[dan]

I'm not entirely sure about this, but I think the 4.3 was taken directly from the precat 4.3 Griff. As the Griff was supposed to get the AJP8 engine, there hadn't been further development to the Rover V8 so the chimp got both of the old engines with a CAT bolted on. When it became clear that the AJP was a little way off a rapid revision in the master plan led to the 500 Griff and Chimp which meant that a 4.5 was a more obvious and natural mid size engine.... ....... or something like that