The VTG trucks with factory EB also come with a 68RFE (6.7 truck, 07.5+). The trans and turbo are all made for the EB function. It works in all 3-6. So yea both OD gears. It does function a lot better in 3-4. More retarding power for sure. I don't think you could easily add a VGT to a 5.9 3rd gen. You can add a normal EB, however on a auto 4x8RE you will need a controller of some kind to deal with the trans though, it should handle it all so it only kicks in when the converter is locked. I doubt it cares about OD or not.Do the dealer installed or newer exhaust brakes built into the variable geometry turbos engage when you're in overdrive?
The instructions from PACBrake said not to use it in OD but the document they reference is from October 1999 so perhaps that limitation was only applicable to 47 or older transmissions.
It does not make sense to me that the transmission would be significantly weaker with relatively low resistance from an EB versus full power from the engine. I checked with my transmission shop and they also did not know of any 'directional' weaknesses in the 48RE that would make using the EB in overdrive a problem. Whenever I'm in overdrive I am obviously empty or towing light so the EB is not trying to hold back a large load either. Also, the brake will automatically engage in overdrive when the torque converter is locked up so they don't programmatically prevent it from operating in overdrive.
I'd like to just leave it on all the time and use it to slow down from highway to town speeds but do not want to risk transmission trouble if that's a possibility.
note that there really isn't an "EB" persay on the 6.7 trucks, not like one you would install in a 5.9 truck. It's just the VGT's ability to close up. Its functions just like an EB. So why not include it in the truck right?