I need help, I cannot decide. I also need to know what i will need to do the conversion for all engines, such as exhaust manifold (sirio/fantom) carb linkage and clutch/flywheel. I know the 2.5 is the cheapest, The Sirio is the HP king, fantom I heard was good. but it has less hp than the 2.5. I have been leaning toward the sirio but i do not know what i need to install it on my rustler. I am not interested in any OS engines they are vastly under powered! I was possibly considering a megatech as a distant backup.