From that famous online encyclopedia:

A naturally aspirated engine is one common type of reciprocating piston internal combustion engine that depends solely on atmospheric pressure to counter the partial vacuum in the induction tract to draw in combustion air. This is in contrast to a forced induction engine, in which a mechanically driven supercharger, or an exhaust-driven turbocharger is employed to facilitate in increasing the mass of intake air beyond what could be produced by atmospheric pressure alone.

WWhunter wrote:What year model of engines are good for this conversion? I guess what I am asking is when did Rotax start 'beefing' up the crankcase. I know of a 912 that can be bought reasonably attached to a Kitfox. I think it is mid 90's vintage.

I believe the bottom end on the 912 was strengthened in about the 2006 - 2007 model year motors.

There are plenty of 914 motors made with the "weak" bottom end that are running just fine. When run in spec to what they are designed for, they will run well for a long time...

Both of my 912 UL motors are of the mid 90s vintage. I would have no issue installing the bully hawk system on either of them. (As long as I get a sponsor to pay for it... )

There would be more of an issue in getting the system to fit. My model IV Kitfoxes will not take the turbo motor or conversion due to the design of the motor mount and oil tank location.