got from wikipedia ...
In 1987, the 6.9 liter engine was supplanted by the 7.3 liter upgrade, with over 300,000 6.9s shipped to Ford and countless numbers installed into medium-duty trucks, school buses, and the like. This engine features numerous improvements over the 6.9 liter, with most of the changes located in the heads; the block received an increase bore and select-fit pistons, while the heads received an enlarged prechamber, enlarged valve stem shields, harder valves, and other minor upgrades. The front cover was revised to reduce seepage.
Finally, in 1993 Ford made available a turbocharged edition of the 7.3. The turbocharging system was produced by ATS, which produced three turbocharging systems for the International-Navistar IDI engine; The ATS 085 and 088 turbo kits, both of which are wastegate-less designs, and the ATS 093, which included a wastegate. The Ford factory turbo system utilized the 093 and was detuned from the ATS kit. ATS formerly sold a turbine housing upgrade kit but this appears to have been discontinued. This detuning job is believed to have been due to Ford's impending use of the T444E (Power Stroke) engine. A 7.3 liter IDI in good tune with an ATS 088 turbo kit makes approximately the same power as the T444E, though it makes it at higher RPMs due primarily to turbocharger design. Factory turbo vehicles received numerous additional improvements. Pistons had an enlarged ring, added intermediate rings, an enlarged ring land and wrist pin, as well as an anodized piston face; other improvements included tweaks to the injection pump and injectors, and an upgraded oil-coolant heat exchanger.
These engines are unusual today in that the fuel system is entirely mechanical, though this was the standard for diesels of the day and indeed for the earliest fuel-injected gasoline vehicles. The fuel system also utilizes indirect injection which made it quieter than its predecessor. The indirect injection fuel system makes this popular among drivers who choose to run vegetable oil rather than petroleum fuel (the dual fuel tanks that some Ford trucks are equipped with further makes running vegetable oil easier). Both 6.9 and 7.3 liter engines use the Stanadyne DB-2 injection pump fed by a cam-driven lift pump.
I guess it really does not explain why though ...