Unless your friend is an experienced racer or extraodinarly coordinated naturally, he will be slower with anything faster than 10.5. A Mamba 5700 is strictly a motor you use so that you have enough power double the tripple at the local track for fun.....worthless for racing. I think the 4600 is the equivalent of an 8.5. I run faster with a 13.5 in my T4 than with a 10.5 and I have a few wins under my belt and some trophies. Honestly, if you are building a car for him it probably means that hes a rookie. All he is going to do with something that fast on a track is wad it up and and crash into everything & everyone on the track.

....just my opinion.