Understanding the FA20 Engine

The FA20 is a 2.0-liter horizontally opposed (boxer) four-cylinder engine developed by Subaru, most famously found in the Subaru BRZ, Scion FR-S, and Toyota 86. Its flat layout gives vehicles a low center of gravity, contributing to exceptional handling. The FA20 features a dual-injection system (port and direct), a high 12.5:1 compression ratio, and a redline around 7400 rpm. While the stock engine delivers around 200–205 horsepower at the crank, its torque curve is relatively flat, with a slight dip around 3500–4500 rpm (the infamous “torque dip”). This dip is largely caused by the restrictive factory intake manifold and the interplay between the two injection systems.

Enthusiasts quickly discovered that improving airflow into the engine can mitigate that torque dip and unlock additional power higher in the rev range. Upgrading the intake manifold is one of the most cost-effective ways to achieve these gains, often adding 10–15 horsepower at the wheels for under $500.

Why Upgrade the Intake Manifold?

The intake manifold's job is to deliver air evenly to each cylinder while maintaining the appropriate velocity for good low-end throttle response and sufficient volume for high-end power. The FA20’s stock manifold is a composite plastic design that prioritizes packaging, NVH, and emissions over peak performance. Its long, narrow runners and small plenum volume create turbulence and uneven distribution, especially as engine speed climbs.

An aftermarket manifold addresses these shortcomings with:

  • Larger plenum volume – stores a bigger air charge, reducing the reversion and helping each cylinder draw a consistent air/fuel mixture, particularly beneficial at higher RPM.
  • Shorter, smoother runners – reduce restriction, improve flow, and shift the torque peak upward, helping fill that mid-range dip.
  • Improved materials – aluminum or carbon fibre dissipate heat faster than plastic, keeping intake air temperatures lower for more dense air.
  • Better distribution – quality aftermarket manifolds use computational fluid dynamics (CFD) to ensure each cylinder sees nearly identical air mass, leading to steadier air/fuel ratios and more consistent power.

For the FA20, the most notable result of upgrading the intake manifold is the reduction or elimination of the 3500–4500 rpm torque dip. Many owners report a smoother power delivery that actually feels like a broader torque curve, not just peaky top-end power.

Real-World Horsepower Gains: 10–15 HP Explained

Claims of 10–15 horsepower gains are based on back-to-back dyno runs of common aftermarket intake manifolds (e.g., FT86 Speedfactory’s intake manifold tests). A typical example is the Skunk2 Ultra Street intake manifold or the Mishimoto intake manifold. These manifolds consistently show 10–15 whp gains on a naturally aspirated FA20 without any other modifications, and the gains increase to 15–20 whp when paired with a proper ECU tune and a catless header.

Important: Gains vary by brand, tuning, and supporting mods. A manifold alone may yield the lower end of that range; a tune optimizes the ignition timing, cam phasing, and fuel mapping to take full advantage of the improved airflow. On an otherwise stock engine, a quality intake manifold plus a custom tune can push the FA20 from ~170 whp (stock) to ~185–190 whp.

Dyno charts also reveal that the torque dip flattens significantly. Many aftermarket manifolds reduce the dip from ~20 lb-ft to only 5–8 lb-ft, with a more gradual climb. For street driving, this means fewer flat spots and better responsiveness when merging or climbing grades.

Cost Breakdown: Under $500

The “under $500” claim holds true for several quality options, especially if you hunt for deals or buy used. Here’s a rough price breakdown of common FA20 intake manifold solutions:

  • Used OEM manifold – $50–100. Not an upgrade, but often scavenged for porting or core exchanges.
  • Port & polish of stock manifold – $150–250. Modest gains (3–5 hp) but retains factory fitment.
  • Budget aftermarket (e.g., JDL, P&L Motorsports, Injen) – $300–450. Aluminum construction, improved runners.
  • Mid-range (e.g., GReddy, Mishimoto) – $400–600. Quality cast or billet units with integrated vacuum ports and better gasket surfaces.
  • Premium (e.g., Skunk2 Ultra Street, Full-Race) – $500–800. High-flow designs, sometimes with integrated velocity stacks.

Many aftermarket manifolds fall within the $400–500 range when on sale or from discount retailers, putting a 10–15 HP gain within reach for under $500. Pair that with a used ECU tune (e.g., Ecutek or OpenFlash) for another $300–500, and the total cost for a 20+ whp increase can stay under $1,000.

Choosing the Right Intake Manifold for Your FA20

Not all intake manifolds are created equal, and the best choice depends on your goals, budget, and whether you plan to boost the engine later. Consider these factors:

Plenum Volume and Runner Length

Manifolds with large plenums (e.g., over 3 liters) sacrifice low-end torque for top-end power. If the car is a daily driver, you likely want a medium-volume plenum that retains some low-end response while filling the mid-range dip. Most aftermarket FA20 manifolds are designed to balance these trade-offs.

Material: Aluminum vs. Plastic vs. Carbon Fiber

  • Aluminum – cheap, strong, easy to weld. Conducts heat more than plastic, so heat soak can be an issue in stop-and-go traffic. Many use thermal gaskets to mitigate.
  • Plastic (e.g., OEM-like) – less heat transfer, lighter, but less durable and often more restrictive.
  • Carbon fiber – lightweight and heat-resistant, but expensive ($800+). Only seen in race applications.

Injection Type Compatibility

The FA20 uses both port injection and direct injection. Most aftermarket intake manifolds retain the port injector ports, but some remove them for a cleaner look or weight savings. You should keep port injection if you plan to run E85 or need extra fuel. Always verify the manifold includes injector bungs.

Gasket and Vacuum Ports

Many kits include new gaskets and silicone hoses. Check that vacuum ports exist for the brake booster, PCV, and EVAP system. Some manifolds eliminate some ports, requiring rerouting of hoses – not a big deal but adds installation complexity.

Installation Process: Step-by-Step

Installing an intake manifold on the FA20 is a moderate DIY job (2–4 hours for a skilled mechanic). You’ll need basic hand tools, a torque wrench, and a set of replacement gaskets (often included). Always work on a cold engine and disconnect the battery.

  1. Remove air intake system – Remove the air filter box, MAF sensor, and intake tube to access the throttle body.
  2. Unbolt throttle body – Disconnect the throttle body from the stock manifold. Keep the coolant hoses attached if still hot; otherwise drain coolant first.
  3. Disconnect sensors and vacuum lines – Label all hoses and electrical connectors (MAP sensor, intake air temp sensor, PCV, EVAP).
  4. Remove fuel rail (optional) – The fuel rail bolts to the stock manifold in some versions. You may need to remove it carefully, depressurizing the fuel system first.
  5. Unbolt stock intake manifold – Usually 8–10 bolts. Work in a crisscross pattern to avoid warping the plastic manifold.
  6. Clean mating surfaces – Scrape off old gasket material. Use a razor blade and plastic-safe cleaner. Do not let debris fall into the engine.
  7. Transfer everything to the new manifold – Install the throttle body, injectors (if using new manifold with different injector ports), and new gaskets. Torque all fasteners to the manufacturer’s spec (usually around 18–20 Nm for aluminum-to-aluminum).
  8. Reinstall – Position the new manifold, install bolts finger tight, then torque in sequence. Reconnect all vacuum lines, electrical connectors, fuel rail, and intake tube.
  9. Final checks – Double-check that no hoses are pinched, all clamps are tight, and the throttle moves freely. Refill coolant if you disconnected heater hoses. Start the engine and check for vacuum leaks (use a smoke machine or listen for hissing).

Pro tip: Replace the rubber vacuum lines with silicone ones while access is easy. And consider upgrading to a high-quality intake manifold gasket to avoid future leaks.

Tuning Requirements for Maximum Gains

To fully realize the 10–15 HP potential, an ECU reflash is highly recommended. The stock ECU may not adapt enough, especially if the manifold’s larger plenum causes the MAF sensor to read a different airflow calibration. A tune ensures:

  • Corrected fuel trims and air/fuel ratios
  • Optimized ignition timing across the rev range
  • Adjusted camshaft timing (dual VVT) to take advantage of the improved intake flow
  • Removal of any torque dip-induced engine timing retard

Popular tuning platforms for the FA20 include Ecutek (custom or off-the-shelf maps), OpenFlash Tablet (DIY tuning with preloaded tunes), and COBB Accessport (though support for FA20 is limited to the BRZ/86). Many tuners offer specific calibration files for popular intake manifolds such as the Mishimoto or Skunk2.

Without a tune, you may still see 5–8 hp from improved flow, but the full 10–15 hp requires recalibration. Also note: if you run a catless header or E85, the manifold upgrade becomes even more effective – you can approach 200–210 whp naturally aspirated with a combination of header, manifold, tune, and E85.

Potential Downsides and Considerations

No upgrade is without trade-offs. Before buying, keep these in mind:

  • Low-end torque loss – A large plenum manifold can reduce below-3000 RPM torque by 2–5 lb-ft. For daily driving, this is minor but noticeable in stop-and-go traffic.
  • Heat soak – Aluminum manifolds in hot climates can soak heat from the engine bay, leading to higher intake air temperatures after a long idle. Consider a thermal gasket and ceramic coating.
  • Clearance issues – Some aftermarket manifolds are taller or wider, interfering with the strut tower brace, hood liner, or engine cover. Check fitment guides.
  • Vacuum leaks – The FA20’s integrated PCV/EVAP system is fussy. A small leak can cause rough idle and check engine lights. Install meticulously.
  • Emission legality – Most aftermarket intake manifolds are not CARB-legal in California. Check local regulations if your vehicle must pass smog.

For most drivers, the mid-range gains and higher peak power outweigh any low-end loss. A tune can often recover some of the lost low-end by adjusting cam timing.

Final Verdict

Upgrading the FA20 intake manifold is one of the smartest power-per-dollar modifications you can make. For under $500, you gain a noticeable 10–15 horsepower, a flatter torque curve, and better throttle response – all while keeping the engine naturally aspirated and reliable. When combined with a tune, the gains are even more impressive, pushing the FA20 closer to its full potential without forced induction.

Whether you choose a budget-friendly used manifold or a premium aftermarket unit, the key is to match the manifold to your driving style and future plans. Research dyno results, check forums, and don’t skip the tune. With careful selection and proper installation, your FA20-powered car will feel more responsive and energetic on every drive.