Camshafts and valve timing

A camshaft is a shaft with an egg-shaped lobe for each valve. As it turns, each lobe pushes its valve open and lets the spring pull it shut again, as you saw in The cylinder head and valves. The camshaft turns at half the speed of the crankshaft, so each valve opens once every four strokes.

Valve timing is when each valve opens and shuts, compared with where the piston is. It’s measured in degrees of crankshaft turn, counted from the two points where the piston stops and turns round:

• Top dead centre (TDC), with the piston at the very top.
• Bottom dead centre (BDC), with the piston at the very bottom.

You might expect each valve to open exactly as its stroke starts and shut exactly as it ends. In a real engine they don’t. Both valves open early and shut late, and for a moment at the top both are open at once. The shape of the lobes and where they sit on the shaft decide all of this, and with it, whether an engine is happiest pottering about town or screaming at the redline.

The demo is the same 2.0-litre engine as the other lessons. The graph shows how far each valve is open through one full cycle, starting at the spark: the intake valve’s lift in Mustard and the exhaust valve’s in Coral. The dark band is where they’re both open. Step through the four phases with the numbered buttons:

1. Overlap. At the end of the exhaust stroke, the intake valve opens before the exhaust valve has shut. For a moment, both are open as the piston passes the top. The exhaust gas rushing out of the cylinder leaves low pressure behind it, which helps pull the fresh charge in. This is called scavenging.
2. Intake. The intake valve stays open well after the piston has passed the bottom. Air has weight, so once it’s rushing down the port it keeps coming, even as the piston starts back up. Shutting the valve late lets more air pack in. The standard cam shuts it 46° after BDC.
3. Both shut. The piston squeezes the charge, the spark lights it, and the burning gas pushes the piston down. Both valves have to stay sealed for all of it.
4. Exhaust. The exhaust valve opens about 46° before the bottom, while the gas is still pushing. By then most of the push has been used, and letting the pressure out early means the rising piston doesn’t have to shove against it.

The Cylinder fill readout shows how full the cylinder gets with the accelerator pressed all the way down, as a share of its size. It’s a simplified model, but it shows what really happens. Now try the cams:

• Set Low revs and press Race cam. The race cam holds each valve open for longer, so the overlap grows from 12° to more than 100°. At low revs the gas moves too slowly to make use of that. Watch the intake: the rising piston pushes charge back out before the valve shuts, and exhaust leaks back into the intake during the overlap. The fill drops to 73%. In a real car, the engine idles with a lumpy, uneven beat and feels weak until the revs come up.
• Press Redline. Now the air is moving fast enough to keep coming all the way to the late shut, and the fill climbs past 100%. The air rushing in packs the cylinder fuller than it would be at rest. The standard cam, shutting its intake too early for these revs, fills only 80%.

Torque follows how full the cylinder gets, so a cam moves where in the rev range the engine makes its best torque, as in Horsepower vs torque. No single cam is best at every speed. A road car’s cam is a compromise that idles smoothly, pulls well low down and passes the emissions test.

Every camshaft comes with a few numbers that describe it, often on a printed cam card.

• Lift is how far the lobe opens the valve. The standard cam in the demo lifts 10 mm (0.39 in). More lift lets more air past, but needs stronger springs and more room between the valves and the pistons.
• Duration is how long the valve stays open, in degrees of crank turn. The valve creeps open and shut on gentle ramps at each end of the lobe, so makers only start counting once it has opened a set amount. Many European makers use 1 mm (0.04 in), and American cam makers use 1.27 mm (0.05 in); some quote “advertised” duration from much less lift, which gives a bigger number. Only compare figures measured the same way. The demo’s cams are measured at 1 mm (0.04 in): 232° for the standard cam, 271° for the fast road cam and 312° for the race cam.
• Lobe separation angle is how far apart the intake and exhaust lobes are on the shaft. Closer lobes give more overlap, which suits high revs but makes the idle rougher.
• Intake centreline is where the intake lobe is at its highest, in degrees after TDC. It sets where the whole cam sits against the crank: moving it earlier is called advancing the cam, and later retarding it.

Overlap comes from the others. A longer duration or a tighter lobe separation both give more of it.

The camshafts are driven from the crankshaft by a cambelt, a chain, or on a few engines a set of gears. The pulley or sprocket on each camshaft has twice as many teeth as the one on the crank, which is what makes the cams turn at half speed.

When the engine is built, marks on the pulleys are lined up with marks on the engine, so that the cams and crank start in step. They have to stay that way. On a typical belt, one tooth is worth more than 15° of crank turn, so even a single tooth out puts every valve event in the wrong place.

The engine’s computer also needs to know where the cams are. A sensor on the crank tells it where each piston is, but every piston passes the top twice per cycle, once at the end of compression and once at the end of exhaust. A cam position sensor tells the two apart, so the ECU knows which cylinder to fire and when to inject its fuel.

Timing belts vs timing chains compares the two ways of driving the cams. Variable valve timing lets the engine move its cams while it runs, so it can have a smooth idle and a strong top end.

• Timing jumped a tooth. A stretched timing chain, a worn tensioner or an old cambelt can let the cams slip out of step with the crank. The engine runs roughly, loses power and may be hard to start. The ECU notices that the cam and crank sensors disagree and turns on the engine warning light, often with a “cam/crank correlation” fault code.
• Snapped cambelt. The camshafts stop while the crank keeps turning. On an interference engine, the pistons hit the open valves and bend them, which means a cylinder head rebuild. Change the belt on time, as Timing belts vs timing chains explains.
• Worn lobes. A cam lobe that wears down lifts its valve less, so that cylinder can’t breathe and the engine misfires or loses power. It’s more common on older engines with flat tappets, especially when the oil is old or low.
• Timing set wrong. After a belt or chain change, a pulley fitted one tooth out gives the same symptoms as a jumped tooth. On some engines it’s enough to make the valves touch the pistons, so turn the engine over twice by hand before starting it.
• Failed cam position sensor. The engine may crank for a long time before starting, cut out, or go into a limp mode with less power until the sensor is replaced.

A performance camshaft is one of the classic ways to get more power from an engine without a turbo. It doesn’t add power everywhere: it moves the engine’s best breathing higher up the rev range, giving more power at the top in return for less low down.

• Fast road cams add a little lift and duration. The engine pulls harder from the mid-range to the redline, idles almost as smoothly as standard, and is still easy to drive in traffic. This is the sensible choice for a road car.
• Race cams have a lot of lift, long duration and big overlap. They make the most power at high revs, but the car idles with a lumpy beat, is weak and jerky at low revs, uses more fuel and is harder to keep clean enough for the emissions test. Low vacuum at idle can also weaken the brake servo on some cars.

A new cam needs other work to go with it:

• Valve springs stiff enough for the extra lift and the higher revs, so the valves don’t float.
• A remap, so the ECU fuels and controls the idle for the new cam. Without one, the engine can run poorly or set fault codes.
• Checking the clearance between the valves and the pistons. More lift and more overlap bring them closer at the top.

Cam timing can be fine-tuned too. Adjustable cam pulleys, sometimes called vernier pulleys, let you turn each cam a few degrees against its pulley. Advancing the intake cam moves the torque lower down the rev range, and retarding it moves it higher. Setting this properly, called degreeing the cam, takes a dial gauge and a big protractor on the crank, and is best checked on a rolling road.

Most modern engines already have variable valve timing, so they get much of what a performance cam offers without its low-rev manners. Cams usually come as part of a bigger build, with the head work, intake and exhaust to match.