Don.
As you know there is a significant difference in the GW and MAN engine, you may say, MAN was the Ferrari and GW the Ford. With regard to the timing of the exhaustcams MAN was more susceptible to back pressure variations as they were equipped with the BUCHI system, an early version of todays turbo supercharger systems. The Buchi system was, as to days turbocharger system, in continious operation over the whole power range. At that time the it was difficult to get the alloys for exhaustgas turbine blades capable of withstanding the high temperatures. The temperatures increases with the back pressure so I guess they had to install the special schnorchelling cams for continious schnorchelling in order to increase the overlapping to get the exhausttemperatures down. This in contradiction to the GW engine having a mechanical supercharger (Roots blower) which was only clutched in at high loads and revs. Thus on the GW engines the whole reversingmechanism including the reversing cylinder and astern cams were removed which simplyfies the engines and give a very welcome weight reduction particulary for the VIIC/41 type.
Tore