The Subaru FA20 engine powers the BRZ, 86, and earlier Forester and Legacy models. It earned a strong reputation for its flat-four layout, low center of gravity, and impressive fuel economy. However, the FA20 also faces well-documented limitations when pushed past 300 wheel horsepower. The stock direct injection system reaches its ceiling, the cast pistons are prone to ringland failure, and the twin-scroll turbocharger becomes a restriction. Breaking the 370 horsepower barrier requires moving past bolt-on modifications and investing in a fully built long block, a comprehensive fuel system upgrade, and high-end engine management. This guide walks through the specific parts, engineering choices, and tuning strategies needed to achieve a reliable 370+ horsepower FA20.

Why 370 Horsepower Is the FA20 Reliability Threshold

The stock FA20 block can support up to 320–340 wheel horsepower on a conservative tune with very good fuel. Beyond that power level, failures occur consistently. The cast pistons crack under high cylinder pressure, the connecting rods bend, and the high pressure fuel pump cannot supply enough volume to support anything beyond 93 octane pump gas at that power level. Pushing past 370 wheel horsepower forces the engine into a fundamentally different operating regime. At 370 whp, the engine demands full control over fueling, compression, and ignition timing. This power level also represents the point at which the chassis comes alive. The FA20 cars weigh around 2,800 pounds, so 370 whp yields a power-to-weight ratio of approximately 7.5 pounds per horsepower, placing the car firmly in legitimate sports car territory.

Forged Internals: The Foundation for 370+ Horsepower

Stock FA20 components are not designed for sustained high boost. Forged internals provide the strength, fatigue resistance, and thermal stability required to survive repeated high-load pulls. The rotating assembly is the single most important investment in a Stage 3 build.

Pistons: Alloy Selection and Compression Ratio

Two primary alloys dominate the FA20 piston market: 2618 and 4032. 2618 alloy pistons (JE Pistons, CP-Carrillo) offer higher tensile strength and better resistance to thermal fatigue. They expand more when hot, requiring larger cold clearance. This makes them ideal for high boost applications where knock resistance is critical. 4032 alloy pistons (Mahle) contain higher silicon content, reducing thermal expansion and allowing tighter piston-to-wall clearance. They experience less cold start noise and last longer in street-driven cars, but they are more brittle under extreme detonation events. For a 370+ horsepower FA20 running E85, 2618 pistons are the standard choice. Compression ratio selection depends on fuel. For E85, 11.5:1 compression works well. For pump gas, 10.5:1 allows more boost without detonation. Pistons should include a ceramic thermal barrier coating on the crown and a friction-reducing skirt coating.

Connecting Rods: I-Beam vs. H-Beam Design

FA20 connecting rods from the factory are powder metal. They fracture under high cylinder pressure. Aftermarket rods are manufactured from 4340 forged steel billets. I-beam rods (K1 Technologies, Carrillo) offer the highest strength-to-weight ratio. They are stiffer in tension and compression, which matters at high RPM. H-beam rods (Manley, Eagle) provide excellent bending stiffness but are heavier. For an FA20 aiming for 370 whp, an I-beam rod with ARP 2000 or L19 bolts is the optimal choice. The rod bolts must be upgraded regardless of rod choice because the stock rod bolts stretch at high RPM, leading to rod bearing failure.

Crankshaft and Bearings

The stock FA20 crankshaft is forged from steel. It is strong enough to handle 500+ wheel horsepower in most cases. However, balancing the entire rotating assembly is mandatory. Factory balance tolerances are loose. Having the crankshaft, flywheel, pressure plate, harmonic damper, and reciprocating assembly balanced as a unit removes vibration that can cause bearing fatigue. Main bearings should be upgraded to ACL Race Series or King Race Series. These bearings use a higher lead-copper content for better embedment and anti-seizure properties. Rod bearings should match. Bearing clearance must be verified during assembly. Typical target clearance for the FA20 is 0.0020–0.0025 inches on the mains and 0.0018–0.0022 inches on the rods.

Case Integrity: Head Studs and Case Bolts

The FA20 engine is an open deck design. The cylinder walls are not supported at the top, which makes the head gasket sealing surface critical. ARP head studs are mandatory for any build exceeding 350 whp. Studs provide more consistent clamp load than bolts and reduce the risk of head lift under high boost. Case bolts (IAG Performance, Outfront Motorsports) reinforce the lower crankcase, reducing main bearing bore distortion at high cylinder pressure. These are often overlooked but are critical for long-term bearing survival.

Fuel System Engineering for 370+ Horsepower

The stock FA20 direct injection system is elegant for fuel economy but limiting for power. The high pressure fuel pump cannot deliver enough volume to support E85 above 350 wheel horsepower. To reach 370+ whp, the fuel system must be fundamentally redesigned.

Port Injection: The Standard Solution

Adding a secondary port injection system is the most common way to solve the fueling problem. A port injection manifold (GrimmSpeed, Bottlehead, Tomei, or custom) replaces the stock intake manifold or adds injectors to the existing manifold. The port injectors handle the additional fuel volume that the direct injection system cannot supply. Common injector sizes for 370+ whp on E85 are ID1300x (Injector Dynamics), FIC 1650, or Bosch 210 lb/hr. The injectors are controlled by an ECUTek 4-port plug-in controller, a Split Second controller, or a standalone ECU like the Motec M1 or Haltech Elite 2500.

Fuel Pumps and Surge Tanks

A single in-tank fuel pump is borderline for 370+ whp on E85. AEM 340 lph or Walbro 450 lph pumps are the minimum. For consistent fuel delivery, many builders run a surge tank system. A small catch tank (Radium Engineering, IAG Performance) sits in the engine bay or under the car. A primary pump fills the surge tank, and a secondary high-pressure pump feeds the fuel rails. This setup eliminates fuel starvation during hard cornering and allows the secondary pump to run at constant pressure. Fuel pressure regulation must be stable. An Aeromotive or Radium regulator set to 43.5 psi base pressure for port injection is standard.

Flex Fuel and Ethanol Tuning

E85 is the fuel that makes 370+ whp achievable on a reasonable turbo. Ethanol provides a cooling effect during intake and combustion, reduces knock tendency, and allows more aggressive ignition timing. A flex fuel sensor (Continental/GM) allows the ECU to read ethanol content in real-time and adjust fuel and timing maps dynamically. For a Stage 3 FA20, flex fuel tuning is the difference between a compromised pump gas tune and a fully optimized high-power setup. Fail-safes must be configured. If ethanol content drops below a threshold or fuel pressure falls, the ECU must immediately cut boost and pull timing.

Forced Induction and Air System Design

370 wheel horsepower requires airflow. The turbocharger, intercooler, intake, and exhaust must be selected to support the power target while maintaining drivability.

Turbocharger Selection for the 370+ Wheel Horsepower Target

The turbocharger must be sized to reach 370 whp efficiently without excessive lag. Three turbo families dominate the FA20 market for this power level.

  • BorgWarner EFR 6258 or 6758: These turbos spool extremely quickly. The 6258 can hit full boost by 3,200 RPM and easily supports 370 whp. The 6758 pushes to 450 whp. Both feature integrated recirculation valves and ceramic ball bearings.
  • Garrett G25-550 or G25-660: The G-series turbos offer outstanding efficiency. The G25-550 delivers 370 whp with excellent transient response. The G25-660 supports up to 480 whp. These turbos require an external wastegate.
  • Precision 5858: This journal bearing turbo is a budget option. It spools slower than the EFR or G-series but can make power reliably. It is heavier and less efficient.

An external wastegate is necessary for boost control above 20 psi. The Tial 44mm MVS or MVR wastegates are standard. A twin-scroll manifold (Full-Race, Killer B, Outsider) improves spool and reduces exhaust pulse interference, but it adds cost and complexity. A single-scroll manifold is simpler and works well with a properly sized turbo.

Intercooler, Radiator, and Oil Cooling

The stock FA20 intercooler is a top-mount design. It heat soaks quickly under high load. A front mount intercooler (ETS, PRL, PWR) is necessary for 370+ whp. Core size should be at least 24 inches wide and 12 inches tall with a 3.5 inch core thickness. Charge pipe diameter should be 2.75 inches.

The radiator must be upgraded. The stock radiator struggles to keep coolant temperatures below 200°F under sustained load. A Koyorad or CSF three-row aluminum radiator with SPAL fans maintains consistent temperatures. Oil cooling is equally important. The FA20 runs high oil temperatures due to its flat design. A Setrab or Earls oil cooler with a thermostatic sandwich plate keeps oil temperature between 180°F and 210°F.

Exhaust System Requirements

The exhaust system must flow enough volume to support 370 whp without excessive back pressure. A 3 inch diameter exhaust is the minimum. A 3.5 inch exhaust provides a safety margin and supports future upgrades. Equal length headers provide better scavenging and reduce exhaust gas temperature at the turbine inlet. Unequal length headers produce the classic Subaru rumble but increase back pressure and reduce spool. For a high horsepower FA20, equal length headers are the technically superior choice.

Engine Management: Tuning the Stage 3 FA20

Building the hardware is only half the battle. The engine control unit must be calibrated to manage the increased airflow, fuel flow, and boost pressure. The tuning strategy is what determines whether the car is reliable or spends its life in a garage.

ECU Selection: ECUTek ProECU vs. Motec M1 vs. Haltech

ECUTek ProECU is the most common choice for FA20 tuning. It uses the stock ECU, which retains all factory features including idle control, cruise control, AC logic, and emissions readiness monitors. ECUTek offers flex fuel support, multi-maps, and a 4-port auxiliary output controller for port injection. It is the most practical option for a street-driven car.

Motec M1 and Haltech Elite 2500 are standalone ECUs. They offer unlimited configuration options, faster processing, and higher resolution fuel and timing tables. They are necessary for cars running fully mechanical throttles, anti-lag systems, or complex boost control strategies. They require professional wiring and calibration.

Boost Control and Timing Strategy

For 370 whp, boost target is typically 20–24 psi depending on the turbo and fuel. Boost control should be closed-loop, with a 3-port boost control solenoid (Mac, Turbosmart, or Pierburg). The ECU adjusts wastegate duty to maintain target boost. Ignition timing must be conservative. The FA20 has a high compression ratio. Even with E85, timing must be pulled under high boost to prevent detonation. Typical timing at peak torque is 15–18 degrees on E85. On pump gas, timing drops to 10–12 degrees with lower boost. Knock control strategies must be aggressive. The FA20 uses individual cylinder knock detection. The ECU should pull timing from the specific cylinder that knocks, not all cylinders.

Fuel Mapping and Safety Systems

Fuel mapping must account for the direct injection and port injection split. The ECU must blend the two injection systems seamlessly. Target lambda is 0.78–0.80 for maximum power on E85. On pump gas, target lambda is 0.82–0.85. Safety systems must be programmed. The ECU should monitor fuel pressure, ethanol content, intake air temperature, engine oil temperature, and coolant temperature. If any of these parameters exceed safe thresholds, the ECU should immediately reduce boost and retime ignition.

Transmission and Drivetrain Upgrades

370 wheel horsepower exceeds the capacity of the stock FA20 clutch and transmission in many cases. Upgrading the drivetrain is essential for delivering power to the ground reliably.

Clutch and Flywheel

The stock clutch slips at 300 whp. A stage 2 or 3 clutch kit (Exedy, ACT, South Bend Clutch) with a sprung hub disc is necessary. A single-mass flywheel reduces rotating inertia and improves throttle response but increases transmission noise at idle. If the car is daily driven, a dual-mass flywheel replacement or a heavy sprung hub disc helps maintain drivability.

Differential and Axles

FA20 cars equipped with a limited-slip differential from the factory can handle 370 whp for street driving. For track use, a 1.5-way or 2-way mechanical differential (Cusco, OS Giken) provides more predictable lockup and reduces inside wheel spin. Axles are a weak point on high power FA20 cars. The stock axles can break under hard launches with sticky tires. Aftermarket axles (DSS, The Driveshaft Shop) provide upgraded CV joints and thicker shafts.

Oil System Upgrades for High Load Conditions

Stage 3 FA20 builds require serious attention to the oiling system. The flat-four design is prone to oil starvation during hard corners and high-RPM sustained operation.

Oil Pickup Tube and Baffled Oil Pan

The stock oil pickup tube has a known failure mode. The tube cracks at the weld, the engine loses oil pressure, and the bearings spin. A Moroso or Killer B oil pickup tube removes this failure point. A baffled oil pan (IAG Performance, Killer B, Moroso) prevents oil from sloshing away from the pickup during hard acceleration and cornering. The baffles also help the oil release air bubbles, reducing the risk of aerated oil entering the oil pump.

Oil Pump Considerations

The stock FA20 oil pump is a gear-type pump. At high RPM (7,000+), the pump can cavitate and lose pressure. Upgrading to a high-pressure or high-volume oil pump (IAG, Outfront, or OEM 2018+ FA24 pump) maintains oil pressure at high RPM. The pump should be verified for proper pressure relief valve operation to avoid over-pressurizing the oil system at cold start.

Real-World Results: What 370+ Wheel Horsepower Feels Like

A Stage 3 FA20 making 370+ wheel horsepower transforms the driving experience. The car no longer feels like a momentum car. It pulls hard from 3,500 RPM to redline. Throttle response is sharp because the turbo is well matched and the port injection system provides immediate fuel delivery. The low center of gravity remains, so the car handles better than almost any front-engine, front-wheel-drive competitor. The car will accelerate from 0-60 in the low 4-second range and trap over 115 mph in the quarter mile. More importantly, the power is predictable and repeatable. The forged internals provide confidence. The upgraded fuel system eliminates high-load fuel pressure drops. The ECU safety systems protect the engine from operator error or bad fuel.

Stage 3 FA20 Cost and Build Considerations

A Stage 3 FA20 build is not inexpensive. A comprehensive short block assembly with forged pistons, rods, bearings, and case bolts costs between $4,000 and $6,000. The cylinder heads require labor for valve spring replacement and reconditioning, adding $1,500 to $2,500. The fuel system costs $2,500 to $4,000 depending on the pump and injector configuration. Turbo selection, intercooler, exhaust, and engine management add another $4,000 to $7,000. Professional assembly and tuning costs $1,500 to $3,000. Total investment ranges from $12,000 to $20,000. This is comparable to the cost of an LS swap but retains the FA20's factory integration, lower weight, and flat-four character.

Conclusion: The 370+ Horsepower FA20 Standard

370 wheel horsepower is the ceiling for a reliable, street-driven FA20. Achieving this power requires moving beyond bolt-on parts and embracing a fully built engine. Forged pistons and rods handle the mechanical stress. A port injection fuel system solves the fueling bottleneck. A properly sized turbo and intercooler manage air flow and intake temperatures. Tuning the combination with ECUTek or Motec ensures the engine operates safely across all conditions. The result is a fast, reliable, and responsive car that rewards skilled driving. For enthusiasts who respect the engineering required, the Stage 3 FA20 is a high watermark for the platform.