I asked someone in the 3W circle once why these two very similar engines are offered by 3W instead of just going to one. I was told that the warbird flyers bought mostly "i" versions because they fit well in those type cowlings and the sidecarb version was prefered by everyone else because of lighter weight. That person said it makes economic sense for 3w to continue producing two type 75cc engines to service the two markets. I think this is the idea behind the intro of the 55cc sidecarb engine, lighter weight, to better compete in the 50cc class of aerobatic engines istead of just the 50i engine that everyone knows is at least a full pound heavier than DA, brison, bme and others.