Perhaps the single most expensive (and destroyed) part of a diesel pickup is the transmission. From the 400-lb-ft Ram back in 1989 to the 800-plus-lb-ft numbers of the current Big Three, diesel transmissions have one of the toughest jobs in the pickup truck industry. What makes matters worse is that with every horsepower added, 1½ to 2 lb-ft of torque also makes its way from the engine to the transmission. This means that once power gets up to the 500hp level, nearly 1,000 lb-ft of torque is unleashed on the transmission. No matter what make or model the transmission is, that’s a big pill to swallow.
| Budget Transmission Upgrades Working On Transmission
Pick the Right Starting Point
Some transmissions are stronger than others right off the bat. The ’06-and-up Allison 1000s and ’08-and-up Ford 5R110 and 6R140s are good examples of OEM transmissions that can handle huge power and torque numbers in stock form with simple tuning. Unfortunately, they also come attached to $30,000 to $50,000 trucks, which aren’t so budget. If the transmission is an older model, the shape it’s in when the truck is purchased is a very important factor. If the transmission is already worn out, there’s little chance it will be able to handle a power upgrade.
Dodge Transmissions: TF727 to 68RFE
Back in 1989, Dodge paired up a 5.9L Cummins engine and a 727 TorqueFlite transmission and then shoehorned it into its line of Ram pickups. Because the TF727 had proven itself behind the power of muscle cars for years, it was up to the task of the power and torque. Even a stock transmission with a bump in line pressure or a shift kit (something that can be performed by any transmission shop) will be able to handle 300 to 400 hp at the wheels if it’s in good shape. At around 450 to 500 hp, these transmissions start having trouble and need parts like billet flexplates, input shafts, and torque converters. We’ve seen TF727 and A518s (later versions with Overdrives found in ’91½ to ’93 Dodges) handle 700 rwhp when rebuilt with good clutches, a billet input shaft, flexplate, valvebody, converter, and steel planetary gears. While that’s at least $2,500 in parts, it’s still less expensive than the new stuff.
| Sled pulling and drag racing can result in fun stuff. Broken shafts, burnt clutches, and trashed converters are all a possibility for the competition enthusiast—no matter what the budget level.
With the body style change for the ’94 model came a new transmission: the 47RH, an updated version of the TF727 with an overdrive gear. With four speeds and a lock-up converter, these transmissions didn’t change much as the years progressed. After a couple of years of production, they switched to an electronically controlled overdrive (47RE), then to an updated gear train and solenoid-controlled valvebody instead of a TV cable (48RE), and the run of these transmissions were found all the way up until the ’07 model year. Much like the TF727 and A518, these later transmissions could handle a good amount of power, but the stock torque converter and valvebody weren’t always up to the task. With just a valvebody and triple-disc converter, we’ve heard of 47/8-series transmissions handling 500 or even 600 hp. Unfortunately, they do have their weaknesses. Boosted launches or locked shifts can break input shafts, rip out the center of flexplates, or even break output shafts. While a triple-disc converter and valvebody can be installed for a few grand, most performance enthusiasts opt for a full rebuild with a billet input and flexplate, just to be safe.
| Alto Red Eagle clutches, a TransGo shift kit, and a Goerend converter were all that was needed to get this 727 TorqueFlite to handle 514 rwhp on Rust Bucket, our ’89 Dodge project truck (lead photo). A triple-disc converter (above), along with a valvebody, can be a transmission-saver for virtually all makes and models and will allow another 100 to 200 hp more than stock to be sent to the ground.
The newest transmission in the Ram line is the 68RFE, which in stock form is limited pretty closely to stock horsepower. That’s not to say there aren’t solutions—one such budget-minded problem-solver is the 68RFE rebuild and upgrade kit, available from Sun Coast Converters. This kit re-works the valvebody and TCM to where the 68RFE can live at 500 to 550 hp, but any more than that and the truck owner will have to start delving deep into the transmission. As of press time, there are 68RFEs that are handling nearly 1,000 hp, but we can tell you they’re not cheap to buy—or build.
GM: 4L80E to the Allison 1000
General Motors equipped hundreds of thousands of pickups, SUVs, and military vehicles with its 6.2L and 6.5L IDI engines between 1982 and 2000. On the transmission front, most 4L80Es will be safe in stock form, because most hopped-up 6.2L and 6.5L engines can only make about 200 to 250 hp at the wheels. If more transmission strength is desired, there is a robust aftermarket available to build these transmissions to handle 1,000 hp and 1,500 lb-ft.
| Making power for short bursts is one thing; towing with it is another. When it comes to towing, the closer to stock power levels, the better. High-horsepower towing usually involves some very stout parts (billet drums and shafts), as well as an all-out battle against heat retention.
In 2001, GM brought out a game-changer of a pickup: the company’s Heavy-Duty 2500- and 3500-series trucks powered by the 6.6L Duramax diesel. These trucks were partnered with the Allison 1000 transmission, which is known for its strength. Unfortunately for the aftermarket, the Allison is also very smart, and it does things like de-fuel during shifts, drop mainline pressure when the converter locks, and go into limp mode if just a few-percent slip is detected. This makes for a transmission that’s hard to fool into accepting more power and also makes transmission tuning very important. Most five-speed Allisons that were paired with the LB7 and LLY engines (’01 to ’05) can handle around 300 to 400 rwhp with proper tuning, and be safe. Some of the newer transmissions (six-speed found with LBZ, LMM, and LML engines—’06 to current) can handle 500 rwhp or more, again, provided the tuning is there. The Allison is hard to push past that in stock-ish form, because the clutches will slip, and the transmission will go into limp-mode to save itself. With a valvebody, converter, and good tuning, we’ve seen Allison 1000s handle 500 to 550 rwhp for years.
| A deep transmission pan like this Pacific Performance Engineering (PPE) unit can help extend the life of your truck’s transmission, as well as keep the fluid cooler during horsepower production.
Ford Transmissions: E4OD to 6R140
If they’re in good shape, Ford transmissions are some of the best power-handlers in the business—especially the newer ones. In the case of both the E4OD and 4R100, the converter clutch is the weak link and will start to slip before anything else gives. In the E4OD, this might happen as soon as 300 rwhp, while the 4R100s will usually hold out until about 400 rwhp. Good transmission tuning is crucial to these early transmissions, and with an aftermarket triple-disc converter (which can have more than twice the clutch area) and valvebody, it’s not uncommon to see relatively stock 4R100s in the 450- to 500-rwhp range.
| These burned up clutches (right) were the result of about a year’s worth of fun at 550 rwhp on a 6.4L Power Stroke-powered Ford. While the transmission still “felt” OK when it was pulled, eventually it would have started slipping.
The 5R110 transmissions (which came in ’03 to ’10 Super Dutys but were beefed up even more for the ’08 to ’10 6.4L-powered trucks) were Ford’s first step away from the rest of the pack. In completely stock form, we have seen these transmissions hold 500 rwhp for years and up to 800 rwhp for short bursts (dyno runs, eighth-mile passes, etc.) and live. Eventually, the direct and overdrive clutches will show some wear, but usually not until at least a year or two of hard driving. And, although there aren’t that many modified 6R140-fitted 6.7L Fords out there yet, the appearance is that 400 to 500 rwhp on an extended basis and 700 rwhp for brief periods shouldn’t be a problem for these transmissions, either.
| Besides triple-disc converters and valvebodies, another universal trick is increasing the clutch count inside the transmission. This allows for more clamping force and can handle a higher torque load.
The Fix-All: A Converter and Valvebody
If it looks like this whole article boils down to: “no matter which make or model you have, you need to have a transmission that is in good shape to begin with and then install a triple-disc converter and valvebody,” you’re basically right. Most mildly modified diesels are in the 500-ish range as far as horsepower, and that’s something a good stocker (or fresh rebuild) can handle on a budget for just a couple thousand dollars.
Manual Transmissions
Diesels came with a variety of manual transmissions from the ’80s until now, and the offerings found in some trucks are rather good at handling horsepower. The ZF-6 transmission can be found in both GMs and Fords, while Ram opted for NV4500s, 5600s, and G56 transmissions. In all these applications, the clutch will be the initial weak link, but if that is replaced, then most of these transmissions are very strong—capable of handling 600 to 800 hp in daily driver situations. If you’re looking to stay on a budget and not have a $6,000 transmission build, a manual might just be what the doctor ordered.
Pushing the Limits
If you’re willing to live with the fact that your transmission might last a few months, a few weeks, or a few days, you can actually make a lot of horsepower on a temporary basis. While these numbers are fun, keep in mind that they’re not going to be permanent, and sooner or later you’ll have to yank the transmission out for a rebuild. With that in mind, we’ve also included a safe range, a power level range (depending on condition and driving habits), that should keep your stock transmission happy for quite some time.
| With a good builder, the possibilities for handling power and torque in the diesel segment are very impressive. Diesel Power Challenge 2012 competitor Jeremy Pierce made a whopping 2,455 lb-ft of torque, and his highly modified Allison 1000 transmission held together!
| Budget Transmission Upgrades Transmission Operating Power Chart
