Picco definitely make the strongest .21's when it comes to power... for a Hyper ST i would say either a good .28 or a Picco .21...I have had the Sirio engines, they run nice and make good mid and top, but they really lack bottom end compared tot he Picco's...for a Hyper ST you need torque as it is a bigger heavier truck.... If i was running a Buggy or a ultra light truggy I would consider a Sirio engine, but for a hyper ST it just doesn't have enough punch for my taste...as a rule any Sirio engine is going to have a soft smooth bottom, and as a rule a Picco has a strong snappy bottom...Sirio's rev good but they sure don't have the balls of a Picco !