Just an update here. I towed our 6500 lb camper over the mountains of West Virginia twice this weekend, and made a few observations about the new setup. As a reminder, I now have the Tru-cool 40K transmission cooler sitting in front of the A/C condenser and a new Cold Case double-row radiator. All of this is cooling a drivetrain that makes an extra 50-60 HP & TQ over the stock 6.2L, thanks to a stage 2 truck cam upgrade.
Before any of those cooling system mods, my engine would run 200º-230º and my transmission would run 180º-235º when towing in 80º weather, depending on terrain.
After adding just the Tru-cool, my transmission ran much cooler, as expected, between 150º-180º when towing over the hills, which was a huge improvement, but my engine would run much warmer than before, between 220º-230º when towing on relatively flat ground in 80º weather, and I stayed out of the mountains as a result until I could sort it out.
Then, after adding the Cold Case radiator, my transmission temps are still nice and cool, and my engine temps now run 200º-210º when towing on flat ground, and it only gets above 220º when pulling up longer, steeper grades. In general, that's a significant improvement but still may not be enough.
On the way home yesterday, we climbed some long hills and because of traffic on I-81 had to keep our speed at 70 mph or above. It was about 78º outside and I noticed a couple of times my engine temp crept up to 240º but only for brief moments before cooling back down to 230º while still climbing the same grade. My guess is that's when the fans would kick into high gear and wrangle the temp back down. Also, the transmission never got above 180º.
Long story short, my conclusion is that while the Tru-cool is fabulous, its exceptional performance comes at the expense of radiator and engine cooling performance because of its positioning. I'll know more as the summer heat ramps up and we take some more trips, but my guess is that I'll have to take some additional steps to restore the balance of cooling system performance between the engine and transmission. I may experiment with a remote-mounted Derale fan-assisted transmission cooler in order to unblock the radiator. I'll update this thread as I experiment.
If any of you have any other ideas or things I should consider, I would welcome them - especially ideas for how/where to mount a Derale cooler, or even to re-position the Tru-Cool.
