The engine sounds are rather ok, but what we really need is proper exhaust sound simulation. Because, in the end, most of the sound you hear comes trough the exhaust. In LFS, the exhaust is more like a straight pipe than a tuba and it sounds very very thin. I actually doubt there is any exhaust actually. :P It's possible to make sounds with grunt at the moment too, but the problem is the awful clipping you will have.
And for crying out loud, the BF1! Can't drive that thing because the sound makes me nuts!