Mazda MX-5 NA/NB ZF 8HP Swap: why we are developing a complete conversion kit

Years ago, we first encountered the ZF 8HP. What particularly stood out was how smoothly it shifted during gentle driving and how quickly it responded as soon as power was requested. When this transmission was subsequently used more and more frequently for swaps in BMWs, Volvos, and other project cars, the idea stuck.

We have now completed a ZF 8HP swap on a Volvo ourselves. During that build, we not only saw how well the transmission can work, but also how much time goes into everything surrounding the transmission. An adapter plate alone does not make a complete conversion.

Around the same period, we were working on our own Mazda MX-5 and that of a befriended engineer. Both cars are now being used as test platforms for this development. The original six-speed gearbox can become a limiting factor with more power, while the ZF 8HP offers a nice combination of strength, comfort, and fast shifting moments.

In addition, less able-bodied MX-5 drivers occasionally visit our workshop. Some have had to search for a very long time for an automatic MX-5. This made it clear to us that this conversion is not only interesting for powerful turbo builds, but also for people who need an automatic or simply seek more comfort.

This is how the idea for the most complete possible ZF 8HP conversion for the Mazda MX-5 NA and NB came about.

Not just an adapter plate, but a package with which the transmission can actually be neatly mounted in the car.

Why a ZF 8HP in an MX-5?

The ZF 8HP45 and 8HP50 are interesting for this application because they are relatively compact. Our research showed these were the only 8HP variants that could be fitted into the transmission tunnel of an MX-5 NA/NB without major body modifications.

For the first test phase, we are using two Mazda MX-5 NAs: a 1.6 and a 1.8. One of the cars is being built as a turbo build with approximately 250 hp.

We are testing both a ZF 8HP45 from a BMW with an N20 engine and a ZF 8HP50. The 8HP45 is controlled with a MaxxECU Race. For the 8HP50, we are using a MaxxECU Race in combination with a CANTCU.

The test cars are initially built for street use. It is precisely there that the conversion must not only shift well at full power, but also function pleasantly at low speed, in traffic, and during longer journeys.

The eight gears also provide a longer overdrive than the original MX-5 gearbox. This allows the engine speed to remain lower on the highway.

For track, drag, and other competitive applications, the speed and repeatability of the shifting moments are particularly interesting. With manual operation or paddles, the driver needs to pay less attention to clutching and shifting and can focus on braking, steering, and the ideal line.

Our frustration with 'plug-and-play'

Many swap kits are sold as plug-and-play, while in practice a surprising amount still needs to be adapted.

A bracket doesn't quite fit. Lines need to be rerouted. An essential adapter is missing. The driveshaft still needs to be fully custom-made, or there's no neat solution for the wiring harness, cooling, or selector lever.

We wanted to approach this differently with this kit.

 

Our starting point is to clearly indicate what is included in the kit, what is optional, and what the customer still needs to arrange themselves. The components must also be designed as one system. A good transmission mount has little value if it means fuel lines have to be moved or there is no room left for the exhaust.

The intention is that both an experienced hobbyist and a professional workshop can install the conversion. Those who prefer not to do the conversion themselves can also have us install it later.

First, digitally record the entire car

The space around the ZF 8HP is limited. Therefore, we didn't start with just manual measurements and immediately drawing parts.

We scanned, among other things, the inside and outside of the transmission tunnel, along with the ZF 8HP transmission, the original BMW selector, the Mazda PPF, the oil cooler connection, the output flange, and the front subframe.

With this scan data, we can accurately position the parts relative to each other in CAD.

In doing so, we don't just look at whether something physically fits. We also consider the fuel and brake lines, exhaust space, assembly sequence, wiring harness attachments, ground clearance, and the angle of the entire drivetrain.

Counting all 3D-printed test parts, dozens of prototypes have now been made. Two versions of various physical parts have been produced that can actually be used on the test cars.

 

Starting the gearbox mount again

The gearbox mount has demanded the most development work so far.

The mount had to use the original ZF 8HP rubber mounts, without running over the fuel and brake lines. At the same time, sufficient space had to remain for the exhaust, and we wanted to prepare the construction for a future protective plate for the transmission oil pan.

The first version was already quite elaborate when it became clear that the fuel and brake lines would have to be relocated.

Technically, that would have been possible, but it didn't align with the kit's objective. Therefore, we decided to redesign the entire mount.

The current construction consists of multiple individually designed components. Each component has its own function or is shaped in a specific way for a particular reason.

It takes more time to develop this way, but ultimately it yields a much cleaner solution. The original lines can remain in place, and the mount integrates better with the car's existing structure.

It should look like it belongs there

This conversion is not just about whether the transmission fits under the car.

We want to retain as many original mounting points as possible, for example, for the wiring harness. Mounts should not unnecessarily run over lines, and existing components should only be modified when absolutely necessary.

We try to extend that line into the interior as well. The original BMW selector is mounted in the original position of the MX-5 shifter. The mounting solution simultaneously provides a neat seal for the transmission tunnel.

For the oil cooler lines, we encountered a similar problem. The lines are standard on the same side as the exhaust, while there is hardly any space available there.

Existing oil cooler adapters directed the lines in a direction that was not practical for this application. Therefore, we developed our own adapter that routes the lines upwards and over the transmission. This creates more space for the exhaust and keeps the lines further away from the heat source.

The PPF is partially retained

The original Power Plant Frame connects the gearbox to the differential and helps align the drivetrain.

In our opinion, the ZF 8HP is too heavy to be connected to the original PPF in the same way. However, we also did not find a complete standard PPF delete ideal.

Therefore, we opted for a compromise. The connection to the transmission is omitted, but the PPF is still used to support and align the differential. The original mounting points for the wiring harness are also retained.

In addition, the angle of the differential can be adjusted to approximately 0.4 degrees. This allows the drivetrain to be precisely aligned during assembly.

Precise alignment is particularly important during road tests. The combination of gearbox mount, PPF solution, and driveshaft will only be released once the drivetrain functions vibration-free.

Below is a photo during the prototyping of our PPF and gearbox mount. We use 3D prints reinforced with plasma-cut steel plates for this.

Drive-by-wire as part of the conversion

To fully utilize all functions of the ZF 8HP, a drive-by-wire conversion is necessary.

For this, we have developed an accelerator pedal adapter and a throttle body adapter. The throttle body adapter consists of two parts. This allows the electronic throttle body to be placed directly on the intake manifold.

With many existing adapters, the throttle body is positioned at a noticeable angle. Our solution is slightly more complex, but it ensures a neater alignment and an appearance that better matches the rest of the engine bay.

A driveshaft that can also be maintained

The driveshaft is a difficult component in many engine and gearbox conversions. It often has to be completely custom-made, and it is not always clear later which parts were used.

For the ZF 8HP45, we are therefore developing our own output flange that can accommodate common driveshaft flanges.

The complete kit comes with a suitable driveshaft. This is suitable for up to 1,000 Nm and equipped with greasable, replaceable universal joints.

The original MX-5 driveshaft does not have serviceable universal joints. In case of wear, the entire shaft often has to be replaced. With our solution, the wear parts can be maintained or replaced individually.

Exhaust and ground clearance

The original MX-5 exhaust no longer fits next to the ZF 8HP. Therefore, an adapted exhaust is always required for this conversion.

A round 2.5-inch exhaust is possible within the available space. A 3-inch exhaust is also possible, but requires a compact custom solution. This will probably involve an oval section at the height of the transmission.

In the future, we may offer our own exhaust solution. In the first phase, the exhaust will be developed as a separate option.

To ensure the transmission oil pan is roughly level with the bottom of the frame rails, the engine is raised by approximately one centimeter.

For extremely low MX-5s, we advise against this conversion. The available ground clearance may simply become too limited.

With supercharger configurations, extra attention must be paid to the space under the bonnet. If the supercharger is already close to the bonnet, the engine may not be able to be raised further. In that case, the transmission oil pan will be slightly lower.

What will ultimately be in the kit?

The exact packages will only be finalized after road testing.

The intended basic package will likely consist of the gearbox mount, adapter plate with flywheel, and our modified PPF solution.

Various options can be chosen around this, including:

• drive-by-wire;
• selector mount with tunnel seal and shift boot;
• ECU and transmission controller;
• oil cooler adapter, lines, and cooler;
• driveshaft with output flange;
• wiring;
• and possibly a complete exhaust solution later.

The driveshaft will be part of the complete kit. The ECU and transmission controller can be supplied by us, but the customer remains free to choose a suitable combination themselves.

A fully custom wiring harness will likely follow later. For the first cars, we are making the necessary wiring specifically for the test configurations.

All vehicle-specific parts are designed by LucaCarMods. We source the adapter plate and flywheel solution from an external manufacturer.

Where are we now?

Most vehicle-specific parts have now been designed.

The skid plate, oil cooler bracket, fan shroud, oil lines, wiring, and exhaust are currently still in production for the test cars.

Full assembly is planned for September. If that phase proceeds without major issues, we aim to start the first road tests in September/October.

We will primarily focus on:

• driveline vibrations
• transmission and differential alignment
• shifting behavior
• ground clearance
• temperatures
• noise
• and daily use.

Some supporting parts may become available earlier. However, the gearbox mount, driveshaft, and PPF combination will only be sold after the road tests have been successfully completed.

The complete kit will be introduced as soon as the drivetrain functions properly and is vibration-free. We do not want to name a fixed release date before this has actually been demonstrated.

We'll keep you updated on the progress

In the coming period, we will share more about the individual components and the choices made during development.

Among other things, the 3D scanning process, the six-part gearbox mount, oil cooler adapter, PPF solution, selector, drive-by-wire, and driveshaft will be covered in more detail.

After that, of course, the full assembly and the first road tests will follow.

On our central project page you will find the current compatibility, available parts, and the latest status of the Mazda MX-5 NA/NB ZF 8HP conversion.