Even a 2014 3500 SRW has a maximum conventional trailering capacity of 13,000 lbs. (compared to 23K for 5th wheel), so to a certain extent weights like these are up against the limit of what's reasonable to tow with a ball hitch.These guys also have a 6L80/90 improvement program. Sounds like they have/use similar hardware...
I called them and they thought that a 6L90 would be able to live behind LML power (350HP/750ft-lbf) with just their 4 disc torque converter and software to optimize shift timing/pressures. Don't go attaching a 15k trailer and expect it to live (per their recommendation), but should move the van around just fine...
FWIW, 2010-2014 3500 vans are rated to tow up to 10k lbs., and long wheelbase DMax vans are rated at 18,500 lb. GCWR. (std. wheelbase only 17,000 lbs. for some odd reason, but it may have something to do with the longer wheelbase providing significantly more leverage against the trailer hitch. Or maybe GM just pulled the numbers out of the air?)
In comparison, 2014 6.0 gas/6L90 3500 pickups are rated at 10,700 lbs. (conventional) and over 14k lbs. 5th wheel, so that should give some idea of what GM thinks a stock 6L90 will handle:
Notice that in the chart above, the GCWR numbers are omitted, but the tow ratings alone indicate that the Allison specs from GM powertrain aren't correct.
Given the tow ratings for new 1/2-ton pickups, maybe the reality is that there's a lot of monkey business being played with tow ratings to meet marketing demand for bragging rights. Automakers probably don't feel a need to exaggerate van tow ratings to the same extent as pickup tow ratings.
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