Cool - thanks all.
Here's the 2.3L summary for anyone who cares to know.

apart from all the other intricacies around compression efficiencies - just getting such a large odd and hollow shape to be balanced at 23,000rpm would be a challenge in its self.

PS - Photo courtesy of Kanters link - but I've taken his main drive gear (46 tooth) and mirrored it and then rotated 180deg to overlay.

b22b