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Step up, Mopar guys. Step up! There’s a whole new world of Chrysler automatic overdrives out there that only a few seem to know anything about. The performance world is changing very rapidly now so we thought we’d venture into this brave new multi-gear world and report what we’ve found. The current approach for OE transmission builders is to keep adding gears and complexity, making this new breed of automatics a far cry from the days of A904’s and A727’s. It’s also about new ways of talking about transmissions so you’ll have to learn a few new alpha-numeric abbreviations with a few odd acronyms thrown in for good measure. But don’t worry – we’ll get you through it.
Among the original Mopar three-speeds is the venerable A727 automatic that was used behind some fairly strong RB big-blocks like the 440 wedge and the 426 Gen II Hemi. This trans used a 2.54:1 First gear ratio.
All Mopar fans know about the original three-speed automatics like the aforementioned A904 and A727. These transmissions should be familiar so we’ll start with them. Much like what other OE’s have done, the engineers began this evolution by renaming transmissions with a new system that still uses letters and numbers but in a more efficient fashion.
If you think about it, the term A904 is not really that informative. So with the new orientation, the A904 became known as a 30RH. The 3 represents the number of forward gears, the 0 is the lowest rating in torque capacity, the R is for rear wheel drive, and H means the transmission is hydraulically controlled. Using this concept, you begin to see how the A727 can be called a 36RH or 37RH with increased torque capacity.
We’ve included a chart that defines all the alphanumeric abbreviations in an attempt to make this a bit less baffling. Most of these are self-explanatory except for perhaps the difference between E as electronic and FE as full electronic. This was necessary because the earliest electronic efforts were aimed solely at adding an electronically controlled overdrive to the existing A727.
We’ll just briefly mention the A904 transmission here as the majority of the late-model transmission swap interest will be with the higher capacity transmissions. The A904 was also called the A500 for awhile and then rebranded again into the 30RH, 36RH, and then to the 40RH/42RH designations that were hydraulically controlled and used in 1990’s Jeeps and light duty trucks. Then came the 42RE and 44RE versions that were electronically controlled. These are an affordable overdrive transmission for stock small-block applications. The A727 briefly morphed into the A518 that then evolved into what became the 46RE that we’ll discuss next. These later transmissions featured an overdrive placed in the extension housing.
The next in this evolution was the 46RE, which is essentially electronic control over the previous 46RH or hydraulically controlled 727. This transmission employed what can best be described as partial electronic control over the transmission. The 46RE can be found in a number of different rear wheel drive Chrysler products from 1995 through 2002, and some early-production 2003 Dodge Ram pickup trucks that had the 5.9L Magnum V8. In its previous iteration, the 46RH was found in earlier vehicles from 1990 through 1998.
The 46RH and 46RE both use the same gear spread as the old A727 but with a 0.69 overdrive. This makes for a great highway combination since this 31 percent overdrive will soften a 4.10:1 rear gear down to the equivalent of a 2.82:1 rear gear ratio. Combined with a lockup converter, this would make an excellent street transmission.
The 46RE uses hydraulic control for the 1-2 and 2-3 shifts based on position of the throttle valve (TV) cable connected to the throttle body. This TV input establishes both load and mph-based shift points related to the position of the TV cable. The shift into overdrive is controlled electronically along with engagement of the torque converter clutch.
Because this transmission can be built to take quite a bit of abuse, it’s a reasonable choice as a swap both for all Mopar engines but especially behind a Gen III Hemi since Holley now offers the ability to control fuel injection with the affordable Terminator-X Max ECU that can easily manipulate the electronic side of the 46RE transmission with the adition of this adapter harness (558-473)
Note: 545RFE four-wheel-drive application pictured.
This next evolution in Mopar automatics expanded the four-speed concept in 2002 in Dodge Rams, Durangos and other vehicles by extending gear multiplication to five forward speeds. The RFE suffix tells us that this is one of the early Chrysler gearboxes to exert full electronic control for a rear wheel drive vehicle. Predating the 545RFE was the 45RFE that was a four-speed box that was normally paired with the smaller 4.7L V8 engines. We’ll concentrate on the five-speed version for this story as it was used behind the more powerful V8 Hemi engines.
The 545RFE is found in a wide variety of truck and SUV applications from 2001 to 2012 with Jeep Grand Cherokee and Jeep Commander models sporting the higher output Hemi engines. This trans can also be found in Dodge Durango models from this time period. The 545RFE uses three planetary gear combinations for added durability that drops the First gear ratio a little deeper with a 3.00:1 low gear, a 1:1 fourth gear, and Fifth gear at 0.67, which is a substantial 33 percent overdrive.
Many of Chrysler’s late model overdrive transmissions now spec what is called ATF+4 fluid, which is also compatible with other OE transmissions. However, because this area is so complex, it’s best to do your own research. For example, the 8HP90 uses a different and specific fluid.
One point worth making with this trans is the big drop in gear spread between First and second gear. This move from 3.00:1 to 1.67:1 is a full 44 percent, not unlike the big drop present in the GM 700R4/4L60E transmissions. While certainly not a deal-breaker, it’s worth noting if you are shopping for an overdrive trans and five forward speeds are attractive.
The biggest hurdle for retro-fitting one of these transmissions is that there is currently no stand-alone controller available that would allow a trans swap in an older Mopar. However, if your swap plans include using a stock Hemi from one of these vehicles, then the stock PCM and harnesses would obviously work just fine. The word on the street is that the 545RFE can be built to handle the power but is questionable behind a larger 6.1L or 6.4L Hemi. However, supporting a mild 5.7L Gen III Hemi this transmission should be fine for long-term use.
As a quick aside, there is also a 66RFE six-speed automatic that was used only in a two year period from 2012-’13. This is a spin-off of the 545RFE that uses a completely different set of gear ratios than the 545RFE. Since this transmission was used only for a short two-year period, we won’t spend any additional space on this iteration.
Here’s where Chrysler’s modern alphanumeric designation takes a small sidestep. The NAG1 designation stands for New Automatic Generation 1. This transmission when built by Mercedes goes by a different W5A580 alias. This decodes as “W” for a torque converter automatic, 5 forward speeds, “A” represents automatic, and 580 references 580 Newton-meters (N-m) for torque capacity (428 lb-ft). At least the Mercedes W5A580 is more informative. Just to muddy the waters even further, German transmission builder ZF Friedrichshafen also builds this transmission but calls it the 5HP30.
Moving past the what’s-my-name escapades, the NAG1 is a fully electronically controlled five-speed automatic that first appeared in 2005 in Dodge Magnum R/T as well as behind the 6.1L Hemi Charger SRT8 and later in the 2014 Charger SRT8 392ci Hemi. As of 2021, this box is still in production in vehicles like the Dodge Charger Pursuit law enforcement vehicles. This usage behind some clearly stout Hemi engines indicates that this transmission can handle the torque. The 6.4L / 392ci Hemi in the ’14 Charger SRT8 puts out 470 lb-ft of torque and 470 horsepower and the five-speed handles this no problem.
This transmission is fairly durable in stock configuration and certain companies are now building performance versions. For example, ShopHemi.com offers several performance upgrade options for this transmission for engines making a lot more power either normally aspirated or with a supercharger.
Of course, this transmission is also fully electronically controlled and the easiest swap solution is to utilize everything from the donor vehicle, though companies such as Sound German Automotive can assist in making the NAG1 (W5A580) work for your project. This sounds daunting but is an option for those willing to manage the Anaconda-sized bundle of wires to make it work. Many road test reviews hint that it might be worth the wait and cost to instead step up to the top rung of the Mopar transmission ladder – the new eight-speed.
Chrysler 8HP70 (Car)
Chrysler 8HP70 (Truck)
Here is where the engineers let it all hang out with a total of eight speeds forward. This ZF-designed transmission built by Chrysler takes the multi-speed gearbox up to the next level. The advantage of adding ratios between low gear and the final overdrive is to reduce the rpm drop between ratios. With more forward speeds, engine rpm remains within a narrow optimized power range for maximum acceleration. The definition of an engine’s power band is that rpm spread between peak torque and peak horsepower. As an example, the 2021 Dodge Chaarger SRT Hellcat Widebody Redeye is rated at 707 lb-ft of torque at 4,500 rpm and 797 horsepower at 6,300 rpm. This means the power band is between 4,500 and 6,300 rpm for an excellent 1,800 rpm spread. Often this power band is much narrower at 1,400 to 1,500 rpm.
Let’s take a look at just one example of rpm drop from 6,300 at the top of Second gear and its shift into Third gear with the 8LHP90 transmission. This represents a 32.9 percent ratio drop from 3.13:1 to 2.10:1. This means that the engine rpm will theoretically drop from 6,300 to 4,227 rpm which is just slightly below peak torque. This is one small reason why this car is so quick because the engine does not have to struggle with a huge rpm drop below peak torque.
As a comparison, an A727 will drop 43 percent from First to Second – a 2,800-rpm drop. This will push the rpm well below peak torque (as low as 3,500 – not taking into account converter slippage) and the engine will struggle (and take more time) to grunt back up into the power band that starts at 4,500 rpm.
The net gain here is the eight-speed narrows that engine speed drop at each shift. Just the addition of these gears will improve e.t. by perhaps as much as two tenths of a second versus a typical three-speed automatic. Plus, the 8HP70 /8HP90 include a pair of overdrive 0.84:1 and 0.67:1 ratios. The rear axle in these cars is 3.07:1 but when the transmission shifts into 8th gear, that effectively drops the final drive ratio down to an astounding 2.05:1 which puts cruise rpm on the highway at 70 mph just under 1,700 rpm. Yet the overall first gear ratio of this Hellcat combination is more aggressive than a 727 trans with a 5.13:1 rear gear! That’s one reason why these big heavy Mopars can launch so hard. They are applying serious leverage to the launch.
It should also be noted here that there is a difference in bolt patterns between the car-based 8HP-series transmissions and the truck-sourced 8HP-series transmissions, so be cautious and know what vehicle donated the transmission you are selecting for your build.
The all-wheel drive traction afforded by the 8HP95 is supported by a torque rating of 700 lb-ft.
Its worthy of note that the 8HP70 and 90 version transmissions are intended for rear wheel drive applications while the 8HP95 is used in the all-wheel-drive vehicles such as the Jeep Cherokee SRT Trackhawk, placed behind a Hemi making 707 horsepower and 645 lb-ft of torque. This Jeep even offers significant towing opportunities with a capacity fo 7,200 pounds in the 2021 model year version so the transmissions needs all of its 900 lb-ft of torque capacity.
This biggest issue with regard to using these late model all-electronic transmissions in an older vehicle like in a Pro Touring application is that we are not aware of an aftermarket controller currently available for these transmissions. Much of this has to do with the complexity of control that these transmissions require which makes designing a stand-alone controller extremely difficult. This obstacle may eventually be overcome in the future, but as of this writing the only practical approach is to use the factory engine and transmission combination from a late model donor vehicle complete with all wiring harnesses and powertrain control modules.
This conversion is not without its challenges since multiple inputs must come from other body control computers that may have to be accommodated. There is certainly plenty a host of excellent reasons to make that happen. A complete Demon Hemi / 8HP90 swap into an early A-body like a ’69 Dodge Dart would be eyeball-flattening quick and extremely fun to drive!
Example: 46RE = Four forward speeds, a torque capacity rating of 6, rear wheel drive, electronically controlled.
These are approximate weights for these transmissions with a converter. These numbers will vary based on converter size, so these weights may and probably will change based on that variable. All weights are in pounds.
