OHV OHV or Pushrod Engine components OHC SOHC engine components DOHC DOHC engine components

Examples of DOHC engines:
1. Ford 3.5L EcoBoost V6 DOHC
2. Ford Mustang Boss 302 5.0L DOHC V8
3. Ford Mustang 5.2L V8 Supercharged
4. BMW S65 DOHC V8
5. Infiniti 3.0L VR30
6. Mercedes-Benz Inline-6 DOHC engine.

In a DOHC engine, camshafts can be installed farther apart from each other. This allows the intake valves to be at a larger angle from the exhaust valves, which results in a more direct air flow through the engine. In other words, a DOHC engine can "breathe" better, which means it can produce more horsepower out of a smaller engine volume. Compare: The 5.0-Liter V8 DOHC Coyote engine with 4 valves per cylinder of the 2018 Ford Mustang GT is rated at 460 hp @ 7,000 rpm. The 6.2-Liter OHV (pushrod) V8 GM L86 engine has two valves per cylinder and produces 420 horsepower at 5,600 rpm.

Technologies like Variable Valve Timing and Variable Valve Lift can be easily implemented in a DOHC engine on both camshafts further improving efficiency. Downsides of a DOHC engine include a larger size and more complex design with a timing belt or chain and related components. A timing belt needs to be replaced at recommended intervals, adding to maintenance costs. Replacing a timing chain is only necessary if it's stretched, or there is another related problem, but it could be expensive.

Conclusion: Currently, the DOHC engine design is the most fuel-efficient, but an old-school OHV engine will last longer in similar conditions and is cheaper to maintain.