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Boiling Point, Part 1: How Far Can You Push Your Ford Diesel’s Transmission?

Upon disclosing each Power Stroke diesel’s breaking point in our Threshold for Pain series, we realized we’d better clue you in as to what each brand’s transmissions are capable of handling, too. After all, most transmissions fail before you’re ever able to realize the full potential of the engine mounted in front of it. As for the Ford camp, its automatic transmission offerings have only gotten better over the years. For example, back in the day an E4OD subjected to 300-rwhp and 600 lb-ft of torque on a regular basis could lunch itself rather quickly. Fast-forward to the present and the current 6R140 TorqShift has no problem harnessing 600-rwhp and 1,200 lb-ft for tens of thousands of miles.

Without going any further, however, we have to address the simple fact that these power thresholds aren’t set in stone. Case in point: the weekend warrior’s transmission might hold up 10 years at the 500-rwhp mark, while Joe “heavy foot” Blow’s bone-stock, yet highly-abused truck could blow through three slushboxes a year. It’s all in the operator as to how long your transmission copes with added power and/or abuse.

After you digest what Ford’s E4OD, 4R100, 5R110 and 6R140 automatics are capable of, stay tuned for our installment on the Allison, the transmission that brought medium-duty robustness to GM’s HD truck line.

1. E4OD (’94.5-’97)


To say the E4OD is the weakest automatic transmission to have ever been mated to a Power Stroke would be true, but you have to remember that it was based off of the C6—a design that was 30 years old (and originally intended solely for gas engines) when the 7.3L Power Stroke came along in the middle of 1994. With the addition of steel planetaries (in most, but not all units) and a different torque converter with a higher torque holding capacity, Ford readied the heaviest duty automatic in its arsenal for the 7.3L Power Stroke. As long as the 7.3L Power Stroke goes unmodified, the factory E4OD will generally live a while. Although, it can still be killed at the stock 210 hp and 425 lb-ft level with heavy abuse.


Looking at things from a performance perspective, all transmissions will need to be rebuilt or reinforced at some point—and the E4OD’s and 4R100’s are arguably the cheapest to build up. Tow-anything transmission builds can be had for as little as $3,000, with all-out competition-ready units going for anywhere from $6,000 to $7,500. Most E4OD’s in good overall health can be fitted with a quality triple disc converter, an upgraded valve body and live in the 350 to 400-rwhp range under the right foot of a sensible driver. However, there are still many Achilles Heel-type weakness lurking inside an E4OD’s case that can surface at any time (more on that below).

Boiling Point: 250 to 350-rwhp

2. 4R100 (’99-’03)


While it’s best to think of the E4OD as an overgrown C6, the 4R100 can be viewed as the new-and-improved version of what is essentially a 1960’s era transmission. Beginning with the 4R100 (which coincided with the debut of the Ford Super Duty), a PTO option, two additional speed sensors and all-steel planetaries were added, as well as a stronger torque converter with a different lockup method. However, it too suffers from many of the same internal weaknesses as the E4OD (namely failed coast clutch drum and Overdrive drum snap rings, center section bolts that are known to back out and a reverse hub that’s notorious for blowing out).


If you’re getting away with hot-rodding around with a 140 hp chip installed on your 7.3L Super Duty (especially a late ’99 and newer version), we wouldn’t throw a set of bigger injectors in the engine and expect all to be well with the stock 4R100. While it can handle 350 to 400-rwhp a little better than the E4OD, it won’t tolerate it forever. At this point, it’s either build it or lay off! As far as failing due to increased horsepower and torque, the converter is usually the first item to go, if not the direct or overdrive clutches (which, aside from the converter are subjected to maximum torque production with the engine under full load). Also like the E4OD, the 4R100 is still relatively plentiful and can be built fairly affordably.

Boiling Point: 350 to 400-rwhp

3. 5R110 TorqShift (’03-’07)


Night and day difference from the four speed automatics Ford offered from ’94.5-’03, the 5R110 TorqShift behind the 6.0L Power Stroke isn’t based on a previous design. Technically a six-speed, but advertised as a five (the extra gear is only used for cold weather warmup), the 5R110 features adaptive control, which monitors and adjusts shift points in real time—to ensure consistent shifting and also to account for wear. In stock form, it can withstand considerable abuse. How does 500-rwhp for extended periods and 550 to 600-rwhp for racing purposes sound? It's torque converter is light-years ahead of what was offered in the E4OD and 4R100’s in terms of holding capacity and even when the early 5R110’s do fail, it’s rarely the converter’s fault. When the TorqShift behind a 6.0L bites the dust it’s usually due to the direct or overdrive clutches failing.

Boiling Point: 550-rwhp

4. 5R110 TorqShift (’08-’10)


Stronger yet was the ’08-’10 version of the 5R110 found behind the compound turbo’d 6.4L Power Stroke. It’s fitted with a different stall, 8-bolt torque converter (vs the 6-bolt employed in the ’03-’07 unit), higher pinion count planetaries and a deeper pan. Unlike the 6.0L version, this second generation TorqShift isn’t on the edge of what it can handle at 550-rwhp (and 1,100 lb-ft of torque). Rather, it can live there for years with sensible driving.


In competition environments, or when the owner only runs his or her truck hard on occasion, the stock ’08-’10 TorqShift can withstand 700-rwhp and more than 1,300 lb-ft of torque. We’ve seen several tune-only 6.4L-powered Super Duty’s roll around all week at 550 to 580-rwhp and then clear 700-rwhp with a whiff of nitrous at the track and live to tell the tale for a few years. No other transmission in the diesel truck segment has proven capable of doing this—except for the newest TorqShift on the market (keep reading).

Boiling Point: 600-rwhp (but will survive an occasional blast of 700 to 800-rwhp)

5. 6R140 TorqShift (’11-Present)


The definition of rugged, the six-speed 6R140 TorqShift that debuted behind the 6.7L Power Stroke in 2011 has proven even stronger than the second generation 5R110. By the way, the “140” in 6R140 is said to be code for “1,400 lb-ft,” the unit’s factory torque capacity. So even though Ford Super Duty’s are now leaving the assembly line packing 925 lb-ft, there is quite a bit of room for error built into the heavy-duty slushbox underneath them.


It’s no secret that Ford’s primary competition in the diesel-specific transmission segment is the Allison 1000. And as such, its 6R140 TorqShift rivals the reknowned, commercial-duty A1000 in many ways. Its input shaft measures 1.18 inches (vs. 1.26 inches on the Allison). It’s output shaft measures 1.61 inches (vs. 1.75 inches on the Allison). Even its overall dry weight mimics the iconic A1000 (325 pounds vs. 330 pounds for the Allison). A one-piece bell housing certainly won’t lend itself to any sort of flex, either. It’s no wonder these transmissions are living, trouble-free, in countless late-model Super Duty’s clearing 650 to 700 hp at the wheels.

Boiling Point: 650 to 700-rwhp

6R140: A Severe Duty Transmission


You’ll also find the 6R140 in Ford’s medium-duty segment. In fact, it’s the only transmission option available for F-650 and F-750 trucks. How’s that for confidence in its six-speed automatic?

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