The cylinder head and valves

The cylinder head bolts on top of the engine block and closes off the top of every cylinder. If the bottom end, from Pistons, connecting rods and the crankshaft, turns the push of the burning fuel into a spin, the head is what lets the engine breathe. Mechanics call it the top end. It holds:

• The ports, passages that bring air and fuel in and take exhaust out.
• The valves, which open and shut those ports at exactly the right moment.
• The valve springs and camshafts, which work the valves.
• The combustion chamber, the roof of each cylinder, with the spark plug in the middle.

The head also carries coolant and oil round its passages, and on most modern engines the fuel injectors too. A thin head gasket seals it to the block.

The demo is one cylinder of the same 2.0-litre four-cylinder engine as the other lessons, cut through the middle. Step through the parts with the numbered buttons, then try the presets.

1. Ports. Smooth passages cast into the head. Air and fuel come in on one side and exhaust goes out on the other, which is called a cross-flow head. The shape of a port matters as much as its size: every sharp bend slows the air.
2. Valves. Each one is a poppet valve, shaped like a mushroom, with a head and a long stem. When it’s shut, the head presses on a hard ring in the head called the valve seat, which seals in the pressure of the burning fuel. The stem slides in a tube called the valve guide. A cam lifts the valve about 10 mm (0.39 in) into the cylinder. At 6,500 rpm, each valve opens 54 times a second and is open for about 7 thousandths of a second each time.
3. Valve springs. The cam can only push. The spring pulls the valve shut and keeps it following the cam back down. That gets harder as the engine revs, because the force needed grows with the square of the revs, just like the pull on the rod. At 6,500 rpm it takes about 24 kg (53 lb) just to turn each valve round, so the springs are fitted already squashed and pull with much more than that.
4. Camshafts. A camshaft is a shaft with an egg-shaped lobe for each valve. It turns at half the speed of the crank, driven by a belt or chain from the crankshaft, so each valve opens once every two turns. In this engine, the lobe pushes on a tappet, a cup that sits over the top of the valve and takes the rub of the cam, so the valve itself only moves up and down.

Now set Redline and press Tired springs. Springs that have sagged with age and heat can’t keep the valves on the cams. The valves fly off the noses of the lobes, open too far and land late. This is valve float. The engine stops breathing properly, so it loses power, and an exhaust valve that’s still open as the piston reaches the top can be hit by it.

The intake valve is bigger than the exhaust valve. Only the falling piston pulls air in, but the rising piston shoves the exhaust out under pressure, so the exhaust can get by with a smaller hole. The next lesson, Camshafts and valve timing, looks at exactly when each valve opens and shuts, and why.

Two valves per cylinder, one intake and one exhaust, is the simple way, used on older and cheaper engines. Most modern engines have four: two intake and two exhaust. Four smaller valves open up more space for air than two big ones can, and each is lighter, so the springs can control it at higher revs. It also leaves room for the spark plug right in the middle of the chamber, so the flame spreads evenly. A “16v” badge on a four-cylinder car means 16 valves: four per cylinder.

The camshafts can be in different places:

• Single overhead cam (SOHC). One camshaft in the head works all the valves, often through rocker arms.
• Double overhead cam (DOHC), or twin cam. One camshaft for the intake valves and one for the exhaust, like the demo. Most modern engines are built this way. A V engine has a head on each bank, so a DOHC V8 has four camshafts.
• Pushrod, or overhead valve (OHV). The camshaft sits low down in the block and works the valves through long pushrods and rocker arms. It makes a compact engine, which is why many big American V8s still use it, but all those moving parts make it harder to rev high.

Most heads are cast from aluminium, which is light and carries heat away quickly. Around the chambers and exhaust ports run passages for coolant, called the water jacket, because this is the hottest part of the engine. An exhaust valve can run at about 700°C (1,292°F), glowing red. It cools itself by touching its seat while it’s shut, passing heat into the head and on to the coolant. Some high-performance exhaust valves are hollow and part-filled with sodium, which melts and sloshes up and down the stem to carry heat away faster.

The combustion chamber is shaped into the underside of the head. On a four-valve engine it’s usually a pent roof, a shallow tent shape with a pair of valves on each slope, as in the demo. Its shape and size decide how fast the fuel burns and set much of the engine’s compression ratio.

The valves need a tiny gap, called the valve clearance, so that they shut fully once they’ve grown with the heat. Many engines use hydraulic tappets that fill with oil and take up the gap themselves. Others use a small metal shim on each tappet, which a mechanic measures and changes at long service intervals.

The head gasket sits between head and block. It has to seal the burning gas in each cylinder, and also keep the coolant and oil passages that cross the join apart from each other. Modern gaskets are thin layers of steel. Long head bolts clamp it all together, so tightly that the bolts stretch; on many engines they’re used only once.

• Head gasket failure. The classic sign is white, sweet-smelling smoke from the exhaust and coolant disappearing with no leak to be seen: coolant is getting into a cylinder and burning off as steam. You might also see bubbles in the coolant tank, or the engine overheating. It’s usually caused by overheating in the first place. Fixing it means taking the head off, so it’s expensive. (A creamy sludge under the oil filler cap can be a sign too, but on a car used for short trips it’s more often just condensation.)
• Warped or cracked head. An aluminium head that overheats can bend out of flat, or crack between the valves. A warped head can be machined flat again, which is called skimming. A cracked one usually needs replacing.
• Burnt exhaust valve. A valve that can’t seal can’t cool, and the edge of its head burns away. The engine misfires on one cylinder and loses compression there. It’s often caused by too little valve clearance.
• Worn valve stem seals. Small rubber seals on top of each valve guide stop oil running down the stems. When they harden with age, you see a puff of blue smoke when the engine starts or after coasting downhill.
• Ticking tappets. A light ticking from the top of the engine that speeds up with the revs. Hydraulic tappets may tick for a moment after a cold start, but a tick that stays usually means old or low oil, or a worn tappet.
• Carbon on the intake valves. On direct-injection engines, fuel never washes over the intake valves, so oily soot builds up on them over many miles. The engine idles roughly and loses power. A garage can blast it off with crushed walnut shells.
• Bent valves. In most modern engines, an open valve and a piston at the top would hit each other. These are called interference engines. If the cambelt snaps, or the engine is badly over-revved, the valves and pistons collide and the valves bend. That’s why changing the cambelt on time matters so much.

The head decides how much air the engine can breathe, so it’s where many tuners look for power. But head work only pays off when the rest of the engine is built to use it.

• Porting. Reshaping and smoothing the ports so air flows in more easily. On a standard engine the gains are small. With high revs, boost or wilder cams they can be worth having, but done badly it can lose power. A mirror polish isn’t needed, and in intake ports a slightly rough finish helps keep the fuel mixed in.
• Bigger valves. Wider valves, with new seats cut to fit, let more air through. They usually come as part of a full head rebuild with porting.
• Uprated valve springs. Stiffer springs keep the valves under control at higher revs, so they’re needed when a tune raises the rev limit or fits cams that lift the valves further. Don’t fit stiffer springs than you need: they add friction and wear the cams faster. Lighter titanium retainers, the caps that hold the springs on, help too.
• Head studs and stronger gaskets. With a lot of boost, the pressure in the cylinders can lift the head and blow the gasket. Stronger studs in place of the head bolts, and a tougher gasket, hold it down.
• Skimming. Machining the face of the head makes the chambers a little smaller, so compression goes up slightly. It’s mostly done for repairs. Take off too much and the valves can get too close to the pistons.

A properly ported and rebuilt head costs from hundreds to thousands, and gives little on its own. It works best with matching cams and a remap; Camshafts and valve timing explains why.