with many of the hsp series cars / buggies stay in between the .16 and .18 anything higher will ruin the center gear or two speed depending if you got the what they call expert version sh makes great engines at a ok price but i say if the engine you have is still running strong and isn't to bad stick with that one :d but if you must buy a new engine i say sh or cxp 18 there a dime a dozen but careful on the tuning it depends on budget