Although this isn't a diesel transmission, we had to show you this photo. The story we heard was that the rear end broke and locked up, which bent up the driveshaft and caused the transmission case to be ripped in half. Although most diesel transmissions don't have to survive 172mph quarter-mile passes like this Mustang's did, the torque they both produce can be similar, so make sure your driveshafts and axles are up to snuff too.
2. How much power can they handle with a converter and valvebody upgrade?
Goerend: They should have no problem up to 500 horsepower with the stock shafts. If you're trying to build boost (drag racing or sled pulling), you might want to upgrade the shafts too.
Lovrich: Allison-about 150 rear wheel horsepower over stock, but that's pushing it. '03 and later Fords (5R110), with a converter and a programmer that will give you line pressure rise, will let you get away with about 120 extra hp. Earlier Fords with a TransGo shift kit can get away with 150 hp. It depends on the condition of the transmission (good working order). Shafts and flexplates are the problem with Dodges. They can hold good horsepower, but you'll start breaking shafts.
3. How do I know if my transmission is going bad?
Goerend: The first thing would be: Is it consistent? If all of a sudden it starts shuddering or shifting at different points, or you have rpm rise while going up a hill, those are good indications that your transmission might have problems.
Lovrich: Several different ways- a check engine light might come on, you could have wrong gear starts, or flared shifts. In a Ford, an overdrive light flashing will signal that you have a code.
4. What is "limp mode"?
Brian Carter had three different transmissions in his truck during Hot Rod magazine's Drag Week. Don't expect things to be easy if you're trying to propel 8,000 pounds of dualie into the 10-second range.
Goerend: "Limp home mode" is what Chrysler originally called it. It's any time the computer gets scared and doesn't see 12 volts going to the transmission (due to various possible problems). The transmission will start in Third gear because no power will be going to the computer.
Lovrich: It means go get it taken care of immediately. It will run max line pressure and no volts to the trans, so you are stuck in one gear.
5. How are the manual transmissions for reliability (Ford, Chevy, Dodge)?
Lovrich: From what I've seen, they're all pretty solid. The clutch is the weakest point, but there are a few companies that build some killer clutches out there.