Don't know why Evolution made both the .455 and the .46. It is very confusing. The .455 (alpha engine) has milder porting than the .46 which allegedly makes it more user friendly, but I have never noticed the difference. These alpha engines are very good engines, very durable, easy to start and seem to last a long time. They come from the factory with a flywheel, a limiter on the needle valve and a three bladed prop. If you open the box and throw all three of those items in the trash and put a proper prop on it, you will probably like it. I am constantly amazed at the number of Evolution .46 engines I see for sale but when I look closely at them, a lot of them have the stylized "a" on the side of the head.