One of the more confusing upgrade objectives in the Discover Japan campaign in Forza Horizon 6 is the Restrictor Plate challenge. Unlike the usual “buy this part” or “install these tires” tasks, this one requires you to actually lower the power of a heavily upgraded engine.
A lot of players get stuck because they look for the Restrictor Plate option on a stock car and can’t find it. The game only makes the upgrade available after installing certain high-performance engine swaps first.
Here’s the fastest way I found to complete it.
How the Restrictor Plate Challenge Works
The objective is tied to the Discover Japan Collection Journal progression system. Instead of simply increasing performance, the game wants you to install a racing engine and then intentionally reduce its horsepower using a Restrictor Plate.
This mimics real motorsport regulations where race organizers limit engine output to keep cars within a specific class.
In practice, you’re basically:

• swapping in a very powerful engine
• reducing its power with a Restrictor Plate
• lowering the car’s PI rating to fit event restrictions

Once the setup is installed, the challenge stamp unlocks immediately.
Step-by-Step Restrictor Plate Completion Method
Go to a Festival Site or Player House
Head to any garage location where you can access upgrades and tuning. Any Festival Hub or owned house works fine.
Open the garage menu and select your car.
Choose a Car That Supports Racing Engine Swaps
This is the important part.
Not every car can use Restrictor Plates. You need a vehicle that supports large racing engine conversions. Sports cars, track-focused cars, and supercars usually work best.
Good candidates are cars that can swap into:

• racing V6 engines
• V8 race engines
• V10 performance engines

If the car only has mild upgrade options, the Restrictor Plate category probably will not appear.
Open the Custom Upgrade Menu
Go to:

• Upgrades & Tuning
• Custom Upgrade

From here, move to the Conversion tab on the far right.
Install a High-Power Engine Swap
Select Engine Swap first.
This step is mandatory because Restrictor Plates are tied to high-output engines.
Popular options include:

• 3.0L Racing V6
• large displacement V8 swaps
• V10 race engines

After confirming the engine conversion, return to the main upgrade categories.
Install the Restrictor Plate
Now go into the Engine category.
Scroll toward the end of the engine parts list and you should finally see Restrictor Plate options appear.
Choose one of the restriction levels. As you increase restriction, you’ll notice:

• horsepower decreases
• torque drops
• PI rating falls

That’s exactly what the challenge wants.
Purchase and Apply the Build
Confirm the setup and buy the upgrades.
As soon as the installation finishes, the Discover Japan Restrictor Plate stamp should unlock automatically.
You do not need to win a race afterward in most cases.
Why Restrictor Plates Matter in Forza Horizon 6
At first the mechanic feels backwards because most players spend the entire game increasing horsepower.
But Restrictor Plates actually become useful later when building competitive class-specific cars.
For example:

• a lightweight chassis with too much power may exceed A-Class limits
• restricting engine output lets you keep better handling parts
• you can balance grip and acceleration more efficiently

This becomes especially useful for S1 and A-Class road racing setups.
Easy Money-Saving Tip
If you are low on credits, check the mini-map for green CT or aftermarket car icons.
Some discounted pre-built cars already come with expensive racing engine swaps installed. Buying one of these can save a huge amount of credits because you only need to add the Restrictor Plate afterward.