Too much motor heat - help!

[VintageMA]

I'm not sure what I am doing wrong here, but I have just been having terrible problems with motor heat lately.

Latest problem - been waiting for almost 3 months for a 1512/3.5D to come in for my XB8EC and when it finally does here is my setup:

6S
15/46 gearing
Moab 1/8 buggy tires

Calculator puts me around 40mph. Confirmed with Nomadio speed sensor during running.

Lot's of cooling holes in the body - almost looks like swiss cheese right - motor temps were hitting 170* within 5 minutes.

I was running a 1515/2.5D (faster motor) with the same setup on 14/46 gearing and getting faster speeds with less heat.

Any thoughts? Drivetrain spins very smoothly, gear mesh is nice with just a little looseness for impacts, only thing I can think of is that I replaced the center diff oil and went from 10k up a little bit to around 15k (hard to say exactly as it was a mix of 10k and 20k - a little more 10 than 20 - also increased the front from 5k to 7k to increase on power steering a bit).

Based on all the setups that I read here this doesn't sound like it should run so hot. I could drop the pinion down to 12 or 13, but it'll be much slower and doesn't seem like it should have to be that low.