Hello Tom

The difference betwen the early one, which is called a side-entry supercharger, and the central-entry type is not easy to see. I have made the two attached diagrams to try and explain it. The side-entry is so called because the air goes through a fairly sharp 90 degree turn into the impeller through six separate openings. The central-entry employs a much more aerodynamic inlet profile which also carries all the air in the same direction rather than into a circular chamber surrounding the impeller opening. The impellers are much the same and the air enters the rotating inducer in the same manner in both designs, hence the confusion about the meaning of “side-entry”.
I`ll leave it at that because all I`ll do is confuse the issue further!

Incidentally, the bulge at the rear of the so-called Babys Bum caused by the aerodynamic curvature designed into it made the Merlin 45 engine longer than the equivalent Merlin III, so the solution was to effectively turn the entire carburettor round to move the main jet mechanisms to the front, under the supercharger.

Pete