The Turbax 46 is for the OS 46VX engine. I think the turbax I is for the older K&B 7.5 DF engine. I had a JHH F86 that had a K&B 7.5 in it and it flew ok. I then upgraded it to the OS 46VX and that plane came alive! With the OS46VX the speed increased a good bit and it did not struggle to do nice large loops like it did with the K&B 7.5. This is a very good plane! I wish that I still had mine ( a brain fart did it in after 2 years.....[] )