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