Intake manifolds

Between the throttle body and the cylinder head sits the intake manifold. Its job sounds simple: take the air coming through the throttle and share it out between the cylinders. How an engine gets air introduced it. Here we look at how it’s built, and at the trick that lets its shape add power.

It has two main parts:

• The plenum, a chamber straight after the throttle. It holds a store of air for every cylinder to draw from, so each one gets a fair share.
• The runners, one pipe from the plenum to each cylinder’s intake port in the head.

Older manifolds were cast in aluminium. Most new ones are moulded from plastic, a nylon strengthened with glass fibre. It’s lighter and cheaper, its insides are smoother, and it doesn’t soak up as much heat from the engine, so the air stays cooler and denser.

The manifold carries other parts too. On engines with port injection the injectors and fuel rail sit on it. It usually has a pressure sensor, called a MAP sensor, that tells the ECU how much vacuum there is, a pipe to the brake servo and connections for the crankcase breather.

The demo shows the manifold from the side on the 2.0-litre petrol engine from the earlier lessons, flat out. The slider sets engine speed, and the graph shows the torque the engine makes at each speed.

Air doesn’t flow into the cylinder in a steady stream. It moves in gulps, and that sets up waves of pressure in the runner:

1. The valve opens and the falling piston sucks on the runner. That sends a wave of low pressure up the runner towards the plenum, shown as a dark wavefront. The wave travels through the air; the air itself keeps flowing towards the valve.
2. The wave bounces back. Where the runner opens into the plenum, it reflects as a wave of high pressure, shown as a white wavefront, and runs back down to the valve. It travels at the speed of sound, about 350 metres a second, so it makes several trips while the valve is open, fading as it goes.
3. Timing is everything. If a high-pressure peak reaches the valve just as it’s shutting, it pushes extra air into the cylinder, about 8% more in the demo. Engineers call this ram tuning or wave tuning.

The runner’s length decides the engine speed where the timing works. Press Long runners: at 700 mm (28 in) they suit about 3,000 rpm, where the engine makes 185 Nm (136 lb-ft). Rev it to 6,000 rpm and the valve shuts before the wave gets back, so torque drops to 190 Nm (140 lb-ft). Short runners, half the length, suit 6,000 rpm and make 205 Nm (151 lb-ft) there, but only 171 Nm (126 lb-ft) at 3,000 rpm. Halve the length and the speed it suits doubles.

So designers have to choose. A family car usually gets long runners for pull low down, and a sports car shorter ones for power at the top.

Variable gets both. A flap in the manifold stays shut at low revs, so the air takes the long way. At about 4,250 rpm, the ECU opens it, the wave bounces back from the plenum after only 350 mm (14 in), and the runners act half as long. Watch the torque line jump from one curve to the other.

The model is simplified, but the effect is real, and the numbers match the earlier lessons.

Variable intakes appeared on performance cars in the 1980s and 1990s and are now common on engines without a turbo. They come in a few forms:

• Two-stage manifolds, like the demo, use a flap to switch between long and short paths. BMW’s DISA, on its six-cylinder engines of the late 1990s and 2000s, works this way.
• Three-stage manifolds use two flaps, for three lengths. Porsche’s VarioRam on the 911 of the mid-1990s changed both the runner length and the plenum.
• Continuously variable manifolds slide or turn the runners to change their length smoothly. They’re rare, because they’re expensive and complicated.

The flaps are worked by a small electric motor or by vacuum from the manifold, and the ECU decides when to switch.

Many engines also have swirl flaps or tumble flaps in the ports. They aren’t about length: at low revs and light loads they partly close off each port, so the air spins as it enters the cylinder. The spinning mixes the fuel and air better and helps them burn cleanly. At higher loads, the flaps open out of the way.

• Turbo engines pump air in under pressure, so wave tuning matters much less. Their manifolds are usually compact, with short runners and a small plenum, so the boost reaches the cylinders quickly and the engine responds fast. Some, like many of Volkswagen’s small TSI engines, have the intercooler built into the manifold, cooled by its own water circuit.
• Diesels don’t have a throttle controlling their power, so their manifolds sit at about outside pressure, or boost pressure with a turbo. Many have the EGR valve feeding exhaust gas into the manifold to cut nitrogen oxides, and the soot in it can build up inside.
• Direct injection engines spray fuel straight into the cylinder, so their manifolds carry only air, and the injectors sit in the head instead. Fuel injection: port vs direct injection explains the difference.

• Air leaks. A cracked plastic manifold, a perished gasket or a split hose lets air in after the airflow sensor, so the ECU doesn’t know about it and adds too little fuel. The engine idles roughly or hunts, may hiss, and lights the engine management light with a code such as P0171, system too lean. Spraying carburettor cleaner round the joints with the engine idling is an old way to find one: the idle changes when it reaches the leak.
• Broken flaps. Variable intake flaps and swirl flaps live in a hot, oily place, and their spindles and actuators wear. A stuck flap costs power at some speeds and usually sets a fault code; on VW and Audi engines P2015 is common. Worse, some flaps can break off and be sucked into the engine. Several BMW diesels of the 2000s were known for swirl flaps doing this, which can wreck the engine, so many owners had them removed or replaced.
• Soot and oil build-up. On diesels, oily vapour from the crankcase breather mixes with soot from the EGR valve into a thick tar that slowly narrows the manifold and the ports. Bad cases need the manifold taking off and cleaning.
• Leaking gaskets on V engines. On some V engines, coolant passes through the intake manifold between the banks. When the gaskets fail, coolant leaks out or into the engine, so watch for a falling coolant level.

• Aftermarket manifolds usually have shorter runners and a bigger plenum, to move the torque higher up the rev range. They can add power at the top, but often lose some low down, as the demo shows. Unless they come with a remap, they rarely help much.
• Port matching smooths and lines up the manifold’s runners with the ports in the head, so there’s no step for the air to trip over. The gains are small on its own, but it’s often done when an engine is being rebuilt.
• Individual throttle bodies replace the whole manifold with a short runner and throttle for each cylinder. Throttle bodies and electronic throttles covers them.
• On turbo engines, a bigger, smoother manifold helps when the engine is making much more boost than standard, and is often part of a bigger upgrade with a larger intercooler and a remap.
• Removing swirl flaps is popular on some diesels to stop them breaking. It needs blanking plugs and usually a remap so the ECU doesn’t set fault codes, and it can affect the emissions.