The 13B rotary engine is a unique platform that requires a different approach to forced induction compared to conventional piston engines. Its distinct cycle, with intake, compression, power, and exhaust occurring in separate chambers as the rotor turns, creates specific challenges for turbocharger matching and system integration. The BorgWarner S300SX 13B single turbo setup has become a benchmark conversion for Mazda RX-7 and RX-8 owners seeking a significant, reliable increase in horsepower. This guide provides a deep, technical look at the entire process, from selecting the correct turbo components to dialing in the ECU tune.

Why the BorgWarner S300SX is Well-Suited for the 13B Rotary

The 13B rotary engine has inherently different airflow characteristics than a reciprocating piston engine. The intake and exhaust events are dictated by the opening and closing of ports in the housings, rather than valves. This results in a lack of intake/exhaust valve overlap, which significantly affects how a turbocharger behaves on the engine.

The BorgWarner S300SX, specifically the SX-E 83-75 or the 82-75 models, offers an excellent balance of spool characteristics and top-end flow for a ported or streetported 13B. Key attributes include:

  • Billet Compressor Wheel: The S300SX family features a lightweight, high-strength billet aluminum compressor wheel. This reduces rotational inertia, allowing the turbo to spool faster than the older cast wheel designs. For a street rotary, this means boost arrives earlier in the powerband, making the car far more responsive.
  • High-Efficiency Turbine Housing: The .83 A/R T4 divided housing or the .63 A/R T3 housing options allow you to match the turbo to your specific porting and displacement. A divided housing paired with a divided manifold capitalizes on the pulsed exhaust flow from the rotary ports, improving transient response.
  • Heat Management: Rotaries generate extremely high exhaust gas temperatures (EGTs), often exceeding 1600°F under sustained load. The robust construction of the S300SX turbine housing and wastegate porting can handle these thermal loads better than many budget-oriented turbos.
  • Boost Threshold: The S300SX 83-75 can reach full boost (15-20 psi) by 3500-4000 RPM on a streetported 13B, depending on the turbine housing selection. This provides a wide, usable powerband without the violent lag of larger frame turbos.

Critical Prerequisites: Building a Foundation for Boost

Before you mount the BorgWarner S300SX, the supporting systems must be up to the task. A rotary engine is very unforgiving of lean conditions or detonation. Proper preparation is non-negotiable for long-term reliability.

Fuel System Capacity

The rotary engine rejects a significant amount of unburnt fuel out the exhaust port to cool the leading spark plug and apex seals. When you add a turbocharger, the fuel demand skyrockets. Your stock fuel system will be woefully inadequate.

  • Fuel Pump: A high-flow 255 LPH or 340 LPH in-tank pump is the minimum requirement. External surge tanks and Bosch 044 pumps are highly recommended for consistent fueling under high boost.
  • Injectors: Standard high-impedance injectors like 750cc primaries and 1000cc secondaries are a starting point. For the power levels an S300SX is capable of (400-550 whp), you should look at 1000cc primaries and 1650cc secondaries. Electronic boost reference fuel pressure regulators (1:1 rising rate) help maintain injector control.
  • Fuel Type: On a high-boost rotary, pump gas is a limiting factor. A mixture of ethanol or race gas is often required to safely run 18+ psi without triggering detonation.

Ignition System Upgrades

The stock Mazda leading and trailing ignitors are prone to failure under the high cylinder pressures of forced induction. A weak spark will lead to misfires, unburnt fuel in the exhaust, and catastrophic detonation.

  • Coil Upgrade: Swapping to a set of GM LS coils or a dedicated CDI (Capacitive Discharge Ignition) system is an industry standard. These systems provide a much hotter, more reliable spark.
  • Spark Plugs: Use a colder heat range plug (e.g., NGK 9 or 10 series). Gapping should be tightened to around 0.025 inches to prevent spark blowout at higher boost levels.

Engine Health Assessment

Do not bolt a BorgWarner S300SX onto a worn-out rotary. A compression test is mandatory. Low compression indicates worn apex or side seals, which will make the engine impossible to tune properly and will lead to hard starts and poor idle.

The Complete Installation Procedure

This process assumes you have a suitable stand-alone ECU, a front-mount intercooler, and a high-quality exhaust manifold designed for the T3/T4 flange pattern of the S300SX.

Step 1: Disassembly and Preparation

  1. Disconnect the battery. Drain the engine coolant and engine oil.
  2. Remove the stock intake pipe, air filter, intercooler piping, and turbo assembly.
  3. Remove the stock exhaust manifold and downpipe. Inspect the studs in the iron side housings. They are known to break. Use penetrating oil and heat if necessary. Replace any compromised studs.
  4. Remove the oil filter housing or sandwich plate. You will need to install a plate with ports for the turbo oil feed and oil pressure/temperature sensors.

Step 2: Turbo Manifold Installation

Securely mount the new exhaust manifold. Use high-temperature thread locker on all studs and bolts. Torque the manifold nuts to the manufacturer's specification in a gradual, cross-hatch pattern to prevent warping the flange. A divided manifold paired with a divided inlet on the S300SX turbine housing is the performance gold standard for rotaries.

Step 3: Mounting the BorgWarner S300SX

  1. Remove the protective packaging from the turbo. Inspect the compressor and turbine wheels for any shipping damage. Spin the shaft to ensure smooth rotation.
  2. Install the correct gasket onto the manifold flange. Carefully lower the S300SX onto the studs.
  3. Torque the turbo mounting nuts to the specified value.
  4. If you are using an internal wastegate, connect the actuator arm to the wastegate flapper. Secure the circlip. If using an external wastegate, mount it to the manifold or turbo scroll and route the pressure reference line.
  5. Clock the compressor housing to align with your intercooler piping. This involves loosening the V-band clamp and rotating the housing. Do not clock the center housing without properly repositioning the oil drain and feed ports.

Step 4: Oil and Coolant Line Routing

This is the most critical step for turbo longevity on a rotary. The 13B has high oil pressure (80-100 psi cold). The turbo bearing system requires a specific oil volume and pressure to operate correctly without pushing oil past the seals.

  • Oil Feed: Use a -3 or -4 AN stainless steel line. Install an oil restrictor in the feed line. A 0.060" to 0.085" restrictor is required for a journal bearing turbo like the S300SX. Without a restrictor, the high oil pressure can overwhelm the turbo seals, causing smoke and failure.
  • Oil Drain: Use a -10 AN line minimum. The oil drain must be gravity-fed. It needs a constant downward slope to the oil pan. A scavenge pump is required if the turbo is mounted very low relative to the oil pan.
  • Coolant Lines: If your S300SX center housing is water-cooled, plumb these into the engine's cooling system. A common method is to T into the heater core lines.

Step 5: Charge Side and Cold Air Piping

  1. Mount your front-mount intercooler (FMIC). Secure it properly to prevent movement under pressure.
  2. Route the hot-side charge pipe from the compressor outlet to the intercooler inlet. Ensure there are no sharp bends.
  3. Route the cold-side charge pipe from the intercooler outlet to the throttle body.
  4. Install a blow-off valve (BOV) on the cold-side pipe, near the throttle body. A 50/50 recirculating BOV helps maintain drivability on rotary engines.
  5. Use silicone couplers and T-bolt clamps on all connections to prevent boost leaks.

Step 6: Intake and ECU Wiring

  1. Install a large air filter (conical style) on the turbo inlet. The filter should be shielded from engine heat with a heat shield.
  2. Wire in the MAP sensor, IAT sensor, and wideband O2 sensor into your stand-alone ECU harness.
  3. Calibrate the boost control solenoid if you are using one.

Tuning Strategy for the 13B Rotary Turbo

A rotary engine has a very narrow window for safe tuning compared to a piston engine. The lack of a knock threshold means detonation can destroy an apex seal instantly. The tuning strategy must be conservative and methodical.

  • Fuel Mixture: Under boost, a rotary requires a rich mixture. Target 11.0:1 to 11.5:1 AFR (Lambda 0.75 to 0.78). This rich mixture helps cool the combustion chamber and prevent detonation. At idle and cruise, rotary engines naturally run rich (12.5:1 - 13.0:1) to cool the leading spark plug.
  • Ignition Timing: The 13B uses leading and trailing spark plugs. The trailing plug fires slightly later and ignites a fuel-rich area near the exhaust port. Trailing timing is very sensitive to detonation. A safe baseline for a S300SX setup is 10-12 degrees leading timing at peak boost (15 psi). Trailing timing should be 5-10 degrees retarded relative to leading.
  • Boost Tuning: Start with low boost (8-10 psi). Log your fuel trims, EGTs, and wideband readings. Only increase boost once the fuel map is stable and safe. A pyrometer in each exhaust runner is an excellent safety tool.

Common Pitfalls and Troubleshooting

Boost Creep

Many S300SX installations suffer from boost creep, especially with internal wastegates. The large turbine housing flow can overcome the standard wastegate port. Solution: Port the wastegate hole and use a larger flapper valve. An external wastegate (Tial 44mm MVR) is the definitive fix.

Oil Leaks from the Turbo Drain

If you see blue smoke on deceleration or after hard driving, the turbo oil seals are being overwhelmed. Solution: Check the oil restrictor. Ensure the oil drain line is -10 AN and has a continuous downhill slope with no tight 90-degree bends.

High Exhaust Gas Temperatures (EGTs)

If EGTs exceed 1600°F, you are running lean or your timing is too aggressive. Solution: Enrich the fuel mixture in the affected load cells or retard the leading ignition timing.

External Resources

For further technical specifications on the turbocharger you are installing, consult the official BorgWarner product page for the S300SX series. For detailed engine management strategies specific to the Wankel cycle, a stand-alone ECU tuning guide such as those provided by Haltech's rotary support documentation is invaluable. Reliable sourcing of engine components and gaskets can be found at specialists like Atkins Rotary.

Conclusion

Installing a BorgWarner S300SX 13B single turbo on a rotary engine is a rewarding project that delivers a dramatic improvement in power and drivability. The success of the project hinges on understanding the unique needs of the Wankel engine: high fuel flow requirement, sensitivity to detonation, and high oil pressure. By meticulously preparing the fueling, ignition, and oiling systems, and by tuning the ECU with a conservative, data-driven approach, you can build a rotary-powered vehicle that is both fast and reliable. The result is a street car or weekend warrior with a linear, potent powerband that embodies the spirit of the rotary.