Where are all the others doing this that haul heavy constantly with their manual trans trucks?
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OP - I didn't buy my truck to race, just tow and be reliable/simple. Aside from the syncros in mine starting to wear a bit now, at 250,000 miles, I'm still happy I made the choice of a ZF6. Besides, I still have fun bangin' gears in this old truck.
First off, when you don't make any power the ZF is a totally different story. I ran one with bs injectors for years and aside from the continual fights with the plastic hydros giving up and the inevitable creeping through lights with the clutch fully depressed, and general slow ass truck syndrome, it was a practical option.
The problem with a ZF comes with increased power. And no, it doesn't break except in extreme cases. That's not the mode of failure. The mode of failure is in one of two ways, that you as the owner gets to pick. You either pick a dual disc clutch that holds, or.... you pick a single disc clutch that shifts, albeit still hopelessly slower than a trans with multidisc hydraulically actuated shifts...
Problem with holding capacity:
The hydros suck... Period. So you can't run a lot of plate, you'll never get the pos into or out of gear, and the syncros will die even faster than usual, plus it will drive like even stinkier dog poo. So you have to run more than one disc to gain the torque capacity. Problem with multiple discs is weight. The heavier the rotating assembly splined to the input shaft is that you must accelerate and decelerate this mass every time you make a shift. The syncro must suck all that spinning crap down in rpm on ever upshift before that gearset can be engaged to the input shaft. Which is why a dual disc of the size in these trucks shifts like dog crap. Anybody that tells you otherwise is clueless, or lying. It's physics.
So... we take the sprung hub out of the discs to save weight. So now it only shifts like horse crap instead of dog crap. Problem is, now the truck sounds like a can of marbles at idle, and the bearings in the trans are getting beat to hell because of the pounding the driveline then takes from the shocks of each cylinder firing with no springs to dampen it. Screwed again...
Okay... so we make a lightweight single disc... and hey, it shifts okay. Still many times longer to complete a shift than a 4R, but livable. If you shifted it with moderation and reserve the syncros would last okay. Problem is... as soon as you roll into 550+hp whoops... you just hit the rev limiter.... It won't hold the power...
The above is the ability to hold power problem you face with the ZF. Which is why I suggested that at or below ~500 wheel, it's doable, above, you start having to pick.
Second problem, unrelated to the first, is continuous duty:
The trans is a 6 forward speed, one reverse. That means you have a minimum of what, 14 gears in constant mesh all the time? That's a lot of oil chewing and slinging. Under normal power loads, like what the unit was designed for, that's no problem. The losses are still pretty good for a unit that size and type. But when you start pushing beyond the designed capacity of the unit (600ft/lbs btw) the losses start climbing. Not coincidentally... when you exceed about 600ft/lbs continuously the thing starts to reach a temperature runaway. It's almost like the people that designed and manufactured the transmission
know it's capacity! Because in my experience, almost exactly beyond this point it starts to overheat in continuous duty. That means at least an hour for all you backroad heros...
Reason hardly anyone sees this is that hardly anyone can
make 600+ ft/lbs for over an hour with a 7.3 and not overheat the engine first. You need a decent nozzle, and decent charger, with a relatively good state of tune to have the engine in the 6 to 700+ ft/lb range at steady state, with no egt control issues. OBS trucks will overheat the water even if the tune, nozzle and charger are spot on. Superduty trucks will almost always overheat the EGT before this point is reached in 99% of the cases due to crap tuning, or tiny little nozzles.
Reason you will not find a ZF in any superduty produced in years? Because the engines now come
factory rated at outputs that enter these ranges. In fact, the last 6 leakers already had ZF upping the rated capacity just to keep them in those trucks. Hence the ZF S650!
A ZF6 as offered in a 7.3 equipped superduty would melt in a single trip behind a bone stock 6.7 if the load and road required the 6.7 to make rated power for any extended period and the ZF6 was not fitted with additional cooling. Neverminding the wear due to continuous torque loads of 800ft/lbs on a 600ft/lb capable trans. And when I say 600ft/lbs capable, that's continuous duty. For short bursts I'm sure it would hold 2000+ ft/lbs if it were not applied as a shock load that blew that number way up.
As for a 4R in a company vehicle that tows heavy, the reason that's hard is because the monkey driving won't let off, and the 4R must make every shift under full power. So it's usually better, since those trucks make no power, to run a ZF, and just deal with the blown syncros and smoked clutches. I actually said that my 550 will not run a 4R for this reason.
Other people driving it. You know they would never let off, and the 7.3 computer doesn't do well with defueling for shifts.
For some reason, I doubt an EX is going to be 30k lbs all the time. Hence my suggestion of a full manual 4R. If you've never driven one, you might be surprised at how well it scratches the full control itch without the bs. Especially as power rises beyond 500 wheel or so.
Sometimes they
don't make sense. Most times they do.
