Is it just me, but it seems that the only thing about the engine sound that changes with the RPM is the pitch?

As far as I know, with real cars, engine sound volume is also related to the RPM. So at 1-2k rpm you have a certain pitch and loudness, but as the RPMs increase, you get a different pitch and higher volume.

Why doesn't LFS do that?

It does. It's even is load dependent.

Hmm, you're right, there is some effect there. I guess it's just not nearly as pronounced as in real life.

For example, I tried the FZ50 and the engine is very quiet at 1-2k, but it gets most loud at 3-5k. There is pretty much no increase in loudness when going all the way to 6-7.5k.

From what I remember with real cars, they are many times more louder at 7k rpm than at 4k, for example.

If you press Shift+A while out on the track, you will open the engine sound editor. In the editor you can experiment with the off power volume, idle boost and exhaust volume to make the engine more quiet at lower RPMs. I haven't tried this myself, but it's my best guess.

Quote from shurcooL :

From what I remember with real cars, they are many times more louder at 7k rpm than at 4k, for example.

there is no general rule here, depends on numerous factors like exhaust length, geometry, cams, resonance frequency of the different parts etc.
My Lupos engine is loudest at roughly 4k rpm, after that it gets quiet again.

The EJ207 in my Impreza "burns fuel and convert it into noise" at its best when I rev near the power peak.

Quote from ACCAkut :

there is no general rule here, depends on numerous factors like exhaust length, geometry, cams, resonance frequency of the different parts etc.
My Lupos engine is loudest at roughly 4k rpm, after that it gets quiet again.

don't buy a honda then... its generally loud after 3k and screams like a 6 year old girl until you shift up...

my audi... it bellows like a bass tennor until the exhaust rattles around 3150-3200 rpm... i should really tighten it up....