When I first got my engine, I also thought those 2 numbers meant you can switch between them. I have a 66/80. But you cannot switch it. Its the displacement of the engine which is measuring in cubic centimeters. Basically the size of the engine. The reason there is 2 numbers is because the engines are made in china and china calls them 80cc when they are really 66cc. So if the zeda100 says its an 80/100, then its the smaller number 80cc.