Rotax 912 vs Continental O-100D vs Lycoming o-233
Development work on the 0200 and 0235 for LSA application was not complete when the RV-12 was being designed and had either of those engines been available, it might be a different story.
I think Van's had a very close look at engines that were available and were about to be available. The power to weight figures of the Rotax 912ULS compared to the Continental O-200D and Lycoming O-233 are the most compelling.
Rotax 912 ULS
100 bhp/141 lbs
Continental O-200D lightweight
100 bhp/199 lbs
Lycoming O-233 lightweight
115 bhp/213 lbs
Continental had to go to some heroic measures to achieve this weight, such as machining the cylinder barrel fins down.
Now look at the RV-12 vs the Cessna Skycatcher
RV-12 - Rotax 912 ULS engine
Empty weight: 740 lbs
Gross weight: 1320 lbs
Useful load: 580 lbs
Fuel capacity: 20 gals (120 lbs)
With full fuel and 2 x 205 lb people and 50 lbs baggage you are at gross
Cessna 162 - Continental O-200D engine
Empty weight: 834 lbs
Gross weight: 1320 lbs
Useful load: 486 lbs
Fuel capacity: 24 gals (144 lbs)
With full fuel and 2 x 170 lb people you are at gross (no baggage)
I think the Rotax 912 ULS engine is the perfect engine for the LSA category