Why engines sound different
An engine note is a pulse train played through a pipe. Three things set it: how often pulses come, how evenly they come, and what the pipe does to them.
How often: firing frequency
A four-stroke cylinder fires once every two turns. So firing frequency in Hz is rpm divided by 60, times cylinders, divided by 2.
| Engine | Pulses per turn | Hz at 800 rpm | Hz at 3,000 | Hz at 6,000 |
|---|---|---|---|---|
| Single | 0.5 | 6.7 | 25 | 50 |
| V-twin | 1 | 13.3 | 50 | 100 |
| Inline-3 | 1.5 | 20 | 75 | 150 |
| Inline-4 | 2 | 26.7 | 100 | 200 |
| Inline-5 | 2.5 | 33.3 | 125 | 250 |
| Six | 3 | 40 | 150 | 300 |
| V8 | 4 | 53.3 | 200 | 400 |
| V10 | 5 | 66.7 | 250 | 500 |
| V12 | 6 | 80 | 300 | 600 |
| 2-stroke triple | 3 | 40 | 150 | 300 |
This is why a V12 at 3,000 rpm has the same pitch as a V6 at 6,000, and why more cylinders sound smoother and higher. The pulses blur into a tone.
How evenly
Even pulses make a clean harmonic series. Uneven ones add notes below the firing frequency, and you hear those as lope or burble. A cross-plane V8 does it per bank, a V-twin does it outright, and a flat-4 with unequal headers does it by making pulses arrive late. A five-cylinder is even, but with 2.5 pulses per turn its note sits between the orders you hear from a four or a six, and that gives it the warble.
The pipe
Exhaust gas is hot, so sound moves through it at around 500 metres a second. A pipe open at one end rings at a quarter-wave, so a 2.5 metre pipe has a first resonance near 50 Hz with more above it. When a harmonic of the engine lands on one of those, the note swells. That is exhaust drone, and it is why a car can be quiet at 2,000 rpm and booming at 2,400.
The muffler then removes highs and adds its own resonances. A turbo sits in the way and smooths the pulses, which is why turbo engines sound softer and more whooshy for the same layout.
Why not just pitch-shift a recording?
You can, and most car games do it with a lot of care. A simulation gets different things for free: the limiter really cuts pulses, a misfire really leaves a gap, and swapping the firing order changes the sound. You lose the last bit of realism that comes from a microphone next to a real car.