Racing cars are loud

Where the noise comes from, and what it tells you about the engine.

Racing cars are loud because each cylinder's exhaust valve opens abruptly onto a large volume of hot, high-pressure gas that has to escape suddenly into a comparatively narrow pipe, and that sudden, forceful release of pressure is exactly what generates loud sound in general, a mechanism racing exhausts share with a firework's bang or a burst balloon rather than with anything gentler.

Ford v Ferrari came out this month, and beneath the racing it is a film about brake changes and things that fail at three in the morning, but the sound design alone makes an argument the story never has to state directly, that an engine's note is a direct broadcast of exactly what is happening inside every cylinder, hundreds of times a second.

A pressure cooker's valve

The sharp hiss from a pressure cooker's valve shows the mechanism at a small, safe scale. Steam inside the sealed pot sits at a pressure well above the kitchen air, and the moment the valve opens, it rushes out through a narrow opening with real force and real noise. Nothing strikes anything and nothing rattles. The sound comes entirely from the sudden difference in pressure between the pot and the room. Opened only a crack, the valve hisses steadily. Released all at once, it gives a much sharper, louder burst.

Pulses of gas leaving the cylinder

Sound is a pressure wave travelling through air, and the loudest, sharpest sounds come from the most sudden pressure changes, which is what an exhaust valve produces each time it opens. A cylinder that has just burned its charge of fuel and air holds gas at a pressure far above the atmosphere's, and when the valve lifts, that gas is suddenly connected to the low-pressure exhaust pipe and leaves in a sharp pulse instead of a steady flow. It is the pressure cooker's full release, repeated at a far higher pressure and far more often than any kitchen pot manages.

How often is easy to work out. In an ordinary four-stroke engine each cylinder fires once every two turns of the crankshaft, so an eight-cylinder engine at 10,000 revolutions a minute sends about 670 pulses a second down its exhausts. At that rate the separate bangs blur into one continuous tone, roughly the pitch of a high note sung by a soprano, and it rises and falls with the engine speed. A four-cylinder road car idling at 800 revolutions a minute produces fewer than thirty pulses a second, slow enough to hear as a low burble with a distinct beat. The pitch and texture of the roar are then shaped by how many cylinders there are, how evenly they fire and how the pipework carries the pulses away.

Silencers, and what open pipes give up

A road car's pulses are smoothed by a silencer, a set of chambers and perforated tubes that spreads each sharp pulse out into a gentler one before it reaches the air. A stripped-down racing exhaust, built for the least weight and the least restriction to flow, leaves the pulses almost unmuffled. That accounts for most of the difference in aggression between a racing engine and a road engine of similar size and power, which lies far more in how the sound is allowed to leave the car than in how violently the fuel burns.

The racing exhaust is engineered as carefully as any other part of the car. The length, diameter and shape of the pipes carrying those pulses affect how completely each cylinder empties between firings, because a pressure pulse travelling down a tuned pipe reflects back as a low-pressure wave that helps draw the next cylinder's spent gas out. That in turn affects how much fresh charge the cylinder can take in and how much power the engine makes, so a racing engine's note is a fairly direct acoustic signature of a system tuned on purpose to move exhaust gas efficiently.

Loudness is a symptom of the pipework

A louder note does not in itself mean a more powerful or more efficient engine. The link between noise and performance runs through how well the pipework clears spent gas from each cylinder, and the loudness does no useful work of its own. Two engines producing identical power can sound very different depending on how their exhausts are muffled or left open, and chasing a louder note for its own sake, without the tuning that made the original engine fast, adds noise and nothing else.

Hearing an engine's health

Because the note is built from individual pulses, one per cylinder per firing, an experienced mechanic can learn how an engine is behaving by listening to it. A new roughness, an uneven beat between cylinders that should be firing identically, or a flat spot where the sound briefly drops usually reflects a real change in how evenly or completely each cylinder is burning its charge. A cylinder that misfires simply leaves a gap in the sequence of pulses, and at idle, with fewer than thirty a second, the ear can pick out that missing beat long before any gauge on the dashboard shows a change.