Exhaust manifolds

The exhaust manifold is the first part of the exhaust. It bolts to the side of the cylinder head, collects the burnt gas from every exhaust port and joins it into one pipe. How an engine gets rid of exhaust gases followed the gas all the way out. Here we look at why manifolds are shaped the way they are.

It has a hard life. The gas leaving the ports can be over 850°C (1,562°F), and on a dyno, flat out, a manifold can glow dull red. Then it cools right down every time the car is parked. Most come in one of four kinds:

• Cast iron, a single heavy casting with short passages. It’s cheap, compact, quiet and lasts. Most ordinary cars have one, or a thin stainless steel version.
• Tubular, often called headers in the US. A separate steel tube from each port, joined further down at a collector. They’re lighter and help the engine breathe at high revs, so performance cars and tuners use them.
• Built into the head, where the passages are cast inside the cylinder head itself. Many new engines are made this way.
• Turbo manifolds, thick castings strong enough to carry the turbo, which bolts straight onto them.

A gasket seals the joint to the head, and a metal heat shield usually covers the manifold to protect the parts round it.

The demo looks at the exhaust side of the 2.0-litre petrol engine from the earlier lessons, flat out, with its four ports in a row. The slider sets engine speed and the graph shows the torque at each speed.

1. The gas leaves in pulses. A four-cylinder fires in the order 1-3-4-2, one cylinder every half turn. When each exhaust valve opens, the gas, still under pressure from the burn, rushes out as a pulse. Watch the pipes light up in turn.
2. Pulses can get in each other’s way. As one cylinder’s pulse leaves, the cylinder before it in the firing order is in overlap: it’s finishing its exhaust stroke with both valves open. In the Cast manifold the pipes meet straight away, so each pulse reaches that cylinder’s port and pushes burnt gas back in. Watch the dots run back up the pipe. That leaves about 10% of each cylinder full of burnt gas, with less room for fresh air.
3. Long pipes keep them apart. Press Tubular. Each cylinder now has its own long pipe, and the pulses only meet at the collector. There, each pulse rushing through leaves low pressure behind it, which helps pull the gas out of the next cylinder. This is scavenging. At 5,500 rpm, only 5% burnt gas is left, and torque rises from 208 Nm (153 lb-ft) to 219 Nm (162 lb-ft).

Slide down to 2,000 rpm and the tubular manifold loses a little: the pulses arrive at the wrong moments, much like the waves in a runner in Intake manifolds. Like intake runners, the length of the pipes sets the speed they suit.

Then press Built into head. The power is the same as the cast manifold, but coolant round the passages takes heat from the gas. At 5,500 rpm the catalytic converter sees about 760°C (1,400°F) instead of 860°C (1,580°F).

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

Tubular manifolds for four-cylinder engines come in two main layouts:

• 4-1: all four pipes meet at one collector. It suits high revs, so it’s popular on race and track cars.
• 4-2-1: the pipes join in pairs first, then the two pairs join. It pairs cylinders 1 and 4, and 2 and 3, because each pair fires a full turn apart, so their pulses are evenly spaced and never meet a cylinder in overlap. It gives a broader spread of torque, so it suits road cars.

Equal lengths matter too. If one cylinder’s pipe is longer than another’s, their pulses reach the collector at uneven moments, and the cylinders breathe differently. You can hear it: the famous rumble of Subaru’s older flat-four engines comes from manifolds with unequal pipes. The burble of an American V8 comes partly from its manifolds too. Each bank’s manifold collects pulses that arrive unevenly, because of the way the cylinders fire, so racing V8s sometimes have a tangle of pipes crossing from one side to the other to even them out.

On a turbo engine, the manifold feeds the turbine, so its job is to get the pulses there with as much energy as possible. Turbo manifolds are short, with small passages, and the turbo bolts straight on.

The same problem of pulses getting in each other’s way comes up here. A twin-scroll turbo has two inlets, and the manifold pairs the cylinders the same way as a 4-2-1, 1 with 4 and 2 with 3. The pulses reach the turbine cleanly, so it spins up sooner, as Twin-scroll turbochargers shows. The turbo and supercharger lesson explains how the turbo itself works.

Many modern engines, such as Volkswagen’s 2.0 TSI and Ford’s 1.0 EcoBoost, have the exhaust manifold cast inside the cylinder head, with coolant passages round it. It has several advantages:

• Faster warm-up. The coolant picks up heat from the exhaust, so the engine and the cabin heater warm up sooner after a cold start.
• Cleaner exhaust. The catalytic converter, or the turbo, can bolt right onto the head, so it’s close to the cylinders and gets hot enough to work sooner.
• Less fuel at full power. Turbo engines used to add extra fuel when working hard, just to keep the turbo and the catalytic converter cool enough. Cooling the gas in the head means they need much less.
• Smaller and cheaper. There’s one fewer part to make, seal and fit.

The cost is a more complicated head and a bigger cooling system to carry the extra heat.

• A cracked manifold. Heating up and cooling down again thousands of times can crack a manifold, especially a cast one. Gas puffs out with every pulse, making a ticking noise that’s loudest from cold and fades as the manifold heats up and the crack closes.
• A blown gasket. The gasket between the manifold and the head can fail and make the same ticking. You might see black soot marks round the joint.
• Snapped or rusted studs. The studs that hold the manifold on rust and seize in the heat. They often snap when someone tries to undo them, and drilling them out of the head is a slow job, so penetrating oil and patience help.
• A rattling heat shield. The thin shield over the manifold rusts round its bolts and buzzes or rattles at certain revs. It’s one of the most common rattles on older cars.
• Wrong fuel readings. A leak before the oxygen sensor can let air in between pulses. The sensor sees the extra oxygen and thinks the engine is running short of fuel, so the ECU adds more and the engine actually runs rich.
• Fumes. A manifold leak puts exhaust gas, with its carbon monoxide, into the engine bay, where it can find its way into the car through the heater. If you smell exhaust inside, get it checked.

• Tubular manifolds usually add a few per cent at the top of the rev range on an engine without a turbo, as the demo shows, and can lose a little low down. They need a remap to get the most from them, and they often make the engine louder and sharper sounding.
• Watch for the catalytic converter. On many modern cars the first catalytic converter is built into the manifold or bolted right after it. Some aftermarket manifolds leave it out, which makes the car illegal on the road.
• Turbo manifolds for bigger turbos are often fabricated from steel tube. They flow well, but welded joints can crack in the heat, so good ones are made from thick-walled stainless steel and supported properly.
• Heat wrap and coatings keep heat in the exhaust and out of the engine bay. Wrap can trap water against the pipe and make it rust, so a ceramic coating is usually a better choice.