the dyno is used with the tuner (diablo or sct) to custom tune your car for the most hp or the best average power, rather than peak power. each engine is different and wont respond exactly the same as others. my dyno guy had a guy come in with 2 cobra replicas with motors built exactly the same (5.0s) the guy wanted one tune so he could use the same tune for both cars. the dyno guy said ok, but ill give you free dyno run on the 2nd car to see what it does. he knew they wouldnt be the same. it ended up that they were 30 hp different, but with a tune for the 2nd car, they were set. my dyno guy is familiar with sct, so i will be buying that one. it sounds like most people are just using generic tunes form the tuner and not getting custom tunes.