I used to have a Mach .21 with my hyper 8 and hated it, never got it hold a tune and went thru 3 pistons. But i have heard of some people that have had good ones, so you must be one of the lucky ones.
Novarossi do make good motors from what i have heard, but i have found that if you're after bang for buck have a look at the Go Engines range of motors.
http://www.go-engine.com/in-products...15P-P820SG.htm
http://www.rchobbyzone.net/front.asp...GOR215P-P820SG