there are 3 difrent sirio 21 pr pro engines first is the older version s21pr pro 7 port aac. The lates relised version is the s21 pr pro evo2 7 ports aac. And the last is the s21 pr pro evo 2 sti 7 ports aac. The best way to tell them apart is the early s21pr pro engine had a black head and there was a second version of the sirio 21 pr pro with a blue head they had a 21 on the head and 21 on the side of the block but most important when you look at the crank it will only have one balancing plug in the bell. Were as the new s21pr pro evo2 and evo2 sti boath have blue heads the sti will have sti on the head above the exhaust port and it will have sti on the bell of the crank and 2 blancing weights in the crank bell and no 21 stamped in the side of the block. The s21pr pro evo2 engine will be the same minis the sti marks and slighty less work to port's and crank but outher than that they are the same. if you post a pic of the head and the block side on ill can tell you defently and send you all the part numbers you will need and were to get them from. Regards Jeremy.