How to Make a Rear Diffuser for a Subaru Legacy Sedan

If you’re looking to upgrade your Subaru Legacy sedan’s aerodynamics, adding a rear diffuser can be a great option. A diffuser serves to reduce the amount of air pressure underneath the car, which can lead to improved downforce and stability at higher speeds. Plus, let’s be honest, it looks pretty cool too!

In this guide, we’ll go over step-by-step instructions for how to make a rear diffuser for your Subaru Legacy sedan. We’ll cover everything you need to know, from materials and tools to the actual building process. By the end of this article, you’ll be ready to transform your car into a sleek, aerodynamic machine.

Key Takeaways

  • Adding a rear diffuser can improve your Subaru Legacy sedan’s aerodynamics and stability at higher speeds.
  • Before you start building your diffuser, you’ll need to gather the necessary materials and tools.
  • The building process involves measuring, cutting, and shaping various pieces of material to fit your car’s specific dimensions.
  • Once your diffuser is complete, you’ll need to install it onto your car using screws or other fasteners.
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Now, let’s get started!

Materials and Tools

Before you jump into building your rear diffuser, you’ll need to gather a few materials and tools. Here’s what you’ll need:


  • Sheet of aluminum or plastic for the main diffuser
  • Additional aluminum or plastic sheets for the support brackets
  • Rivets and/or screws for fastening the diffuser and brackets together
  • Sealant to prevent water from getting into the trunk


  • Measuring tape
  • Saw or Dremel tool for cutting the aluminum or plastic sheets
  • Drill for making holes for the fasteners
  • Rivet gun or screwdriver for installing the fasteners
  • Heat gun or hair dryer for heating and shaping the plastic sheets (if using plastic)

Building Process

Now that you have all your materials and tools gathered, it’s time to start building your rear diffuser. Here’s a step-by-step guide to help you through the process:

Step 1: Measure and Cut the Main Diffuser

  • Using your measuring tape, measure the length and width of your car’s rear bumper.
  • Transfer those measurements onto your sheet of aluminum or plastic and mark the cutting lines.
  • Use your saw or Dremel tool to cut out the main diffuser shape.

Step 2: Create the Support Brackets

  • Cut additional aluminum or plastic sheets into strips for the support brackets.
  • Bend the strips into an L-shape to fit against the underside of the diffuser.
  • Mark and drill holes for the fasteners where the brackets will attach to the diffuser.

Step 3: Fasten the Brackets to the Diffuser

  • Line up each bracket with its corresponding hole on the diffuser and fasten them together using rivets or screws.
  • Make sure the brackets are evenly spaced and aligned correctly.
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Step 4: Shape the Diffuser (If Using Plastic)

  • If you’re using plastic, you’ll need to heat the material with a heat gun or hair dryer until it becomes pliable.
  • Once the material is pliable, shape it to match the curvature of your car’s rear bumper.
  • Hold the material in place until it cools and retains its shape.

Step 5: Install the Diffuser onto Your Car

  • Apply sealant around the edges of the diffuser to prevent water from entering the trunk.
  • Line up the diffuser with your car’s rear bumper and mark the location of the screw holes.
  • Drill holes into your car’s rear bumper where you marked the locations.
  • Fasten the diffuser to your car’s rear bumper using screws or other fasteners.

Congratulations, you’ve now successfully built and installed your own rear diffuser for your Subaru Legacy sedan!


How much does it cost to make a rear diffuser for a Subaru Legacy sedan?

The cost of materials for a DIY rear diffuser can vary depending on the type of materials you use and where you source them from. However, you can expect to spend a few hundred dollars on materials and tools.

Do I need any special skills to make a rear diffuser?

While some basic DIY skills are required, such as cutting and drilling, you don’t need any specialized skills to make a rear diffuser. Just take your time and follow the instructions carefully.

Will adding a rear diffuser affect my car’s fuel efficiency?

In general, adding a rear diffuser can actually improve your car’s fuel efficiency by reducing drag and improving airflow. However, the exact impact on your car’s fuel efficiency will depend on a number of factors, including your driving habits and the specific design of your diffuser.

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Can I install a rear diffuser myself, or should I hire a professional?

While it’s possible to install a rear diffuser yourself, it can be a fairly involved process that requires some technical knowledge. If you’re not comfortable working with tools or don’t feel confident in your abilities, it may be best to have a professional install your diffuser for you.

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Joseph Weaver

With a background in engineering and a passion for cars, Joseph brings a unique perspective to our blog. He's particularly interested in the technology and innovation behind Subaru's advanced safety features, and his articles on this topic are always insightful and informative. When he's not writing, he enjoys playing video games and spending time with his family.

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