Man you got to find out the compression ratio your engine has. If the ratio is below 8.5 you can use 87 gas all right. If you have 9.0 and higher the 93 gas is definitely better. Some highspeed 50 cc engines have compression ratios around 12, so they run best on 95 and 98 gas. So, the compression ratio of your engine is the most important value to choose the correct fuel. Also keep in mind that feeding a low-compression engine with a high-octane fuel is a sheer waste of money and it doesn't make it run faster but is not likely to harm the engine. On the contrary, if you try to use low-octane fuel in a high-compression engine this will cause knocking (or detonation) which is going to kill the engine very soon.