I find the technical aspects of engines, especially car engines, fascinating, but don’t know much about them. I know the basic principles of an internal combustion engine and that’s about it.
But when I listen to different cars, there seem to be typical sounds for different engines: V8 cars, like the Boss 429 Mustang or Alfa Romeo Montreal, tend to have this deep thundering sound with the cool “stutter” while idling. Cars with V6 engines, like the AMG C43 or Audi S4 (to me) have a more aggressive humming sound, while cars with V10 (e.g. the Huracan) seem to have a more high-pitched screeching sound. Is the cylinder architecture the reason for that or is it something else that affects the sound? Or is my perception just off and the engine type has no correlation to sound?
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Of course the cylinder architecture affects the sound. Most of the sound comes from the ignition and the exhaust stroke of the engine. Each cylinder have an exhaust stroke and an ignition every two cycles. So in an 8 cylinder engine you have 4 exhaust strokes per rotation and 4 ignitions while in a 10 cylinder engine you have 5.
But it is not as simple as that because the pistons are not spaced equally to each other as they need to be balanced. So you tend to have pairs of cylinders opposite sides of the engine move together. The “worst” configuration comes from a V8, or an I4. In these engines the outside two cylinders move together with one exhausting as the other is igniting, and then the two centre cylinders will be exhausting and igniting. So for each engine cycle you get 4 distinct sounds in a V8. Compare this with a V6 where the three cylinders in each bank can balance themselves and therefore give a nice humming sound. Or the V10 where the cylinders are more equally spaced again so you end up with 10 distinct exhaust and ignition events per cycle. This is what creates the nice high pitch sound as the events all blend together. It also helps that more cylinders mean smaller cylinders to get the same volume which means you can run the engine at higher speeds.
So the biggest thing that you might not think of is the exhaust system. Physically each piston is a different distance from the tail pipe. So if the exhaust pipes were straight lines you would get a lot of noise (turbulent uneven bursts).
If you’ve ever seen a photo of exhaust systems and see the pipes in all kinds of weird bends it’s because they’re trying to make them equal in length to make it quieter.
You can tweak those lengths to get the sound you want.
This is a complicated subject but I’ll ELI5 it to keep it short. There’s two main things that affect a car’s sound. The first is the engine arrangement and firing order, that is how many cylinders it has, what arrangement they’re in, and the order in which they fire, and the second is the exhaust system, and to a lesser extent the intake. As far as the engine arrangement and firing order goes there’s a myriad of practical reasons to choose one over the other before you get to sound but that’s not to say it’s not important. A great example is parallel twin engines in motorcycles. Long considered to be dull and boring, they’re experiencing a resurgence in popularity, thanks to no small part to the 270degree firing order, which in layman’s terms means that those parallel twins sound like V-Twins, which are considered characterful and torquy. As far as the exhausts go of course there’s a ton of considerations there too since no production vehicle can have a free flowing straight pipe exhaust but even so they have engineers working specifically on the acoustics of the engine so that they get a specific desirable sound profile as much as they can, considering of course there’s legal limitations to noise and also catalytic converters drown out a lot of the sound.
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