The Audi B5 S4, launched in the late '90s, remains a cult icon among performance enthusiasts. Powered by a 2.7-liter twin-turbocharged V6, it delivered a respectable 250 horsepower from the factory. But its true potential lies in its robust engine block and sophisticated all-wheel-drive system, making it a prime candidate for serious power upgrades. While factory K03 turbos can be upgraded to K04s or hybrid units, they quickly hit airflow limits. For those seeking a genuine leap to 480+ horsepower and beyond, the BorgWarner S366 turbo kit has become a proven solution. This article dissects how this single, large-frame turbocharger transforms the B5 S4, covering the engineering behind the kit, installation nuances, tuning requirements, and the supporting modifications needed to unlock reliable four-digit-wheel-horsepower potential.

The BorgWarner S366 Turbocharger: A Technical Deep Dive

At the heart of this upgrade is the BorgWarner S366 turbocharger, part of the company’s popular S-series line designed for high-horsepower street and race applications. Unlike the factory twin-turbo setup, the S366 kit replaces both small turbos with a single, larger unit mounted in a custom location (usually on the passenger side of the engine bay). The S366 features a 66mm compressor wheel and a 62mm turbine wheel, with a T4 or T3 turbine housing depending on the kit variant.

Compressor and Turbine Specifications

  • Compressor Inducer/Exducer: 66.4mm / 88mm (7-blade or billet options available)
  • Turbine Wheel: 62mm (79.4mm exducer), often in SuperCore or A/R .70 or .82 housings
  • Bearings: Available in journal bearing (standard S366) and ball bearing (S366 SX-E or SX-S) versions. Ball-bearing options reduce spool time by 500–800 RPM.
  • Flow Capacity: The S366 can flow over 85 lb/min, supporting 850+ horsepower on gasoline given adequate fuel and boost. For a B5 S4, 480–600 hp is easily achieved with conservative boost (20–25 psi).

BorgWarner’s S360 and S369 siblings have slightly different trims, but the S366 strikes a balance between responsiveness and top-end flow. Its 60mm vs 66mm comparison is often discussed on forums like Audizine. The larger S366 is preferred for those targeting over 500 whp without excessive backpressure.

Kit Compatibility and Installation Considerations

While “BorgWarner S366 Turbo Kit” for the B5 S4 is a generic term, the most common configuration uses a custom manifold, downpipe, and intake piping. Several vendors (like SRM, Trackspec, and JHM) offer complete single-turbo conversion kits. Installation is not a bolt-on affair; the entire factory twin-turbo assembly must be removed, along with the airbox, Y-pipe, and associated plumbing.

Key Installation Steps

  1. Remove Front Bumper and Intercoolers: The S366 setup typically uses a larger single front-mount intercooler (FMIC) in place of the factory side-mounts.
  2. Detach Exhaust System: To extract the stock downpipes and turbos, the exhaust must be unbolted from the Y-pipe back to the downpipes.
  3. Install Turbine Manifold: Most kits use a stepped or equal-length manifold to merge the two exhaust banks into a single T4 flange. Proper orientation is critical to clear the frame rail.
  4. Mount the Turbo: The S366 bolts to the manifold, often requiring an oil feed from the engine’s oil filter housing and a drain line back to the oil pan.
  5. Custom Intake and Intercooler Piping: A single cone filter feeds the compressor via silicone hoses. The cold side connects to a large FMIC, which then splits back to the throttle body.
  6. Oil and Coolant Lines: The S366 journal-bearing version requires a restrictor in the oil feed (ideal pressure 40–60 psi). Ball-bearing versions have specific lubrication requirements.
  7. Downpipe and Wastegate: A 3-inch or larger downpipe exits the turbine housing, often with a 44mm or 45mm external wastegate to control boost accurately.

Many builders recommend replacing the motor mounts and transmission mount during the installation because the engine torque path changes significantly. The whole process can take 40–80 hours for a first-timer; professional installation is advisable unless you have extensive fabrication experience.

Tuning the S366: Unlocking 480+ Horsepower

After the hardware is in place, tuning is the single most critical factor. The stock Motronic ECU (M-box, A-box, or later variants) requires a custom chip tune or a standalone ECU (e.g., Syvecs, Vi-PEC, or Haltech). Most tuners use ME7.x tuning suites or MAF-less MAP-based strategies to scale the fueling and boost maps.

Fuel System Upgrades

  • Injectors: Stock 32 lb/hr injectors will max out quickly. Upgraded 52 lb/hr (550cc) or 60 lb/hr (630cc) injectors are mandatory. Many owners run ID1050x or FIC 1000cc for room to grow.
  • Fuel Pump: A single Walbro 450 lph in-tank pump or a Bosch 044 inline pump is typical. For 500+ whp, a dual-pump setup may be needed.
  • Fuel Pressure Regulator: A 1:1 rising-rate regulator (e.g., Fuelab) helps maintain consistent pressure under boost.

Boost Control and Ignition

The S366 can easily produce 25–30 psi of boost, but the B5 S4’s 2.7L engine’s stock compression ratio of 9.3:1 limits safe boost on pump gas. A good tuner will use boost control solenoid (MAC valve) and a 3-port or 4-port controller to manage the external wastegate. Ignition timing is pulled back under boost to avoid knock; quality 91 or 93 octane fuel allows around 20–22 psi for 480 hp. With methanol/water injection or E85, 25–28 psi and 550+ hp are attainable.

Dyno tuning is strongly recommended. Many shops post results on AudiWorld’s B5 S4 forum. Typical numbers for a well-tuned S366 with supporting mods range from 480–560 whp (crank horsepower of 580–680) with torque peaking around 450–500 lb-ft.

Performance Characteristics: Spool and Drivability

The B5 S4 with a single S366 drives differently than the original twin-K03 setup. The smaller stock turbos spool almost instantly by 2500 RPM, but they run out of breath around 5500 RPM. The S366, being a larger single unit, starts building boost later – around 3000–3500 RPM and hits full boost (20+ psi) by 4000–4200 RPM. This “lag” can be mitigated with ball-bearing center sections and optimized exhaust manifolds.

Once spooled, the S366 pulls linearly and strongly all the way to 7000+ RPM, with power still climbing past the stock rev limiter. The torque curve is broader than a small-frame single turbo, making it more streetable than a monster S400 series. Owners describe it as “laggy below 3500, then violent after 4000.” For those chasing drag strip times or high-speed rolls, this trade-off is acceptable. For daily driving, a 2.7L stroker engine or porting the heads can improve low-end response.

Essential Supporting Modifications

To safely handle 480+ hp, the B5 S4’s drivetrain and cooling systems need upgrades. The stock clutch will slip at anything over 350 lb-ft of torque. A single-mass flywheel and a Stage 3+ or twin-disc clutch (e.g., South Bend, Clutch Masters) is mandatory. The six-speed manual gearbox (01E) is relatively strong, but at 550+ whp, the differential and axles become weak points – upgraded DSS axles are common.

Cooling System

  • Intercooler: A large core (3–4 inches thick) with low pressure drop – essential to keep intake air temperatures (IAT) below 120°F during pulls.
  • Radiator: Upgrade to an all-aluminum radiator (e.g., CSF or Mishimoto) to cope with extra heat load.
  • Oil Cooler: A thermostatic Oil cooler kit (Setrab or Mocal) helps prevent oil breakdown under sustained boost.

Exhaust System

A 3-inch cat-back exhaust with a high-flow catalytic converter (or race pipe) reduces backpressure. Header-back systems are common.

Reliability and Maintenance Tips

A properly built S366 B5 S4 can be reliable for daily driving and track days, provided maintenance is strict. The 2.7L engine is known for timing belt/chain issues – at this power level, replace the timing belt, water pump, cam seals, and tensioners every 60,000 miles. Oil changes should be every 3,000–5,000 miles with full synthetic 5W-40. Heat management is critical: wrap the downpipe and manifold with thermal blankets to reduce under-hood temperatures, which can degrade wiring and hoses.

Also, the single-turbo setup removes the factory “turbo bleed” lines and PCV system. A catch can (e.g., Radium) prevents oil vapor from coating the intercooler. Regular boost leak testing every 10,000 miles keeps the system efficient.

Real-World Results and Community Feedback

Numerous B5 S4 owners have documented S366 builds online. For example, a well-known member on Audizine reported 520 whp on 93 octane with 60 lb injectors, Walbro 450, and a 3-inch exhaust. Another on the B5 S4 Facebook group ran 11.7 @ 124 mph on a race tune. These results confirm that the S366 kit can exceed 480 hp with proper preparation.

For an official product overview, BorgWarner’s website provides technical data sheets. Additionally, performance shops like 034Motorsport offer custom ECUs tailored for single-turbo setups.

Conclusion: Is the S366 Kit Right for You?

The BorgWarner S366 turbo kit is a proven path to 480+ horsepower and beyond for the B5 S4, but it demands commitment. The installation is involved, the supporting modifications are extensive, and the spool characteristics favor high-RPM driving. However, for those who crave a dramatic power increase that transforms the car into a serious street/strip performer, the S366 delivers. With proper tuning and maintenance, this setup can provide years of adrenaline-filled driving. If you’re willing to invest the time and money, the S366 will reward you with a B5 S4 that pulls harder, sounds more aggressive, and turns heads at every gear change.