Where Are The Two Camshafts Located On The Cylinder Head?

DOHC stands for double overhead camshafts

You have probably seen the expressions in written form: SOHC, which stands for single overhead camshaft, and DOHC, for double overhead camshafts. There is a good reason to have two separate camshafts, one for the intake valves and one for the exhaust.

If you get involved in engine tuning trying to get more power—why you would want it is always a question—having two separate camshafts allows you to change the timing of the intake or the exhaust without disturbing the timing of the other camshaft.

In this example, we can see the tappets where all 16 valves would go. These saddles are the bearings in which each camshaft revolves. They are plain bearings, not ball or needle bearings. Plain bearings are very durable provided they get plentiful, filtered, cool oil.

How are the camshafts driven? By an inverted-tooth silent chain. If I had both camshafts in the engine, you would see the chain passes over their sprockets, goes down a slot, and loops over a smaller gear at the end of the crankshaft.

Plastic dampers press against the runs of the chain to prevent them from bow-stringing. Remember, this engine is powered by a series of combustion thumps, so the crankshaft does not rotate smoothly like a mill wheel in a beer commercial.

Instead, it accelerates when the combustion in one cylinder begins and then it slows as that combustion dies away. One hundred eighty degrees later it feels another combustion event and accelerates again.

That yanking on the cam chain would set the chain to vibrating and many stress cycles of such vibration would soon wreck the chain. So soft plastic “shoes” press against the runs of the chain and damp out any vibration.

Making the camshaft drives stable is always a question in any new engine design. By the time that engine reaches the marketplace, such problems have been solved.