A camshaft opens and closes the engine’s valves. It is a shaft with egg-shaped lobes, turned by a timing belt or chain at half the speed of the crankshaft. As each lobe rotates, it pushes a valve open, directly or through lifters, pushrods and rocker arms, and the valve spring closes it again. The shape of the lobes sets how far the valves open, how long they stay open and when, which shapes where in the rev range the engine makes its power.
How a camshaft works
- The crankshaft turns the camshaft through a timing belt, chain or gears, at half crank speed.
- Each lobe pushes its valve open at a set point in the engine cycle.
- Valve springs close the valves as the lobe rotates away.
Because the timing must be exact, a stretched chain or broken belt causes serious problems; see timing chain symptoms and timing belt.
Overhead cam vs cam-in-block
| Overhead cam (SOHC, DOHC) | Cam-in-block (pushrod) | |
|---|---|---|
| Location | In the cylinder head | In the engine block |
| Valve operation | Directly or through followers | Through lifters, pushrods and rockers |
| Examples | Subaru EJ and FA, most modern four-cylinders | GM LS and LT V8s, Hemi V8s |
What a camshaft does for performance
A performance camshaft opens the valves further (more lift) and longer (more duration), letting more air in at high rpm. That typically moves power higher in the rev range and can make the idle rougher and low-speed torque softer. Big cam changes often need stronger valve springs and supporting changes. See horsepower vs torque.
Variable valve timing
Many modern engines can shift camshaft timing while running, using oil pressure and solenoids, to balance low-speed torque, high-rpm power and efficiency; see VVT solenoid.
Camshaft vs crankshaft
The crankshaft converts the pistons’ up-and-down motion into rotation and drives the wheels through the transmission. The camshaft is driven by the crankshaft and controls the valves. Because the camshaft turns once for every two crankshaft turns, each valve opens once per four-stroke cycle.
Camshaft problems
- Worn lobes or lifters cause ticking and misfires; see lifter tick.
- Camshaft position sensor faults cause starting and running problems; see camshaft position sensor.
- Low oil pressure or dirty oil wears lobes and bearings.
Cam swaps
Changing a camshaft is a common upgrade on pushrod V8s because the cam sits in the block and can be replaced without removing the heads on many engines. A cam swap usually goes with new lifters, valve springs and pushrods sized for the cam, and it is usually done by a shop experienced with the engine. Check your state’s emissions rules before modifying a street vehicle.
Related
See crankshaft position sensor, LS1 problems and what is a dyno? for measuring the result of a cam change.
More guides: what does a crankshaft do? · what is compression ratio? · bent pushrod symptoms · bad rocker arm symptoms.
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