Diesel fuel burns much slower than gasoline, which is why they idle much lower, and don't rev anywhere near as high as a gas engine. I have made cars sound like diesels with tweak, which run say 300HP at 3000RPM and 650 TQ at 1200RPM. No car in LFS has a powerband like that, so good luck really getting a diesel sound until/unless the devs make a diesel for LFS.