I have a Magnum .91 4s in mine and it seems the perfect combo.. I considered the 120 but the big jump in weight IMHO negates the small gain in power... If you have a model that needs a lot of nose weight then the 120 looks a little better.. With the .90 the Extra66 balances with the battery (1400 mah nicad) right behind the fuel tank... Works out perfect... This is one sweet flying plane... I was amazed at how easy it seemed to penetrate the air... The .90 will give you plenty of verticl (with nearly perfect up and down lines). Plenty of power for nice big loops too.. If you check the weight jump between a Mag 90 and a Mag 120 you wil find it significant... Likewise between a Saito 100 and a Saito 120.... The increased wingloading IMHO is detrimental to flight performance... You want something wild go for a YS 110......