How the Restrictor Plate Upgrade Works in Forza Horizon 6

In Forza Horizon 6, most upgrades are designed to make your car faster, louder, and more powerful. The Restrictor Plate upgrade works differently. Instead of increasing performance, it intentionally reduces engine power so your car fits into a lower Performance Index (PI) class.

At first, this sounds pointless. Why would anyone want less horsepower in a racing game? But once you start building competitive cars or working through the Discovery Journal challenges, the Restrictor Plate becomes surprisingly useful.

What the Restrictor Plate Actually Does

The upgrade simulates a real-world restrictor plate system that limits airflow going into the engine. Less air means less combustion, which means lower horsepower and torque.

In gameplay terms, installing a restrictor plate lowers:

Horsepower
Acceleration
Top speed
Overall PI rating

This gives you more flexibility when tuning a build.

For example, you might have a car sitting at 905 PI in S1 Class. That puts it slightly above the legal limit for some events. Instead of removing expensive suspension or tire upgrades, you can install a restrictor plate to bring the car back down to S1 900 while keeping the handling setup intact.

That makes the upgrade especially useful for competitive class building.

Why Players Use It

Most players first encounter the restrictor plate while completing the Discover Japan mechanic challenges, but experienced tuners use it regularly for balancing builds.

Here are the main reasons people install it:

PI Management

This is the biggest reason.

Sometimes a car has excellent grip, aero, and handling, but just a little too much power. A restrictor plate lowers the PI without forcing you to downgrade important performance parts.

It is basically a tuning shortcut that helps squeeze strong cars into lower racing classes.

Better Balance

Some cars become difficult to control after aggressive engine swaps. Reducing power slightly can actually make the car faster around technical tracks because it improves traction and stability.

A lower-powered but balanced build often beats an overpowered car that struggles for grip.

Challenge Completion

Several Discovery Journal tasks require players to equip unusual upgrades, including the restrictor plate. Installing one counts toward progression and helps unlock cosmetic rewards like the Mechanic Outfit.

Where to Find the Upgrade

One thing that confuses many players is that the Restrictor Plate option does not appear on most normal street cars.

It is usually tied to racing engines or specialized race configurations.

To access it:

Go to a Festival Site or Player House
Open Upgrades & Tuning
Select Custom Upgrade
Navigate to Conversions
Install a racing engine swap
Return to Engine Upgrades
Look for the Restrictor Plate category

Without the correct engine, the menu option simply will not appear.

Best Engine Swaps for Unlocking It

The easiest way to unlock the upgrade is by using race-oriented engine swaps. Two of the most reliable options are:

1.6L I4 Turbo Rally engine
2.6L 4-Rotor racing engine

These swaps commonly enable restrictor plate tuning on otherwise standard road cars.

Dedicated race cars, drift cars, and time attack builds may already include the option by default.

Easy Cars to Use for the Mechanic Challenge

If your goal is simply finishing the Discovery Journal requirement quickly, there are a few cheap and reliable setups players are using.

Mazda RX-7 Setup

The Mazda RX-7 is one of the easiest solutions.

Simply:

Buy the RX-7
Swap to the 2.6L 4-Rotor engine
Open Engine Upgrades
Install the Restrictor Plate

The challenge should complete immediately after purchase.

Miata Method

The Mazda MX-5 Miata is another popular option because it is inexpensive and widely available.

Players typically:

Buy the Miata
Install the 1.6L I4 Turbo Rally swap
Return to Engine Upgrades
Equip the Restrictor Plate

This is one of the cheapest ways to finish the requirement.

Honda NSX-R GT Barn Find

The Honda NSX-R GT Barn Find also works well with the rally engine swap method.

Because the car already has strong racing potential, many players keep using it after completing the challenge instead of selling it immediately.

Is the Restrictor Plate Worth Using?

For casual free-roam driving, probably not. Most players naturally prefer more horsepower.

But for tuning, competitive racing, and PI optimization, the Restrictor Plate is surprisingly important. It gives builders more control over class limits and helps create cleaner, more efficient setups.

Once you start experimenting with higher-level tuning in Forza Horizon 6, you realize the best builds are not always the most powerful ones. Sometimes removing a little power is exactly what makes a car competitive.