How to Complete the Restrictor Plate Challenge in Forza Horizon 6

[FrostPhoenix]

The Restrictor Plate upgrade challenge in the Discover Japan section of the Discovery Journal can be confusing at first. A lot of players open the upgrade menu, search through engine parts, and never actually see the Restrictor Plate option appear. The trick is that the upgrade only becomes available on certain engine swap combinations.

After testing a few setups, the easiest solutions are using either the Mazda RX-7 or the cheap MX-5 Miata build. Both methods work quickly, but one is faster while the other costs fewer Credits.

Why the Restrictor Plate Option Does Not Show Up

By default, most cars in the game simply do not support Restrictor Plate tuning. The option only appears after installing specific engine swaps that unlock additional racing engine configurations.

Once the correct engine is installed, the Restrictor Plate category becomes visible inside the Engine upgrades menu. You only need to install or modify the part once for the journal challenge to complete.

Fastest Method: Mazda RX-7 Build

The quickest and most reliable approach uses the Mazda RX-7.

Players usually recommend the Mazda Savanna RX-7 or the RX-7 Type R because the upgrade path works consistently.

Step-by-Step Setup
Buy or select a Mazda RX-7.
Drive to a Horizon Festival site or your player house.
Open:
Upgrades & Tuning
Custom Upgrade
Conversions
Engine Swap
Install the Racing 2.6L 4-Rotor engine.
Return to the main upgrade categories.
Open the Engine section.
You should now see the Restrictor Plate option.
Install any available setting, even “No Restrictor Plate.”

As soon as the upgrade is applied, the Discovery Journal challenge should complete instantly.

This method is popular because the 4-Rotor swap unlocks the option immediately without requiring additional tuning work.

Cheapest Method: MX-5 Miata Build

If you want to save Credits, the Mazda MX-5 Miata is the better choice.

The car costs only around 14,000 Credits from the Auto Show, making it one of the cheapest solutions for the challenge.

Step-by-Step Setup
Purchase the Mazda MX-5 Miata from the Auto Show.
Go to:
Upgrades & Tuning
Custom Upgrade
Conversions
Engine Swap
Install the Inline 4 Turbo Rally engine.
Return to the Engine upgrades category.
Locate Restrictor Plate.
Install or modify the part.

That immediately counts toward the Discovery Journal requirement.

Which Method Is Better?

The RX-7 route is easier if you already own one because the upgrade path is straightforward and widely tested by players.

The MX-5 route is better for newer players with limited Credits since the car is extremely cheap and still unlocks the challenge quickly.

In practice, both methods only take a few minutes once you know the correct engine swap.

Common Mistakes Players Make

A lot of players get stuck because they:

Search for the Restrictor Plate before swapping engines

Use the wrong engine conversion

Look inside aspiration upgrades instead of Engine upgrades

Try random cars that do not support the part

If the Restrictor Plate option is missing, the engine swap is usually the problem.

Once the challenge completes, you can keep the upgraded build or simply remove the parts later. The journal only checks whether you installed or modified the Restrictor Plate once, so there is no need to keep the setup permanently.

For most players, the RX-7 4-Rotor method is the fastest overall, while the MX-5 Miata method is the cheapest and easiest for beginners.