RE: traxxas vs hpi engines
Let's see, the HPI 4.6 is a big block motor (.28 I think) and the 2.5(r) is a small block motor (.15).
By all reports the 4.6 is a good motor.
The 2.5(r) is a good motor with somewhat of a bad rep. It can be a bit of a bear to tune but when tuned properly it really is a good running, very powerful .15 engine. Unfortunatly it is often compared to big block motors and that's not really a fair compasison. Of course if you put a big block in a truck designed for a small block the performance difference will be dramatic.
I believe that the only difference in the newest versions of the 2.5 and the 2.5r is the head. The 2.5 is a cast alum head and the 2.5r has a machined head. The machined head should provide a slight improvement in cooling.