The 2.5 and 2.5r version are the same excpet the 2.5R is a little better engine. the 3.3 version has a larger, .20 (still smallblock for some reason, dont know wh traxxas doesnt switch). But its a little bigger then the 2.5R's, so they made a new longer chassis for it. from what ive heard its quite a bit qukcier then the 2.5's, and is a little more stabile on the track too.