Nickiphorosn, its all got to do with ifmar rules. the sirio t3 meaning 3 intake ports i legal for state title national titles and world titles providing the you get the ifmar or efra crank version. The 12 5t is faster but its classed as an outlaw engine and not legal to run it events that requrie ifmar legal engines. most countries have classes were out law 12 engines can be used. the Sirio 5t sti is a 5 intake port engine and will produse much more horse power and will be faster over all. If you are in an rc car club ask them if they run a class were outlaw 12 engines can be used. If you just use your car for fun get the 5 port as the paformance difrence is a bit better because the engine dose not have to conform to ifmar engine limations on number of ports and size of crank port. Regards Jeremy.