Why the ZF 8HP Is Perfect
for Your E9x LS Swap

This is everything we've learned running the ZF 8HP behind an LS in an E9x or E8x, put on one page. It covers which donor transmission to buy, how the adapter and converter go together, mounting and the driveshaft, cooling, wiring, and the one-time TCU flash.

Why Choose the ZF 8HP?

The 8HP runs eight speeds, from a deep 4.70 first down to a 0.67 eighth, which is a spread of just over 7:1. That range is wide enough that the same transmission leaving hard off the line will also hold the highway at under 3,000 rpm, where an older three or four-speed automatic has to give up one end to get the other. Factory shift times are around 200 milliseconds, and on the paddles it feels a lot closer to a dual-clutch than to a classic automatic.

ZF rates the 8HP70 at 516 lb-ft from the factory, but a healthy stock one will happily live in the 600 to 800 lb-ft range, and a built transmission goes well past 1,000, so you have a lot of room before power is the thing holding you back. Running it on MaxxECU also gives you a transbrake, launch control, kickdown, and shift points as high as 9,200 rpm on the factory mechatronics.

The 8HP fits into the E9x tunnel with our mount and crossmember without any modifications. And because ZF built these by the millions for BMW, Dodge, and Jaguar Land Rover, donor cores and service parts stay cheap and easy to find. For a street and track BMW that is a hard combination to beat: a modern eight-speed with great torque capacity at an affordable price.

Picking Your Donor

Two things decide whether a donor will work before you look at anything else, and the first is the generation. MaxxECU only controls the Gen 1 8HP45, 8HP70, and 8HP90, so the Gen 2 units (the 8HP50, 8HP75, 8HP95, and Chrysler's 850RE) are not an option with MaxxECU. Some applications moved to Gen 2 around 2019, the new-body Ram being the obvious one when it went to the 8HP75, while plenty of others stayed Gen 1 for years, so read the model tag on the left rear of the case before you pay for anything.

The second is the case itself. The bellhousing is cast into the main case and every manufacturer used its own bolt pattern, so a BMW or Jaguar 8HP70 won't bolt up to any of the LS adapter kits, which are all built around the Dodge Hemi pattern. What you want is a Dodge-sourced transmission specifically, not just any 8HP70.

Even within the Dodge family, the cases differ:

A couple of smaller things apply to any donor. The torque converter pilot changed partway through the run, with 2013 to 2017 units using a 34 mm small pilot and 2018 and up using a 46 mm large pilot; the adapter kits come with pilots for both, though aftermarket converters like Circle D are built around the smaller 34 mm size.

Whatever you end up with, it's best to buy a running-condition pull with reasonable miles and service it on the bench before it ever goes in the car: pan, filter, fresh fluid, and a new seal on the mechatronics sleeve while everything is easy to get to.

What You'll Need

Adapter, Flexplate, and Converter

The adapter kit is what mates the Dodge-pattern 8HP to the LS. It bolts the transmission case to the block and adapts the factory Dodge torque converter to your flexplate. The Dodge converter stays, and the kit's converter adapter bolts it to a standard GM flexplate.

Most of the fitment comes down to matching a few parts correctly. Any OEM or aftermarket LS flexplate works as long as it's one of the common patterns: 3-bolt 10-3/4 inch, 3-bolt 11-1/16 inch, or 6-bolt 11-1/2 inch. The converter pilot follows the same 34 mm and 46 mm split from above, with pilots for both in the kit and a little work needed on the 2018-and-up converters. Starter fitment depends on the donor case, with the Ram and Durango cases taking smooth-nose LS starters directly and the Charger and Challenger cases needing a little trimming or an unshrouded high-torque starter. The kit comes with dowel pins for both sides plus the right hardware and threadlocker.

While the transmission is out, there are a couple of clearance items worth doing. Grinding the unused bolt bosses off the bellhousing buys you tunnel clearance, and on a naturally aspirated car with long-tube headers you can take the left starter bubble off for header room.

Mounting and Driveshaft in the E9x

The transmission mounts from our LSe9x DCT/8HP crossmember with the Guru billet upper mount bracket on the transmission side, and that bracket takes either a GM TH350/TH400-style mount (the Energy Suspension 3.1108G poly mount) or BMW-style bushings, so you can set it up to match the rest of the car.

The output side is where the donor case comes back into it. The 8HP70 output is 43-spline, and the Ram truck flange takes a 1350 U-joint directly using the OEM Dodge 1350 flange (part 68417915A), while the Durango and Charger flanges need an adapter to get there. If you'd rather keep the stock BMW driveshaft, you swap the output flange off a BMW 6HP automatic onto the 8HP and the factory shaft bolts right up. Going custom, run a single-piece shaft with a 1350 joint and only measure for length once the drivetrain is sitting at final ride height on torqued mounts; our driveshaft guide covers how to measure. Rear gear choice ties straight into the 8HP's output shaft speed ceiling, which is covered in gears, tires, and output speed below, with a calculator for your own setup.

Gears, Tires, and Output Speed

Wheel speed is really just a chain of three numbers. Engine rpm divided by the transmission ratio gives you driveshaft rpm, that divided by the rear gear gives wheel rpm, and wheel rpm times tire circumference gives you speed. Here are the Gen 1 8HP70's ratios (the 8HP90's are within a hundredth or two):

That deep 4.70 first gear changes how you should think about rear gear. On 3.08s an 8HP already launches at a 14.5:1 overall ratio, where a 4L60E on 3.91s comes out to 12.0:1 and a TH400 on 4.10s to 10.2:1. Steep rear gears earned their reputation behind three and four-speed automatics that needed the help off the line, but the 8HP already has that multiplication built into first, so on this transmission a numerically high diff mostly trades away top speed for launch gearing you were going to have anyway.

Where the eight speeds really shine is between gears. The ratio steps are small, so each upshift in the upper gears only drops the engine 1,000 to 1,500 rpm and the motor stays right in its powerband through a whole pull, where a four-speed like the 4L60E gives up more than 3,000 rpm on the 1-2 shift alone (where converter slip is very important for a good combo). Overlay one in the calculator below and you can see it as a much finer sawtooth. The double overdrive is the other half of it: on 3.08s the 8HP is turning just under 2,000 rpm at 70 mph, where a TH400 on 4.10s would be spinning close to 3,900 at the same speed.

For some reference numbers: Dodge gears the Hellcat's 8HP90 with 2.62s and still runs it to 203 mph, the F90 M5 tops out on a 3.15, and even the 700-horsepower Trackhawk carries only 3.70s, every one of them leaning on the deep first gear instead of a steep differential. That's the same logic that makes a 2.81, 3.08, or 3.15 the natural fit in an E9x.

With gearing this deep, first gear is optional in street driving. MaxxECU has a gear 1 lockout input that starts the transmission in 2nd and never lets it drop into 1st, which is an easy way to calm the launch down if you've ended up with a numerically higher differential. Even locked out of first, an 8HP on 4.10s still leaves in a 12.8:1 overall ratio.

Top speed is the one place you need to pay attention, because the driveshaft turns at output shaft speed and that shaft has a ceiling. The output bearing is rated to 8,500 rpm and the factory Dodge TCU starts its output-speed protection just above 8,100, but treat those as the absolute limit rather than somewhere to spend time: the only documented output-bearing failure we've been able to find came from repeated 175 mph pulls at roughly 8,400 output rpm.

For anything sustained, keep driveshaft speed at or under 6,500 to 7,000 rpm. That's about where the factory 8HP cars sit at their own top speeds (a Hellcat Charger is turning around 6,100 rpm at 203 mph on its 2.62s, and the F90 M5 around 7,200 at 190), and it's also where most driveshafts run out of critical speed anyway, since a 50-inch 3-inch mild-steel shaft is only good for about 6,000 rpm and a 3.5-inch chromoly one for about 7,100. Because driveshaft rpm is wheel rpm times the rear gear, a numerically higher diff reaches those numbers at a lower road speed, so on a typical E9x tire a 4.10 hits 7,000 output rpm around 130 mph while a 3.15 doesn't get there until about 165. The short version is not to gear the car past where you actually plan to use it.

A taller tire moves the same math in your favor, lowering rpm at any given speed and raising the road speed where the output shaft tops out.

For the factory-style BMW differential, the 3.08, the diesel 2.81, and the M3's 3.15 are the ratios that pair well with the 8HP, and if you're running a Ford 8.8 rear, a 3.08 is the one to use.

Gearing Calculator

Put in your tire, rear gear, and shift rpm. The gold trace is what the engine does accelerating through the gears at your shift rpm, the faded lines are each gear on its own, the table shows the shift drops, and the two vertical lines are the output shaft speeds for your setup: amber for the 7,000 rpm recommended ceiling, red for the 8,500 rpm bearing max. You can also overlay another transmission's trace at the same rear gear, tire, and shift rpm; that comparison is gearing only. There's a standalone version of this calculator that runs any transmission, including your own ratios. All speeds assume a locked converter with zero slip. On the 8HP that's most of the drive: the lockup clutch closes early, typically by 2nd or 3rd gear in normal driving, and stays locked through shifts, so outside of launch these numbers are what the car sees.

Shifters

The 8HP is fully shift-by-wire, so there's no shift cable to run; the shifter only sends CAN messages, which leaves the choice fairly open.

The Dodge shifter (part 04670717AB) plugs straight in with our Dodge shifter cable and is the natural match if you started with a Dodge donor. If you'd rather keep the BMW interior looking factory, the F-series GWS electronic shifter from F2x/F3x/F8x cars (61319296896) or F0x/F1x cars (61319296904) works with the BMW shifter cable, and our shifter mount sits flush with the OE trim in the E9x console. You don't strictly need a physical shifter at all, since MaxxECU's shifter emulation can run P/R/N/D off a CAN keypad or plain switches, and there's a sequential option as well. Whichever way you end up selecting gears, you can wire the factory BMW paddles (or any buttons) to MaxxECU GP inputs and turn on shift-up and shift-down.

Wiring and the TCU Flash

The factory TCU stays bolted to the transmission and MaxxECU commands it over CAN. Our universal 8HP swap harness handles the physical side: a DT 6-pin main connector for switched 12V, ignition, ground, and CAN high/low, a pre-terminated TCU plug, a 4-pin shifter breakout, and an OBD breakout with a PT-CAN tap for flashing and diagnostics.

The flash itself is a one-time reflash of the factory TCU with MaxxECU's firmware, done on the bench (or in the car) with the ACDP-2 tool (rent ours or buy it). Once you're set up it's about a fifteen-minute job: read the TCU, send MaxxECU the identification, then flash their model-specific files in order. Two things to keep in mind, both straight from MaxxECU, are that the firmware is matched to the exact TCU (so check the Bosch part number before buying a donor if you can) and that a flashed TCU only works with MaxxECU afterward, so it won't run behind a stock Dodge or BMW computer anymore.

Frequently Asked Questions

Guides & Resources

• MaxxECU official 8HP documentation (wiring, flashing, transbrake, every setting)
• Guru Autowerks adapter FAQ (donor identification, starters, converters)
• Transmission section of the LS swap parts list
• ECU comparison: Holley vs MaxxECU vs Haltech
• MaxxECU terminated harness install guide
• Driveshaft basics for E9x swaps
• LSE90 Garage on YouTube (build videos for these swaps)

Ready to 8HP-swap your BMW?

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