Table of Contents
Understanding the Stock Subaru Legacy Turbocharger
The factory turbocharger on the Subaru Legacy—typically a Mitsubishi TD04 or IHI VF40/52 depending on the generation and trim—is engineered for a specific balance of daily drivability, emissions compliance, and reasonable performance. It works well within the engine’s stock parameters but leaves substantial headroom unutilized. Key characteristics of the stock unit include:
- Boost pressure: Factory wastegate actuation holds boost around 14.5 psi, with a hard cap at roughly 16 psi under aggressive conditions.
- Power output: On a typical 2.5L Legacy GT (2005-2009), stock horsepower ranges from 250 to 275 at the crank, with 260-280 lb-ft of torque.
- Response: Moderate lag—full boost arrives around 3000-3200 RPM, which is livable but far from optimal for a quick spooling performance setup.
- Compressor and turbine: Small A/R ratio housings limit top-end flow, causing power to fall off sharply past 5500 RPM.
For many owners, the stock turbo is the primary bottleneck when chasing more than 300 wheel horsepower. The small turbine wheel and restrictive outlet design heat-soak quickly, especially during repeated pulls or track use.
The BorgWarner EFR 8374: A Technological Leap
The BorgWarner EFR 8374 is a next-generation turbocharger that redefines the performance envelope for the Subaru platform. It belongs to the Engineered For Racing (EFR) series, which incorporates aerospace-derived materials and advanced aerodynamics. The 8374 designation indicates an 83mm compressor wheel and a 74mm turbine wheel. Standout features include:
- Boost capability: Sustain up to 30 psi with a properly built engine and tuning, though most street setups settle at 25-28 psi for reliability.
- Power ceiling: With appropriate supporting mods, the 8374 can produce 500-600 wheel horsepower on pump gas and over 700 whp on race fuel or ethanol.
- Spool characteristics: The EFR’s patented Gamma-Ti turbine wheel (titanium aluminide) drastically reduces rotational inertia, enabling full boost by 3600-3800 RPM on a 2.5L—remarkably close to stock spool despite the massive flow capacity.
- Integrated features: The cartridge includes a recirculating wastegate port and a built-in blow-off valve, simplifying installation and reducing part count.
- Billet compressor wheel: The 5-axis CNC-machined billet wheel (83mm inducer) offers excellent efficiency across a wide map.
Why the EFR 8374 for a Legacy?
The Subaru Legacy is heavier than the Impreza WRX/STI, so it benefits from a turbo that can move air without sacrificing low-end torque. The 8374 strikes a sweet spot: it spools fast enough for daily driving yet flows enough for high-horsepower builds. Compared to a GTX3076R or a Precision 6266, the EFR series generally shows better transient response and thermal management.
Head-to-Head Performance Metrics
To quantify the upgrade, we compare the stock TD04/VF40 against the EFR 8374 on a typical 2005-2009 Subaru Legacy 2.5GT with a built block, upgraded fuel system, and a quality tune. All figures are based on real-world dyno results from owner reports and reputable tuners.
| Metric | Stock Turbo (TD04/VF40) | BorgWarner EFR 8374 |
|---|---|---|
| Peak Horsepower (wheel) | 220-250 whp | 450-550 whp (typical street tune) |
| Peak Torque (wheel) | 240-260 lb-ft | 400-480 lb-ft |
| Boost Pressure | 14-16 psi | 25-30 psi |
| Full Boost RPM | 3000-3200 RPM | 3600-3800 RPM |
| Redline Power Drop-off | Significant after 5500 RPM | Holds strong to 7500+ RPM |
| Compressor Efficiency | ~72% peak | 78-80% peak |
Spool and Lag: Real-World Differences
Many enthusiasts fear that upgrading to a large turbo will ruin daily drivability. The EFR 8374 defies that expectation. Thanks to its lightweight titanium aluminide turbine wheel and dual ball bearing cartridge, it achieves boost only 4-600 RPM later than stock. In a 3500-lb Legacy, that translates to a momentary pause before the surge—far from the “lag monster” that older T3/T4 hybrids would deliver. Once spooled, the 8374 pulls linearly to redline, whereas the stock turbo runs out of breath at 5500 RPM.
Torque Curve and Drivability
The stock turbo’s torque peaks early and then drops off, giving a “hit” then fade. The EFR 8374 produces a broad plateau of torque from 4000 to 6500 RPM. This makes highway merging and passing effortless: you don’t need to downshift from 6th gear on the interstate—the turbo just builds boost smoothly.
Supporting Modifications: What You Need to Run an EFR 8374
Swapping in the 8374 is not a direct bolt-on affair. The stock fuel, intake, exhaust, and engine management are insufficient. Here is a breakdown of necessary upgrades:
Fuel System
- Injectors: Minimum 1000 cc/min (or 1300 cc/min for E85). Top-feed conversion recommended.
- Fuel pump: A Walbro 450 LPH or AEM 340 LPH in-tank pump, plus a surge tank or upgraded lines for high horsepower.
- Fuel pressure regulator: An adjustable FPR (e.g., Aeromotive or Radium) to maintain steady pressure under high boost.
Intake and Intercooling
- Cold air intake: A 3.5-inch or 4-inch MAF-less intake (or speed density setup) is required; the stock MAF housing will restrict flow.
- Intercooler: A front-mount intercooler (FMIC) is mandatory. The stock top-mount can barely handle the stock turbo’s heat load. The 8374 will saturate it instantly, leading to high intake temps and knock.
- Charge piping: Upgrade to 2.75-inch or 3-inch aluminum piping with silicone couplers.
Exhaust System
- Turbo-back exhaust: A 3-inch downpipe (catless or high-flow cat), 3-inch midpipe, and 3-inch cat-back. The stock 2.25-inch exhaust creates a serious bottleneck.
- Wastegate routing: The EFR 8374 has an integrated recirculating wastegate port, but for high boost you may want an external dump tube for better boost control.
Engine Management and Tuning
- Standalone ECU: The factory Subaru ECU cannot properly fuel or control timing for a turbo of this size. Options: Haltech Elite 2500, Link G4+, Motec, or a reflash on Cobb Accessport with a custom tune (limited capability).
- Professional tuning: Must be done on a dyno or via e-tuning by a Subaru specialist who knows the EFR series. Improper tuning can destroy the engine in minutes.
Engine Build Requirements
The stock Subaru 2.5L short block will not survive beyond 400 whp on pump gas reliably. For the EFR 8374, you need at minimum:
- Forged pistons (Manley, JE, CP-Carrillo)
- Forged connecting rods (Manley H-beam or I-beam)
- ARP head studs
- Upgraded valve springs and retainers (for 7500+ RPM)
- Reinforced block (semi-closed deck or aftermarket deck plate)
If you plan to run less than 450 whp, a built block is still recommended; a stock block on the 8374 at 350 whp is living on borrowed time due to the torque curve's peak cylinder pressures.
Installation Challenges and Tips
Fitting the EFR 8374 on a Legacy is more involved than on an STI due to chassis differences. Key considerations:
- Clearance: The EFR frame is larger; you may need to dent the inner fender well or use a different downpipe. A reversed intake manifold helps fit the compressor inlet.
- Oil and coolant lines: The EFR uses a cartridge with different -AN thread sizes than stock. Custom braided lines are required.
- MAP sensor and boost control: Upgrade to a 3- or 4-bar MAP sensor and use a three-port boost solenoid for precise control.
- Intercooler piping: The Legacy's narrow engine bay demands custom routing—many builders use a rotated turbo mount to simplify piping.
Expect the labor time for a DIY mechanic to be 40-60 hours, including fabrication. A professional shop might charge 30-50 hours at shop rate.
Cost Analysis: Stock vs. EFR 8374 Upgrade
Upgrading is expensive. Here is a realistic budget for a full turbo system conversion, not including the engine build:
| Component | Estimated Cost |
|---|---|
| BorgWarner EFR 8374 turbo kit | $2,500 - $3,200 |
| Upgraded fuel system (pump, injectors, FPR, lines) | $1,200 - $2,000 |
| Front-mount intercooler kit | $800 - $1,500 |
| 3-inch turbo-back exhaust | $600 - $1,200 |
| Standalone ECU + tuning | $2,000 - $4,000 |
| Intake and charge piping | $400 - $800 |
| Boost control and gauges | $300 - $600 |
| Engine build (if needed) | $4,000 - $8,000 |
| Total (no engine build) | $7,800 - $13,300 |
This is not a cheap mod, but for enthusiasts who want a unique Legacy that can embarrass many modern sports cars, the cost-per-horsepower is excellent.
Reliability and Longevity: A Trade-off
The EFR 8374 itself is extremely robust—BorgWarner uses quality bearings and materials. However, the weak link becomes the rest of the powertrain. The Subaru 5-speed manual (if equipped) can handle around 350-400 lb-ft before failing; the 6-speed STI swap is highly recommended. The automatic transmission is not rated for sustained high torque either.
With proper tuning and maintenance, a built engine with an EFR 8374 can last 50,000+ miles of street driving. Track use and frequent high-boost pulls will shorten that interval. Heat management (oil cooler, bigger radiator) is essential.
Real-World Driving Impressions
Owners who have completed the swap report a transformative experience. On a typical Legacy GT:
- Low RPM (below 3000): Feels similar to stock—civilized, with mild throttle response.
- 3000-4000 RPM: The turbo begins to sing; boost comes on strong but smoothly, unlike a sudden kick.
- 4000-7000 RPM: A relentless surge that pins you in the seat. The car becomes genuinely fast, pulling harder than a new STI or even some 6-cylinder performance sedans.
- Highway passing (5th gear at 65 mph): The turbo spools quickly under load, overtaking without a downshift.
One enthusiast on LegacyGT.com described it as “the car Subaru should have built—if they weren’t afraid of replacing transmissions under warranty.”
Is the EFR 8374 Right for You?
Consider this upgrade if:
- You are prepared for a full engine build and supporting mods.
- You have a budget of at least $10,000 (including labor).
- You want a streetable 500+ whp Legacy that can also see occasional track days.
- You accept that fuel economy will drop to single digits under boost (though highway cruising can still yield 20+ mpg).
Avoid the 8374 if:
- You only want a mild upgrade (a VF52 or a BorgWarner EFR 6258 would be better).
- Your engine or transmission is already tired.
- You cannot arrange professional tuning.
Final Thoughts
Upgrading from the stock Subaru Legacy turbocharger to the BorgWarner EFR 8374 delivers a night-and-day difference in power, response, and driving excitement. It transforms the Legacy from a comfortable daily driver into a high-performance machine capable of competing with much newer and more expensive vehicles. While the conversion requires significant investment in supporting modifications and a robust engine build, the result is a reliable, fast, and unique car that rewards the owner every time the boost gauge climbs.
For more technical details, refer to BorgWarner’s official EFR series page and consult with specialty Subaru tuners like Prime Drive or Boosted Craft Tuning for custom tuning solutions.