OHC, SOHC and DOHC: Which is which?

Our tech guru has the answer
by Ferman Lao | Aug 1, 2011

Hi, Ferman!

Good day. I'd like to ask about the difference between the OHC, SOHC and DOHC engines.

I'm planning to buy a car this year and your explanation will definitely be helpful in my car-buying decision.

Thanks and more power to you and all the staff of Top Gear Philippines!


Hi, Boyet!

OHC stands for overhead camshaft. Here, the valves are opened and closed directly by the cam lobes on the camshaft. This is in contrast with the OHV, or overhead valve, configuration where the valves are actuated by pushrods. The OHC configuration allows for lower total mass of the components that drive the valves. OHC configurations also allow for better optimization of cylinder head, intake and exhaust port design compared to OHV engines.

Meanwhile, SOHC stands for single overhead camshaft (per cylinder head), while DOHC means double overhead camshaft (per cylinder head). An SOHC engine will usually be less complex than a DOHC engine albeit, usually, at the cost of slightly less power.

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Most modern SOHC and DOHC engines are usually multi-valve engines. DOHC has either three, four or five valves per cylinder while the SOHC has both intake and exhaust valves.

DOHC engines usually optimize one cam for the intake valves and another for the exhaust valves. Sometimes, variable valve cam timing is employed in DOHC engines. This means the opening of the intake valve in relation to the exhaust valve can be varied for those with a single-variable cam mechanism. What happens in these instances is that the cam gear of the cam shaft is attached to a sliding pulley. The sliding pulley allows cam timing and, on certain engines, valve overlap to be varied to allow for better power and efficiency at all engine operating speeds. There are vehicles that have this on both intake and exhaust cams, which allow an even greater degree of adjustability.

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I hope that helps clear things up for you and good luck on your car purchase.

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Ferman Lao
Technical editor

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