And by the way, it's CI, not CU. (Or Cu In)

It stands for Cubic Inches of displacement within the combustion chamber of the engine. Most of those you are talking about are 0.40, 0.46 or 0.50 CI.

Car engines were always described in cubic inches of displacement in the past; such as Chevy's 283, 350, 396 and 427, or Ford's 289, 352.... etc., etc. Today they tend to label in Litres, like 4.5L which translates into about 275 cu in.

When you get into larger gasoline powered RC airplane engines they'll be in CC's. Such as the Quadra 35, or Desert Aircraft 150. (35 cc's is about 2.1 CI, 150 cc's equals about 9.15 CI.)

Not nit picking, just thought you'd want to get your facts straight.

Good advice on engines so far, by the way.
Dennis-