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How To Pick The Right Transmission For Your 5.9L Cummins

Written By, Mike McGlothlin

 

So you’ve done your homework and scored a 5.9L Cummins for your project vehicle. But what about the transmission tasked with handling the abundant low-rpm torque the inline-six turns out? For a factory Chrysler automatic—a logical choice for budget friendliness and ease of integration with the 5.9L Cummins transmission—it’s a burden that may be too much to bear. For anyone seeking utmost simplicity and solid bang-for-the-buck in the durability department, Big Bear Engine Company recommends the NV4500 five speed and NV5600 six speed manuals. Then there are the budget-blowers: the built automatics with billet internal parts and the Allison swaps.

This time, we’ll discuss the myriad of transmission options facing a 5.9L Cummins owner, even the slushboxes used in GM and Ford applications, and help you sort through them. In the end, you’ll find that transmission selection boils down to two overriding things: 1) the intended use of your vehicle and 2) the amount of money you’re willing to spend. Either way, expect to perform several reinforcement modifications on an automatic or add an appropriately rated aftermarket clutch to the manual transmission you choose. The most commonly chosen transmissions for the job are detailed here.

It Boils Down To Your Specific Needs

Choosing the right transmission to bolt behind your 5.9L Cummins should always be based on your individual needs, driving style and according to how you’ll be using the vehicle. Daily drivers, vehicles used regularly in stop-and-go traffic, and drag racing applications all play into an automatic transmission’s strong-suits. On the other hand, those that are towing, off-roading, and truck pulling tend to prefer a manual shift transmission. Of course, there are manual die-hards as well as automatic die-hards—and either type of transmission can handle the job—but certain situations tend to make one style shine more than the other.

Pick A Transmission That Fits

If it fits it ships, right? The same holds true when pairing a transmission with your Cummins. Unless you’re in the midst of a ground-up or complete, one-off build, you likely don’t want to add transmission tunnel work to your task list. It’s important not to try to reinvent the wheel here—stick with a transmission you know will fit (with ample clearance) in the recipient vehicle. You can complicate matters tremendously by attempting to shoehorn a larger automatic (such as an Allison 1000, for example) into a transmission tunnel that was originally designed to accommodate a much smaller unit, such as an A727, 46RH, 47RH/RE or 48RE, for example. As a general rule of thumb, when a 5.9L Cummins transmission fits under your vehicle’s hood, a Chrysler automatic or Dodge-specific New Venture manual will likely fit behind it.

Mandatory Chrysler Four-Speed Mod #1: Upgrade The Converter

It’s important to know that the guts of the Chrysler 46RH, 47RH, 47RE and 48RE four-speed automatics are very similar—and they all fail in similar fashion when pitted against the torque of a Cummins. First and foremost, the factory torque converter will have to be upgraded. The OEM torque converter clutch is a weak link in every one of these transmissions, but it’s also stalled extremely high. From the factory (and depending on the specific model year), stall speed ranges from 2,300 to 2,700 rpm on ’89-’07 Dodge trucks. This was done in an effort to limit the amount of low-rpm torque being sent through the transmission (and slipping allowed this to be possible). A lower stall (1,700 to 2,200 rpm) triple disc converter allows you to tap into the 5.9L’s vast torque curve, which starts at very low engine speed, a feature emphasized by Big Bear Engine Company in their 5.9L Cummins transmission models.

A518/46RH, 47RH, 47RE, 48RE Gear Ratios

First: 2.45:1

Second: 1.45:1

Third: 1.00:1

Fourth: 0.69:1

*Reverse: 2.21:1

* = 2.35:1 on A518/46RH

Mandatory Chrysler Four-Speed Mod #2: Upgrade The Input

Whether you’re talking about the 46RH, 47RH, 47RE, 48RE or even the A727 three-speed, the factory input shaft is an infamous weak link. It can twist or (worse) snap at power levels as low as 400-rwhp (roughly 800 lb-ft of torque). So basically, if you’re planning on adding any extra power whatsoever to your 5.9L, upgrade the input shaft. It’s worth noting that 400 hp can be achieved with relative ease on ’94-’07 engines—and isn’t beyond the realm of possibility for the early, lower horsepower (and VE-pumped) ’89-’93 engines. Even if you plan to tow heavy with a 350hp 5.9L Cummins, opt for the added insurance that comes with a stronger, billet input shaft. Remember, the torque converter and input shaft are the first components to see the brunt of the Cummins’ immense torque production.

Further Mandatory Four-Speed Mods

It goes without saying that if you’re electing to place a Chrysler automatic behind the 5.9L Cummins you should engage in a full-on transmission build, complete with the proper reinforcements in place. A higher-pressure valve body (higher line pressure allows for more clutch-holding capacity) and a host of stronger material replacement parts—including but not limited to a billet 1-2 shift band apply lever, a billet servo, a billet strut and anchor, a billet accumulator and increased clutches in the direct drum, to name a few—are all highly recommended if any sort of added power is in play. At the 400rwhp mark (again, a horsepower range that’s easily obtainable), billet or oversize bands should also be on the table.

Five-Speed Manual Option: NV4500

If cost and simplicity are your thing, you can’t go wrong with the New Venture Gear 4500. Better known as the NV4500, it’s a heavy-duty, synchronized five-speed transmission and was offered behind the 5.9L Cummins in Dodge Rams from 1994 to 2005. It’s important to note that the NV4500 used in diesel and V-10 equipped Dodge Rams was a heavier duty version than what was used in GM trucks (it boasted a larger diameter input shaft and output shaft). This, along with its cast-iron case, made it highly durable—even in high torque applications. It’s a favorite among countless sled pullers, off-roaders and truck owners who spend a lot of time hooked to a trailer.

NV4500 Gear Ratios

First: 5.61:1

Second: 3.04:1

Third: 1.67:1

Fourth: 1.00:1

Fifth: 0.75:1

Reverse: 5.61:1

Six-Speed Manual Option: NV5600

Like the NV4500, the New Venture Gear NV5600 offers a single overdrive. However, thanks to its addition gear it doesn’t have the steep ratio drop-off the NV4500 does between First and Second, Second and Third, and Third and Fourth. When heavy towing, especially with a 5.9L Cummins transmission, this makes the NV5600 the better candidate for keeping big loads moving. And thanks to its larger, beefier internals, the cast-iron case, aluminum bellhousing six-speed gearbox outweighs the NV4500 by a whopping 165 pounds (360 pounds vs. the NV4500’s 195 pounds). The NV5600 was first placed in service in Dodge Rams for the ’01 model year, behind the high output version of the 24-valve ISB Cummins. It was in use in Rams through the ’05 model year.

NV5600 Gear Ratios

First: 5.63:1

Second: 3.38:1

Third: 2.04:1

Fourth: 1.39:1

Fifth: 1.00:1

Sixth: 0.73:1

Reverse: 5.63:1

The Budget-Friendly, Infinitely Durable Choice: NV4500 Manual

If money is tight and you don’t mind shifting your own gears, the NV4500 arguably provides the best all-around package for 5.9L Cummins transmission systems. There are no torque converter or valve body-related issues to iron out in order to enjoy reliability, and thanks to being offered in tens of thousands of trucks from the early 1990s to the mid-2000s parts are easy (and fairly cheap) to come by, as noted by Big Bear Engine Company. For higher horsepower applications, an input shaft upgrade (1-3/8-inch diameter vs. 1.25-inches stock) and a reputable dual disc clutch will be your only substantial upgrade requirements (as opposed to spending several thousand dollars on a performance 46RH, 47RH, 47RE or 48RE automatic). The NV4500’s one drawback is that the fifth gear lock nut is prone to backing off, which renders overdrive unusable but otherwise doesn’t trash the rest of the gearbox.

Allison Swap

One of the more alluring names in the diesel pickup segment is Allison, and the 1000 series Allison automatics that came in ’01-’19 GM HD trucks are often thought of as the answer to Cummins-related transmission problems. On paper, the Allison 1000 swap looks great. It’s big. It’s robust. And its market saturation means one can be found anywhere. But, believe it or not a factory Allison won’t hold up to much additional grunt from the 5.9L either. Couple that with the fact that the Allison, a clutch-to-clutch transmission, doesn’t like to shift fast and you quickly realize it’s not the best candidate when performance and speed is the goal. Then to add insult to injury, integrating one gets expensive in a hurry. You’ll need a transmission control module, the proper wiring harness, and a Cummins-spec’d torque converter, as well as the appropriate adapter plate and flex plate to make it all work. To be sure, it can be done (and done well), but the cost of getting it right usually turns most people away.

A1000 Gear Ratios

First: 3.094:1

Second: 1.809:1

Third: 1.406:1

Fourth: 1.00:1

Fifth: 0.711:1

*Sixth: 0.614:1

Reverse: 4.48:1

* = 2006-2019 models

TorqShift Swap

It may not sound as glamorous as an Allison swap, but Ford’s 5R110W TorqShift automatic can be made to work behind a 5.9L Cummins. And because Cummins conversions are so popular on the ’03-’10 model year Super Duty’s (the years the 5R110W was used), a lot of folks retain them when they ditch the 6.0L or 6.4L Power Stroke. Similar to the Allison, a specific adaptor plate, flex plate, and a different stall converter has to be sourced for the 5.9L Cummins transmission. And the TorqShift is also fully electronic, so measures will have to be taken in order to fine-tune and control its shift points. This can be done via stand-alone controller, but we’ve also seen it accomplished using HP Tuners software in conjunction with the factory Ford transmission control module and powertrain control module still in the mix.

5R110W TorqShift Gear Ratios

First: 3.11:1

Second: 2.20:1

Third: 1.54:1

*Fourth: 1.09:1

Fifth: 1.00:1

Sixth: 0.71:1

Reverse: 2.88:1

* = Fourth gear is only employed during Ford’s cold shift schedule (1-2-3-5-6 is the normal pattern)

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