I noticed when I ran the Viper 17.5 that it ran kinda hot and also that compared to most other 17.5 motors the kv rating is lower than most other faster 17.5 motors that are out.
You want a high out put fan that either runs off your receiver at 6 volts or the better fans that are capable of running at 8.4 volts that you wire into your speedo where you plug the battery in. This will lower your temps a ton. From my other Tamiya cars you will want to run that belt as loose as possible will also help with freeing up the drive train.
I'm geared at 3.88 overall with my VBC Wildfire and I use a 6 volt fan on my Thunder Power 17.5 with the motor timing set right at 35 degrees and my temps are 156 degrees after a 6 minute race.
I would really like to try one a TA)6 on a Exotek or VBC graphite chassis setup and see how it compares to my current car.