Selecting the Right Size Turbo for Your Gr86: 45mm Vs50mm Turbo Kits Explained

Selecting the right size turbo for your GR86 can significantly impact performance and driving experience. Among the popular options are the 45mm and 50mm turbo kits. This article explores the differences between these two turbo sizes to help you make an informed decision.

Understanding Turbo Sizing and the GR86 Engine

The Subaru FA24D engine found in the GR86 and its sibling BRZ is a naturally aspirated 2.4-liter flat-four. While it produces a respectable 228 horsepower from the factory, forced induction is the most effective path to significantly more power. Turbocharger sizing is typically defined by the compressor wheel inducer diameter (the measurement in millimeters used in "45mm" and "50mm"). This number directly influences how much air the turbo can move and at what engine speed it reaches its peak efficiency.

Selecting the correct turbo size for a GR86 requires understanding the relationship between compressor flow, turbine flow, and the engine’s air requirements. A smaller turbine housing and compressor wheel allow the turbo to spool quickly at lower RPMs, building boost early. Conversely, a larger turbo can flow more air at higher RPMs, supporting greater peak horsepower but often at the cost of increased lag. The FA24 engine has a relatively high displacement for a four-cylinder, which gives it strong exhaust energy to drive a turbo, but the choice between a 45mm and 50mm kit shifts the power curve dramatically.

For a deeper dive into turbocharger fundamentals, EngineLabs offers an excellent guide on reading compressor maps.

45mm Turbo Kits: The Responsive Street Build

The 45mm turbo kit is often the go-to choice for GR86 owners who want a noticeable power increase without sacrificing the car’s excellent low-end drivability. The smaller compressor wheel and turbine housing allow the turbo to reach boost threshold very early, often as low as 2500–3000 RPM. Full boost can be achieved by 3500–4000 RPM, making the car feel responsive and punchy in everyday driving.

Typical power targets for a well-tuned 45mm kit on a stock FA24 engine range from 300 to 380 wheel horsepower. With supporting modifications like a higher-flow fuel system and an upgraded intercooler, some kits can push toward 420 horsepower. This level of power transforms the GR86 from a momentum car to a genuinely quick street machine without overwhelming the chassis or requiring extensive drivetrain upgrades.

Advantages of 45mm Turbo Kits

• Quicker Spool Time: The smaller compressor and turbine wheels spin up faster, building boost 1500–2000 RPM sooner than a 50mm kit.
• Improved Driveability: The broad, flat torque curve makes the car effortless to drive in traffic and on back roads. Throttle response remains sharp, and the car feels naturally aspirated below 3000 RPM before the boost smoothly comes in.
• Less Turbo Lag: Significantly reduced time from pedal input to full boost. This translates to better throttle modulation for daily driving and autocross.
• Lower Heat Generation: Smaller turbos generally push less hot air into the intake charge, making thermal management simpler and reducing the risk of heat soak during extended drives.
• Simpler Tuning: The boost curve is more predictable and easier to calibrate on a stock ECU with an aftermarket flash tune or piggyback controller.

Disadvantages of 45mm Turbo Kits

• Lower Peak Power: The compressor runs out of flow capacity above 400–420 wheel horsepower. If your goal is 450+ horsepower, the 45mm wheel becomes a restriction at high RPM.
• Limited Top-End Performance: Power falls off more noticeably past 6500–7000 RPM as the turbo cannot sustain the required mass flow for the engine’s high-RPM needs.
• Potential for Overspeed: Pushing a small wheel to its maximum flow can lead to reduced reliability and compressor surge if not properly matched to the engine.

Many GR86 owners on forums report that the 45mm kit provides the most fun-to-drive experience on public roads. A popular discussion on GR86.org highlights real-world experiences with both sizes.

50mm Turbo Kits: The High-Power Performer

For enthusiasts who plan to take their GR86 to the track, drag strip, or simply want the highest possible horsepower figures, the 50mm turbo kit is the clear choice. The larger compressor wheel flows significantly more air, allowing the engine to produce power well beyond what the 45mm can support. However, the larger rotating mass and bigger turbine housing mean the turbo needs more exhaust energy to start spinning, resulting in later boost onset.

A typical 50mm kit tuned on pump gas (93 octane) will produce 380–450 wheel horsepower. With E85 and upgraded internals, outputs well over 500 wheel horsepower are achievable. The power delivery is more aggressive once boost hits, often coming on with a surge around 4000–4500 RPM and pulling hard to the 7500 RPM redline. The torque curve is more peaky compared to the 45mm, which requires more attention to gear selection and throttle modulation.

Advantages of 50mm Turbo Kits

• Higher Peak Power: The 50mm wheel can support 450–600+ horsepower depending on the specific compressor map and turbine housing selection. This is the choice for high-horsepower builds.
• Better for Track Use: On a road course or drag strip, the ability to maintain boost at high RPM translates to stronger acceleration out of corners and down straights.
• Enhanced Top-End Performance: Power does not fall off; instead, it continues climbing until redline, giving the car a proper race-car feel.
• Upgrade Headroom: If you plan to build the engine in the future, a 50mm kit can grow with you. Many kits use a T3 or T4 flange that allows swapping to a larger turbine housing or even a different compressor wheel later.

Disadvantages of 50mm Turbo Kits

• Slower Spool Time: Full boost may not arrive until 4500–5000 RPM. Below 3000 RPM, the car feels relatively flat, requiring you to keep revs higher for responsive power.
• Increased Turbo Lag: The delay between throttle input and boost delivery is more pronounced. This can make daily driving less enjoyable, especially in stop-and-go traffic.
• Less Suitable for Daily Driving: The car needs to be driven more aggressively to stay in the powerband. Fuel economy also suffers because the engine is working harder off boost.
• Higher Heat Output: More boost pressure and higher flow create more exhaust gas temperatures. Heat management becomes critical, requiring larger intercoolers, oil coolers, and possibly water-methanol injection.
• More Complex Tuning: The steeper boost curve and higher airflow demand careful fuel and ignition mapping to avoid knock. A standalone ECU like a Motec or Haltech is often recommended over a simple flash tune.

Head-to-Head Comparison: 45mm vs 50mm on the GR86

To help visualize the differences, here is a direct comparison of key performance metrics:

Parameter	45mm Kit	50mm Kit
Boost Threshold (RPM)	~2500–3000	~3500–4500
Full Boost (RPM)	~3500–4000	~4500–5500
Peak Horsepower (Pump Gas)	330–380 whp	400–480 whp
Peak Horsepower (E85 / Race Gas)	380–430 whp	480–600+ whp
Torque Curve Shape	Broad, flat plateau	Rising, peaky curve
Ideal Use Case	Daily driver, autocross, canyon carving	Track days, drag racing, high-horsepower builds
Supporting Mods Needed	Fuel system upgrade, intercooler, oil cooler, tune	Fuel system, larger intercooler, oil cooler, upgraded clutch, often engine internals
Typical Kit Price	$3,000–$4,500	$4,500–$7,000

Supporting Modifications Are Critical Regardless of Size

No turbo kit works in isolation. To safely and reliably run either a 45mm or 50mm turbo on your GR86, you need to invest in supporting modifications. The FA24 engine uses direct injection, which limits fuel flow at high RPM. Upgraded high-pressure fuel pumps or port injection systems are mandatory for anything over 350 whp. Most 50mm kits will require a full fuel system upgrade including a larger fuel pump, injectors, and a flex-fuel sensor for E85.

Intercooling is another critical area. The 45mm kit can function with a standard front-mount intercooler, while the 50mm kit demands a large core with efficient end tanks to manage intake air temperatures under sustained boost. Oil cooling is also essential, as the turbocharger itself adds heat to the engine oil. An oil cooler with a thermostat should be considered mandatory for any turbocharged GR86, especially in warmer climates or track use.

For a detailed breakdown of the required upgrades, DrivingLine’s GR86 turbo build guide covers the entire process from start to finish.

Tuning and ECU Considerations

The stock GR86 ECU can be reflashed using tools like EcuTek or OpenFlash, but these solutions are limited for higher boost levels. The 45mm kit at moderate boost (6–9 psi) is well within the capability of a good flash tune with a speed-density conversion. However, for the 50mm kit, especially at boost levels over 10 psi, a standalone ECU is strongly recommended. The stock MAF sensor cannot accurately meter the high airflow, and the factory knock detection system becomes a liability when trying to push the envelope.

Standalone ECUs offer advanced features like boost control, anti-lag, launch control, and full flex-fuel mapping. They also provide better safety monitoring with multiple knock sensors, wideband oxygen sensors, and data logging. Budgeting $1,500–$3,000 for tuning and ECU hardware is realistic for a 50mm build. For a 45mm build, a $700–$1,200 flash tune often suffices.

Installation Complexity and DIY Considerations

45mm Kits: Most 45mm turbo kits for the GR86 are designed as bolt-on systems with minimal modification. They typically include a cast manifold, downpipe, intercooler piping, and a wastegate. A competent DIY mechanic with basic tools can complete the installation over a weekend. Oil feed and drain lines need to be connected to the engine block, and the intercooler requires removal of the front bumper cover. No cutting or welding is usually required.

50mm Kits: The installation is more involved. The larger turbo often requires custom fabrication for the downpipe, intake piping, and intercooler mounts. The added heat shielding is more extensive, and careful routing of coolant lines is needed. Many 50mm kits come as universal fitments, requiring welding for the exhaust manifold and downpipe. Expect a professional installation to cost $1,000–$2,500 in labor on top of the kit price.

Cost Analysis: What Does Each Option Really Cost?

When budgeting for a turbo system, consider the total package. A 45mm kit often runs $3,000–$4,500 for the turbo system, fuel upgrades, and intercooler. Adding a quality tune brings the total to around $4,000–$6,000. This is an affordable path to 350+ horsepower.

A 50mm kit starts at $4,500 for the turbo system alone. With the necessary fuel system ($1,500–$2,500), standalone ECU ($1,500–$3,000), larger intercooler ($600–$1,000), and installation labor, the total easily exceeds $10,000. For those aiming for 500+ horsepower, forged pistons and rods add another $2,000–$4,000. This is a significant investment, but the rewards in performance are substantial.

Making the Decision: A Proven Decision Matrix

Use this framework to decide which size is right for your GR86:

• Primary Use: Is the car your daily driver or a dedicated track toy? If daily, choose 45mm. If track, consider 50mm.
• Power Target: Under 400 whp? 45mm will exceed your needs. Over 450 whp? 50mm is the only option.
• Budget: Under $6,000 total? Stick with 45mm. $10,000+ allows a proper 50mm build.
• Tuning Skills: Do you have access to a dyno and tuner experienced with standalone ECUs? If not, the simpler 45mm flash tune is less risky.
• Future Plans: Do you plan to build the engine and chase big power later? Starting with a 50mm kit saves you from buying a second turbo system.

For many GR86 owners, the 45mm kit offers the best balance of fun, cost, and usability. It transforms the car without compromising its daily-driving character. The 50mm kit is for those who are serious about power and are prepared to invest in chassis and drivetrain upgrades to handle it. Whichever you choose, proper installation and a quality tune are non-negotiable.

For further reading on specific kit brands like HKS, GReddy, and ARM, check out FT86Club’s turbo kit comparison thread where owners share dyno sheets and feedback.

Conclusion

Both the 45mm and 50mm turbo kits offer unique advantages and disadvantages. Understanding your driving preferences and performance goals will help you select the right turbo for your GR86, ensuring an optimal balance between power and drivability. The 45mm kit delivers immediate, street-friendly power with minimal lag, while the 50mm kit unleashes the full potential of the FA24 engine for track-focused builds. Evaluate your budget, intended use, and willingness to invest in supporting modifications before making your choice. With the right setup, your GR86 can become an exhilarating, high-performance machine that rewards you every time you push the throttle.