The thing about the PRO MX 50 you can do with it you cannot with a nicely ported Zen is you can run VERY tall gearing. By crunching the numbers they are calculating speeds of 80mph or more with a single speed in a HPI baja. Which top speed does not mean a whole lot to me. I prefer good handling on the track.
Here is a quote from a post and a newer video. Stock gearing is 17/57
[Now that my 50 has loosened up it has pulled up to just over 18200rpm on 26/48 with the 20tooth trans gear.]
http://s1237.photobucket.com/albums/...5tpower001.mp4