So, I think Live for Speed's sound engine is very great, but there are somethings missing. Like exhaust crackling and pops.

I think the effects of exhaust crackling can already be achieved currently. You just need to have a damaged engine. With that, the engine will give off a very very subtle crackling noise (even when on throttle)

I wish to hear this detail on LFS, being off throttle always sounded way too clean

please refer to previous posts, many times this has been suggested and currently the main focus is the next 2 major overhaul updates. With that noted, yes sounds can be improved, however if you take a look at the addons thread, there is a lot of engine sounds that you are able to download and load them on any vehicle through Shift + A in game,(If you wish to use it on default vehicles, they must be labeled as their name tag e.g. "BF1_[choice]")
The vehicles that have turbos installed, can be used to play around with the sound, and the dump valve of the turbo can make it sound like backfire, this is done by loading customized sound waves on each audio wave. other ways can offer a "hybrid sound" or a very rough sound. Air intake can be changed to make the car sound supercharged. There are a lot of ways sound can be customized

If interested, I have many sound files I am happy to share and provide instructions on how to load each sound wave on to the game, so feel free to send a message

One thing that would allow a wider variety of sounds without having to rewrite code would be to allow extended limits for unevenness and pipe tone in shift A panel

I have been using hackzor techniques to bypass those parameters limit for years, sometime with amazing results.

- unevenness is safe to increase and usefull until value of 0.5 (current limit 0.05). It will add more bass as the value increases. Clipping is not worst than current limits.

- pipe tone is safe and usefull until 350 at least (current limit 50). It will add more punch and crisp when increasing unevenness (wich drowns the sound a bit in the bass only)

Safe = sound never crashed with those limits, and clipping is manageable as usual.

I think another way to try and improve engine sound specifically, would be to know how LFS uses the WAV sample and how the parameters affect it. When looking at the waveforms for default cars' engines, they all fade out and the second half of the sample is almost silent. I did manage to create a couple "proper" engine sounds in Audacity by layering sine waves and some white noise then fading out, but they're not very good, and don't seem too different or unique anyway, plus they tend to clip very easily.

If anyone knows more about this, I'm interested (would also love to know how Scawen or Eric generated the official ones).

Quote from Bokujishin :

I think another way to try and improve engine sound specifically, would be to...

The best way to improve engine sounds would be to allow longer, higher quality samples. As far as I know 8KB is the limit right now. Other older games use up to 300KB for samples. I checked WR2 (because it's structure is very transparent and it's of similar age) and it uses three samples per engine for different RPM ranges. But these samples include all additional noises such as gear whine, intake noise and wind.

I actually tried to cut and compress a recorded sample of a diesel engine to 8KB but it just sounded like sh** . LFS seems to require just a single "pulse" with a very specific wave structure. So far I've not been able to find these in the game folder...

Quote from LE_TANCKER :

LFS seems to require just a single "pulse" with a very specific wave structure. So far I've not been able to find these in the game folder...

You can export the default pulse wave file by pressing the S button, if that's what you're looking for.

Quote from Bokujishin :

I think another way to try and improve engine sound specifically, would be to...

I recall seeing somewhere that a cork was used during development, and that was the starting point for the wav file that is looped as a pulse.

I spent a considerable amount of time understanding the default cars, and recreating a pulse in garageband using a cork. Then I used audacity to tweak it after I got the main structure done. Check out my wip thread for the supra for some other details about it:
https://www.lfs.net/forum/thread/108021

Honestly the existing sound engine works pretty good, though I should work with Jake_Blasted to do some more tutorials for them, some of the sliders aren't worth touching since it cheaply changes the sound.

I'd love some rev limiter functionality, with separate wav and slider for hitting the rev limit. Seems pretty simple and would add quite a bit of realism for the amount of effort (though not fully understanding the object sound is made in, maybe there isn't any room). I've done a fake kinda rev limit sound, but is very difficult to get that and a decent sound at the same time. I'd be curious if there are any sound engineers in the lfs community that would know more on what specifically could be added to alter the wav in a better way than it is.