The real issue with the the older Mercedes diesels are the are already maxxed out when it come to making power. There is no wiggle room, on most diesels you can crank up the fuel, but the fuel is already cranked up on these dumb things. You can make all the mods in the world but if you can't increase the fuel, it's all in vain. So not only are they slow, there is nothing you can do about it, not too mention they are not very efficient when compared to other diesels. On the bright side they are very reliable. The 4D56 is you best option if you are considering an engine swap.