I don't own a HMMWV, however I do believe I can contribute a few helpful comments to this thread.
For the past 7 years I have been completely rebuilding/restoring my 1984 M1028 CUCV pickup. Although I am pretty much finished with the project, I continue to occasionally make performance and comfort modifications to suit my whims. In 7 years I have driven the truck less than 2,000 miles.
The truck has its original 6.2 with TH400 and NP208 transfer case, and just recently turned over 14,000 original miles. It cranks and runs perfect, and I could not be more happy with a vehicle. However last year I began to drive it more, and quickly realized the 4.56 gearing would limit my highway speeds to around 50 mph with a safe RPM value. I decided to pursue a 4-speed transmission upgrade, and for several months read everything I could about the pros and cons of the differences between a 4L80E and a 700R4. I waffled back and forth between the two, and here is what I learned - The 4L80E is a stronger unit when both transmissions are compared in their stock forms. The 700R4 is a later version of GM's TH350 and was offered from GM in their Blazers and half-ton pickups during the 1980's. In my opinion the 700R4 would not hold up, in its stock form, for heavy off-road use or rock crawling. And in all probability would not hold up in an HMMWV environment in its stock form due to the heavier vehicle weight and resulting performance demands. However, the 700R4 does offer advantages over the 4L80E if built correctly. For instance, the 700R4 is a "dumb" transmission similar to the TH400, that is, it requires no electronics to operate. It receives it's instructions from the throttle demands via a cable (the TV cable) and, if the TV cable is adjusted properly, is fool-proof and fail-safe. In contrast, the 4L80E requires an electronic control module which can add considerable cost and complexity to a proposed transmission upgrade. And like any electronic device, the electronic control module can malfunction, resulting in improper transmission operation or, in a worse case, catastrophic transmission failure. While doing my research comparisons on the 2 transmission models, I found myself stepping away from the HMMWV and CUCV forums and I began to read about what transmissions were being used in drag racing. To my surprise, I learned that the 700R4 is the transmission of choice for many drag racers due to lower first gear of the unit design. Granted, a 700R4 to be used for drag racing would obviously need to be built with stronger internal components, however I also learned that such strength increases were readily available and commonplace from the drag racing community of transmission builders. The 4L80E and TH400 both have a first gear ratio of 2.48:1, while the 700R4 has a first gear ratio of 1.96:1. This is why the drag racers prefer the 700R4. I then realized that the lower first gear would be advantageous in a military or rock-crawler application as well, as long as it was up-built equal to or exceeding the strength of the 4L80E. Another advantage the 700R4 had over the 4L80E was size. The 700R4 is the exact same length as the TH400, thereby eliminating the requirement for crossover modification and lengthening/shortening the drive shafts. In the spring of 2016 I made the decision to go with an up-built 700R4 for my M1028 transmission upgrade.
My next task was to explore the numerous transmission builders which catered to the drag racing community, to make a decision on which one I would procure my 700R4 from. This process in itself required several weeks and countless hours on the phone vetting each builder. I concluded that all custom transmission builders catering to the drag racing community were good, reputable outfits, because word-of-mouth on the back-side of drag strips can make or break a business in short order. I narrowed my selection of transmission builders to just a few, and was struggling on my final choice. That is until I called Bowtie Transmissions for the third time and spoke with their general manager. After hearing that my application for an up-built 700R4 would be for a military truck, he got his shop manager on the phone with me, a fellow named Chris Gardner. As it turned out, Chris owns a M1008 CUCV and was running an up-built 700R4 in it and was using it on the weekends as a rock crawler. This fellow knew exactly what was needed to build a true bolt-in 700R4 for my M1028. He knew about the diesel torque converter and governor, and told me that he would need to use an Advance Adapters 32-spline rear output shaft to correctly mate to my NP208. He recommended Bowtie Transmission's Level 2 K-case unit which would safely handle 450 horsepower and 450 foot-pounds of torque. I was too embarrassed to tell him that my stock 6.2 would only be sending 155 horsepower through the 700R4 at any given time. The total costs of the unit including the new diesel torque converter, brake pedal lock-up switch, Advance Adapters 32-spline output shaft, and freight from California to Kentucky would be $2550.00. I gave him my credit card number.
Here is where I am at in this process - I have the Bowtie 700R4 on hand; I have a new GM TV cable; I have the 6.2 diesel TV bracket which bolts on the rear driver's side head (i paid $100 for this from another SS member, however they are regularly offered on Ebay), and I have the taller transmission-to-transfer case adapter. I have the GM part number if anybody is interested. As soon as I get time, I am going to swap the 700R4 into my M1028. I plan to photo-document the entire process in my rebuild thread.
I hope this helps.
