These are the considerations on Rotax & Jabiru of a member of this NG posted
some months ago from Jez:

Hi

Mirco
****

I've used/flown behind both, and they are very different. I have no firm
preference for one over the other, but have formed the following views based
on observations and experience.

The Jabiru is simple, light, easy to install and performs OK in a tractor
configuration. There have been reported overheating and reliability
problems when used as a pusher on some aircraft. It turns the prop at 3,300
rpm to produce it's 80hp, which is fine for a relatively high cruise speed
aircraft. Fitted to a slow (60kt cruise) aircraft the Jabiru did not
perform well, as the small prop that the relatively high rpm demanded gave
relatively poor efficiency. The most notable deficiency was in take-off and
climb performance, which could not be easily corrected by reducing pitch
without the danger of overspeeding the engine in level flight at full
throttle. On a higher cruise speed aircraft, like the Jabiru kit, the
engine peroms well, and gives adequate climb and take off performance.

The 912 has a gearbox so will turn a bigger prop at lower rpm. This gives
much more static and low speed thrust, so improves take off and climb
performance. The downside is that the engine is more complex (gearbox,
water cooled heads, air cooled barrels, plus oil cooler) and heavier (it's
also more expensive!). It has a proven reliability record though, and is
relatively insensitive to mounting configuration due to the water/oil
primary cooling.

We found that fitting the Jabiru with a bigger prop and pitching it to limit
rpm to 2,900 gave better performance than the standard small prop when
fitted to an Easy Raider high wing, low speed, UK spec microlight. This
limited the engine power output to around 70hp I guess, but the gain in prop
efficiency more than made up for the power loss in practice.