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What does a camshaft do?

What does a camshaft do?

About the author

Chris Pollitt is an award-winning automotive word-wrangler, editor of the website Not 2 Grand and a keen collector of crappy old cars.

Your engine is a wonderfully choreographed dance of metal and oil. If we were like it to, say, an orchestra, the crank would be the brass section, the valves would be the strings and the camshaft would be the conductor, stood at the front making sure everything is working harmony.

It’s this seemingly innocuous rod of steel that acts as the conduit through which an engine can either be a delight, or an absolute mess. 

When you look at a camshaft, you’ll see the egg-shaped protrusions along it. These are called lobes, and as the crank rotates, in turn giving motion to the camshaft, so too do the lobes. And it’s their shape that is key to everything.

Those lobes act upon the valves, allowing them to open and close. If the valves can do that, fuel and air can get in, and exhaust gasses can get out. The valves are the piston’s gatekeepers, the camshaft is the valve’s boss. 

In most modern engines, the cam sits at the top of the engine. This is why you’ll often see OCH in car terminology, which means Over-head Camshaft. In the case of OHC engines, you’re dealing with just the one cam, because for many engines one cam is enough.

If you take an eight-valve engine, for example, that means two valves per cylinder – one for inlet, another for exhaust. One cam can handle eight valves with ease.

In fact, one cam can operate up to twelve valves without too much stress. Though unless you drive an old Proton, you probably won’t have 12 valves to worry about. 

When you see the moniker of DOHC, it means Double Over-head Camshaft. So, same as OCH, but multiplied by two. Two camshafts mean the possibility of more valves, more valves make for a more efficient and powerful engine, which is why 16V is often seen as a positive, performance quality.

This of course means four valves per-cylinder, usually three inlet and two exhaust. However, companies like Volkswagen have pushed the idea even further by creating engines with 20 valves. 

In overhead applications, the cams normally act directly upon the valves. In other, usually older engines, the camshaft is seated deep within the engine. However, the valves are still up in the cylinder head.

So what’s the solution? Pushrods, that’s what.

These long rods of medal are acted upon by the cam in the base of the engine, and then in turn act on the valves, usually with the aid of a rocker which translates the upward motion of the pushrod into a downward motion via a central fulcrum. 

The other thing to remember is that you’ll get one cam per bank of cylinders on the case of OHC engines.

So, a V6 can still be single cam despite actually having two cams. That’s because it’s one cam per bank of three cylinders. For it to be a true DOHC, you would have to have two cams per bank. 

The bottom line is that your camshaft is a deeply important, and deeply clever part of your engine. It’s the conductor of combustion proceedings through the entire four-stroke cycle. Not bad for a humble-looking rod of steel.