I think it would be fitting for the D and H series to have the option for a diesel powerplant. It would suit the more commercial and heavy duty packages of those cars. It would bring a cool aspect of a more torquey option along with tuning them for massive boost. The ability to roll coal with a modified exhaust could be fun. would be cool. It would also be nice to get a sound overhaul for the T-Series's engine so it doesn't just sound like a low revving I6.
