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What do SOHC and DOHC mean?
A camshaft’s lobes control when the engine’s valves open and close. Intake valves let the air-fuel charge into the cylinder, while exhaust valves let combustion gases out. In an overhead-cam engine, the camshaft or camshafts are located in the cylinder head, above the valves.
- SOHC: One overhead camshaft actuates both the intake and exhaust valves.
- DOHC: Two overhead camshafts are used. In the common arrangement, one actuates the intake valves and the other actuates the exhaust valves.
Those are the basic layouts. The precise way a camshaft operates the valves can vary by engine, so the acronyms alone do not describe every detail of an engine’s design.
Does DOHC mean more valves per cylinder?
No. Camshaft count and valve count are separate specifications. A DOHC engine does not automatically have four valves per cylinder, and an SOHC engine is not limited to two. Honda’s history of its C20A and C25A engines, for example, describes SOHC cylinder heads with four intake and exhaust valves per cylinder. Honda’s account of the C30A and related engine development is a useful illustration of why the two terms should not be treated as interchangeable with valve-count descriptions.
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When checking an engine, look up its valve count separately rather than trying to infer it from SOHC or DOHC.
What practical difference can two camshafts make?
With separate intake and exhaust camshafts, engineers have distinct mechanisms for designing how each valve group operates. That flexibility can support a particular engine’s goals, but it does not guarantee a particular result. The engine’s overall design determines how that flexibility translates into power, torque, efficiency, or driving character.
Honda’s development of the NSX’s C30A V6 is one specific example: Honda describes choosing DOHC for separate actuation. The C30A was a 3.0-liter engine (2,977 cc), but that figure belongs to that engine alone; it says nothing general about the advantages of either camshaft layout. Honda’s engine history documents the decision as part of that model’s development, not as proof that DOHC is universally superior.
SOHC vs. DOHC at a glance
| Comparison | SOHC | DOHC |
|---|---|---|
| Camshafts in the cylinder head | One | Two |
| Typical valve actuation | One camshaft actuates intake and exhaust valves | One camshaft commonly actuates intake valves and the other exhaust valves |
| Valve count per cylinder | Not determined by the acronym | Not determined by the acronym |
| Guaranteed power, economy, or reliability advantage | None established by the layout alone | None established by the layout alone |
Which is better: SOHC or DOHC?
Neither layout is inherently better for every driver. To compare two actual engines, use specifications and service information for the exact engine and vehicle rather than treating camshaft count as a verdict.
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- Power and torque: Compare published figures or curves, including where in the rev range the engine produces them.
- Efficiency: Use vehicle-specific manufacturer specifications and comparable test conditions. The layout alone does not establish which engine uses less fuel.
- Intended use: Consider how the whole engine delivers its performance and whether that suits the vehicle and your driving.
- Ownership and service: Check the exact engine’s service schedule, procedures, and parts requirements. Camshaft count alone does not establish maintenance cost or reliability.
A DOHC design offers separate intake and exhaust camshaft control; whether that is an advantage for you depends on the complete engine and how it is used. An SOHC engine may also be designed with multiple valves per cylinder. The right comparison is therefore between the engines you are considering, not just their acronyms.
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