mm + 4600 motor temp near 200 with rustler

Is there any reason the cms4600 motor should approach 200 degrees on a rustler with 13/90 gearing and stock 2.2 tires running a123 3s batteries. The esc stays around 120 tops I believe. Batteries barely warm.

I've asked on other forums and even talked with Thomas at Castle a few times on the subject and nothing has brought down the temps. The old 4600 was just replaced with a brand new one by Castle (excellent support I gotta say), gearing was changed to 16/90 and 20/90 to see if the 13/90 was simply undergeared, but the temps rose even quicker than using 13/90. I have the stock plastic transmission housing and no heat sink to help spread the heat. I drive it at the clay rc track almost non stop except for battery changes and some other delays. After the first 15 minutes temps get to 160 then every pack afterwards is in the 180 - 200 range. There is no binding in the transmission since the car easily rolls down even the slightest incline.

I have another mm with vxl motor on the way from an ebay purchase so I plan to try out the vxl with my current esc first. Then will swap out the esc if the vxl runs hot. Any other suggestions?