How to Swap an 8HP Transmission into Your Chevy

The days of settling for old 3 or 4 speed automatics are over. Swapping a modern 8 speed automatic into a classic Chevelle, Camaro, or C10 truck, completely changes the vehicle.

We were surprised by how much we liked the 8HP in our 1986 Chevy C10 truck. The power in first gear sets the tires on fire instantly. Then cruising even with loud exhaust is now quiet due to the many gear shifts to keep the rpm low at all times.

When you want to party, the transmission drops the gears and is constantly in a peak power band with lightning fast shifts. Who could ask for more? 

Below is what we learned from our own swaps that will make installing an 8HP into your ride very simple.

What is a ZF 8HP?

ZF is the manufacturer name and 8HP means 8 forward gears with Hydraulic converter and Planetary gearset. For short we just call it 8HP.

These transmissions were found behind 3 to 12 cylinder engines, in a range of makes including: VW, Alfa Romeo, Land Rover, Dodge, Jeep, Porsche, BMW, Audi, Jaguar, Bentley, Aston Martin, Rolls-Royce and more. These transmissions are very reliable and are underrated for the power they can hold, thus why so many OEMs use these.

Model Numbers

To understand their power ratings, let's use, for example, an 8HP90 transmission found in a Dodge Challenger Hellcat. The numbers after 8HP (45, 50, 70, 75, 90, 95, etc) is the torque rating measured in Newton meters x10 (daNm).

An 8HP90 holds 1000 Nm which converts to 738 lb-ft of torque. Now knowing this torque rating for a Hellcat car, it seems awfully low for cars we've seen turned way up and not having any transmission issues.

What Parts to Use?

To eliminate a lot of variables, we are going to recommend the most common parts, which are the parts we use on our cars, the parts that we stock and sell because they work the best. There are endless possibilities out there of donor car models with each adding its own variables such as bellhousing patterns, driveshaft flanges, transmission cooler line fittings, case sizes, transmission mount bolt patterns, etc.

Regardless of which route you go, the cost is nearly the same. The biggest cost is in the standalone controller (TurboLamik $2500, PCS, Maxx), then the bellhousing adapter w/ flexplate (LOJ $1500 USA made for example), then the transmission itself and finally the driveshaft.

If you want to budget swap an 8HP you will need to chop the money out of one of these 4 areas with the trans itself being the biggest opportunity if you can find a reliable source for a used trans and converter (see links at bottom for new Mopar trans).

We, along with most people, thought this swap was on the pricier side, but believe us it's more expensive to not have this in your life! Save a daily Starbucks and just buy the damn thing.

We recommend a transmission out of a Dodge vehicle due to the availability of those transmissions and more common bellhousing adapter options. We used a stock trans and converter from a Ram 1500 8HP70 in our C10 with an LS 6.0L engine and Hellcat supercharger. The 8HP70 holds nearly the same power as 8HP90 after a good rebuild (see link to reference https://www.puredrivetrainsolutions.com/zf-8hp-transmission-upgrades).

Next we recommend using a TurboLamik controller which allows transbrake amongst other features and is easy to tie into the CAN wiring on a GM ECM or popular Holley ECU (Dominator, TerminatorX) which we recommend since so many people use this combo and can tune it together, such as Andrew over at Dr EFI.

Transbrake

Because the 8HP features a massive 4.71 first gear, launching a high horsepower car in 1st gear instantly sets the tires ablaze resulting in a huge smile on our faces. If we were trying to be more serious we could get the tires hot and launch the car off the transbrake, which BTW is a default feature of the Turbolamik. It does this by applying all forward clutches at same time, thus holding the car in place.

To apply the transbrake with the car not moving. Move the shifter to the left (manual mode) then select the gear you want to launch from (yes ANY gear can be selected). Now pull the shifter towards you to activate and hold the brake on. If needed, tap the park button to bump forward like a bump box. When ready to launch, let go of the shifter and the transbrake shuts off and enjoy the ride.

Let's say you were driving in Mexico and wanted to get a good street launch, but the traction was not great. Well you're in luck because the TurboLamik lets you launch off the transbrake out of any gear, which we think is amazing. I recall a time I paid a few thousand dollars so I could launch my 4L80E out of 2nd gear.

Let's apply how we could use this to our advantage. If you launched the 8HP out of 3rd gear with 2.11 ratio this is similar to a TH400 and 4L80E first gear of 2.48. If you launched the 8HP out of 4th gear with 1.67 ratio this is similar to a Powerglide 1.76 ratio. This highlights how many more gears are in this transmission and how versatile it can be.

For the Drifters: Electronic Clutch Kick

Also in the TurboLamik, you can clutch kick (take the car out of gear, rev the engine, and dump the "clutch") to get the car going sideways. This can be done with a button or adding a switch on a fake clutch pedal. The Turbolamik will see the signal and release the transmission to rev the engine. This may not replace many manual transmissions, but we have already seen drift cars with 8HP's which is really cool to have more options for those guys, or those of us who hate tires.

Plumbing & Drivetrain Integration: Getting Power to the Axle

When installing a Dodge 8HP70/90 core into a classic Chevy layout, you have to account for the physical connections changing from an old GM slip yoke to a fixed position flange.

• Output Shaft: Dodge Ram truck cores (8HP70/8HP75) utilize a 4-bolt output flange with a center, single dowel pin to center the yoke on the transmission. Dodge passenger cars use a round flange to mate to a CV joint on the driveshaft.

• Driveshaft: Because a classic Chevy factory drivetrain expects a slip yoke sliding inside the transmission tailhousing to absorb rear axle suspension movement, swapping to a 8HP means you must add a collapsible slip section directly into the driveshaft.

  • The Simple Solution is to call driveshaftshop.com and tell them: Transmission model, year, application it came from, then driveshaft length, and your rear end flange type.

• Trans Fitting Block: Space is tight inside classic Chevy tunnels so we use a 90 degree transmission line adapter.

• Thermostat: A transmission fluid thermostat is needed since the 8HP needs to be at a proper operating temperature to work correctly.

• Filling the Fluid: This can be a pain, so let's walk through the ideal process. For a brand new or completely rebuilt trans:

  • First, add at least 1qt to the torque converter before installing into the transmission and trans into the car

  • Fully install the transmission

  • Raise the vehicle on level ground, use a pneumatic fluid extractor/dispenser that is filled with 10 quarts of Mopar 8&9 Speed ATF fluid

  • Remove the transmission fluid fill plug on the passenger side rear just above the pan

  • Install our TL-8HP-FILL 8HP fill tool, put the hose from the fluid dispenser onto the fill adapter and do an initial fill of the transmission until it comes out of fitting (this will be around 3 quarts)

  • Start the vehicle and with foot on brake, select reverse,then drive, bring trans fluid to over 100°F and continue to fill the transmission until full with the engine running in park. (total fluid with a stock converter and stock trans pan will be around 9 quarts if it was completely new and empty to start. For used units not completely drained this will take 5-6 quarts).

Parts Checklist