1940/7 getting hot...help?

[BP-Revo]

Ok...so I've had my 1940/7 powering my G2R. Have ran it on everything from 4S to 6S and on 6S it gets a bit toasty but it's fine on 5S all day long. A full run through a set of packs at full race pace and I'm at 150*F.

I recently picked up a 1950/7 from another RCM member so I put the 1950 into the G2R and decided to use the 1940 in my RC8.2 that I got a killer deal on. At first I geared conservatively with a 13T pinion and it was definitely undergeared and temps were really high (165*F). So I jumped up to a 14T. Still felt undergeared but temps dropped to about 155*F. I then went to a 15T pinion and I came back from a track session to find it at 180*F!!! HOLY CRAP.

What the heck is going on here? I'm running it on 4S. How is it that this motor pushed around my bigger, heavier G2R but is having issues in an RC8?

Any input appreciated...