There is a difference, and the reasons are: 1- Different fuel injection pump timing, 2- Differents injectors nozzles, 3- Differents pistons. These changes have been made to reduce emission by keeping the exhaust gas temperature higher. A friend of mine replaced nozzles and pistons with 1666's parts and re-time the fuel pump according to 1666's spec. And it work ! ( 2-3 gal per hour difference with same capacity )