If ya want a .15 and an os you have two good choices: an os .15cv(x) or a os.15cv-r(x). The cv(x) is thought of very highly as a great all around bashing engine. The cv-r is a race engine. Some people love it, others think the cv is a better choice for the maxx. Anyone with experience want to chime in here as to the benfits of one versus the other?