Does compression ratio affect sound?

[Checkmate]

Just pondering, as above. When my peers and myself first started driving, me and a friend both had old Minis. Both started with 998s. His always sounded a bit different to mine, though we did have different exhaust systems. Mine was HC, his the slightly tamer LC spec, shy a few BHP on mine. His sounded a bit more "airy", not as metallic and rowdy, if that makes sense. Came to my mind as I have been recently using my 325i as an everyday car, and compared to the engine in the Alpina, which has a raised CR above standard, the difference in noise seems similarly different, in a way, but then that has a different exhaust system too... Is there a good technical explanation?

[Pasteurised]

[digger the goat]

Nice alternative to 'Whoosh Parrot'


Possibly air filter btw

[Checkmate]

Pasteurised said:

Thanks for the intelligent and informed response. What is that even meant to mean?

[s p a c e m a n]

I like the pigeon

Are we talking different sound from the exhaust, induction or engine? Either way I doubt that compression ratios are the main factor.

[Checkmate]

s p a c e m a n said:

I like the pigeon

Are we talking different sound from the exhaust, induction or engine? Either way I doubt that compression ratios are the main factor.

Different sound from everywhere, really, when driving along in the car. Certainly the engine, and the exhaust too. Induction seems to sound the same, on both of the examples I used. Certainly more of an agressive note in the higher compression variants.

[GroundEffect]

You have different exhausts. They sound different. What a surprise.

[Checkmate]

GroundEffect said:

You have different exhausts. They sound different. What a surprise.

The only difference in the exhausts, on the Mini at least, was that my friend's had a dinky little silencer in the through pipe, which mine lacked, again that exhaust made a different note when I fitted it to a different friend's car.
Perhaps it's cam profiling, duration, lift etc. In the way that a VTEC will dramatically change note when it shifts over.

[Tegis]

Higher pressure will give a louder bang with slightly differnt frequency components when the exhaustvalve opens so compression ratio matters

[Checkmate]

Tegis said:

Higher pressure will give a louder bang with slightly differnt frequency components when the exhaustvalve opens so compression ratio matters

Bigger bang was only about 10% power difference in our earlier engines. Makes sense though I guess. Higher compression engine, more power, different noise.
Those different frequency components being the size of the exhaust ports, manifold, different resonances in the exhaust system due to differing materials? Differing pulses on the exhaust timing?

[GroundEffect]

Checkmate said:

The only difference in the exhausts, on the Mini at least, was that my friend's had a dinky little silencer in the through pipe, which mine lacked, again that exhaust made a different note when I fitted it to a different friend's car.
Perhaps it's cam profiling, duration, lift etc. In the way that a VTEC will dramatically change note when it shifts over.

If you have different cam profiles then yes you should expect a different noise.

[Krikkit]

Checkmate said:

Tegis said:

Higher pressure will give a louder bang with slightly differnt frequency components when the exhaustvalve opens so compression ratio matters

Bigger bang was only about 10% power difference in our earlier engines. Makes sense though I guess. Higher compression engine, more power, different noise.
Those different frequency components being the size of the exhaust ports, manifold, different resonances in the exhaust system due to differing materials? Differing pulses on the exhaust timing?

All of that will change the noise, some of it might be subtle, but that's all it takes!