Converting a high-performance commercial turbocharger, such as the BorgWarner S366, into a turbojet engine is a complex engineering project that requires a deep understanding of thermodynamics, fluid dynamics, and rotational mechanics. The S366, typically utilized in high-boost internal combustion applications, provides a robust rotating assembly capable of handling the high rotational speeds (up to 120,000 RPM) required for jet propulsion.
The S366 features a forged milled wheel (FMW) compressor and a robust turbine section. To convert this unit, one must understand the factory tolerances and limitations.
It is imperative to maintain these clearances. Exceeding the endplay limit will result in contact between the compressor wheel and the housing at the high pressures required for self-sustaining combustion.
The most challenging aspect of the conversion is the fabrication of the annular or can-type combustion chamber. The goal is to introduce fuel into the pressurized air stream exiting the compressor before it enters the turbine housing. For a turbojet conversion, propane or Jet-A fuel is typically used.
The combustion chamber must be constructed from heat-resistant alloys, specifically Inconel 625 or 310 Stainless Steel, to withstand sustained operating temperatures exceeding 850°C. The internal pressure must remain stable to prevent compressor surge. The bypass ratio and air-to-fuel ratio must be finely tuned; a stoichiometric mixture will cause turbine blade ablation. Engineers typically target an air-to-fuel ratio of approximately 50:1 to 60:1 for safe operation.
The BorgWarner S366 relies on a pressurized oil feed system for cooling and lubrication. In a turbojet application, the oil must be kept below 100°C to prevent coking. A dedicated external oil pump (capable of 30-50 PSI) and an oversized oil cooler are mandatory. Failure to maintain oil pressure will cause the shaft seals to leak, leading to catastrophic failure of the bearing housing.
If the rotating assembly must be disassembled for inspection, the following torque values must be strictly adhered to during reassembly:
Before initial firing, the unit must be balanced dynamically. Any imbalance in the rotating assembly will lead to immediate shaft failure at high RPMs. Always utilize a blast shield when conducting the first test runs. Electronic monitoring of Exhaust Gas Temperature (EGT) is critical. Use a Type K thermocouple placed directly at the turbine inlet. If the EGT exceeds 950°C, the fuel supply must be cut immediately to prevent melting of the turbine wheel blades.
The conversion of a BorgWarner S366 is an exercise in precise measurement and thermal management. While the S366 offers an excellent platform for experimental jet propulsion, the user must prioritize structural integrity, proper lubrication, and accurate EGT monitoring to ensure a successful and safe operation. Always consult the specific BorgWarner service manual for your model revision to ensure component compatibility.
The hydrodynamic journal bearing system utilized in the BorgWarner S300SX3 architecture (e.g., part numbers 177281, 177275) relies on a constant oil wedge to maintain radial stability at high shaft velocities. When repurposed for turbojet applications, the bearing housing must be modified to prevent oil coking caused by heat soak during shutdown, as the absence of a cooling internal combustion cycle leads to rapid temperature spikes in the center housing rotating assembly (CHRA). Utilizing a synthetic, high-viscosity index oil—such as a 15W-50 ester-based racing lubricant—is recommended to maintain film strength under extreme thermal loading. Furthermore, the oil return line must be gravity-drained with a minimum internal diameter of 12mm to ensure no backpressure exists, which would otherwise force oil past the piston-ring style dynamic seals, leading to hazardous ingestion into the compressor or turbine stages.
Precision balancing of the rotating assembly is non-negotiable for units reaching 120,000 RPM. The S366 features extended tip technology on the compressor wheel, which significantly increases mass inertia at the blade peripheries compared to standard-geometry wheels. During reassembly, the alignment of the compressor wheel to the shaft must follow the factory index marks; failure to align these precisely shifts the center of gravity, initiating low-frequency vibrations that lead to rapid fatigue of the turbine shaft material. Engineers should utilize a vibration analyzer to ensure the dynamic balance remains within the O-grade ISO 1940 standard. Additionally, verify that the turbine wheel exducer does not encounter thermal growth contact with the turbine housing shroud; clearance must be checked at ambient temperature using feeler gauges to ensure a minimum of 0.50mm, accounting for the differential expansion coefficients of the nickel-alloy turbine wheel and the cast iron or stainless steel turbine housing.
The integration of a fuel delivery system requires a calibrated, multi-point fuel injector manifold located upstream of the primary combustion zone to ensure atomization before the airflow enters the turbine housing. Given that the S366 is designed for pressurized manifold applications, the housing inlet—often a T4 divided twin-scroll configuration—creates significant backpressure, which can induce compressor surge if the burner nozzle calibration is incorrect. To mitigate this, a blow-off valve (BOV) calibrated for high pressure, such as a Tial Q series or similar, should be plumbed into the compressor discharge pipe to act as a surge protection device during rapid deceleration or fuel cut events. Monitoring the pressure ratio across the compressor using a differential pressure transducer (e.g., Honeywell PX2 series) is mandatory to stay within the "island" of the compressor map, preventing catastrophic surge cycles that induce axial oscillation and destroy the thrust bearing stack, specifically the delicate brass or copper-lead thrust washers that locate the shaft within the CHRA.
To optimize the thermodynamic efficiency of the S366-based turbojet, the integration of the combustion chamber must account for the specific discharge velocity of the FMW compressor. Utilizing a 13007110005 major rebuild kit is critical, as the stock journal bearings are not designed for the sustained high-load, non-pulsed thermal environment of a Brayton cycle conversion. When assembling the CHRA, ensure the application of a high-molybdenum-disulfide assembly lubricant to the thrust bearing stack—comprising the 360-degree thrust collar and twin brass thrust washers—to minimize friction during initial spool-up. Any deviation in the axial stack height will lead to catastrophic impeller-to-housing contact, particularly given the aggressive lean angle of the S366 extended tip compressor vanes. It is essential to perform a high-speed core balancing procedure at a certified facility using an IRD or Schenck balancing machine to ensure that the rotor assembly meets the G2.5 balance quality grade at 100,000+ RPM; failing to verify this will induce harmonic resonance, resulting in high-cycle fatigue (HCF) of the turbine shaft material.
The management of the Turbine Inlet Temperature (TIT) is the limiting factor for structural longevity, as the inconel turbine wheel is prone to creep-induced blade lengthening. To prevent thermal runaway, implement a staged fuel injection strategy using high-impedance Bosch EV14-style injectors, which provide superior atomization patterns compared to simplistic single-point propane nozzles. The combustion liner design must incorporate dilution air holes (primary, secondary, and tertiary zones) specifically engineered to manage the flame tube internal pressure drop. By utilizing a total pressure probe upstream of the turbine housing (T4 entry), engineers can monitor the pressure ratio; if the backpressure exceeds 2.5:1 relative to ambient, the turbine nozzle area is effectively too small, forcing the compressor out of its stable operating window. This necessitates a precision nozzle ring or vane adjustment—if the housing allows—to shift the turbocharger away from the surge line and maintain a stable combustion core during transient state transitions.
Addressing the oiling requirements, the shift from internal combustion engine (ICE) pressure profiles to turbojet operation renders standard 30-50 PSI oiling insufficient for sustaining the bearing wedge under continuous load. A dedicated, scavenged dry-sump oil system utilizing a Tilton or Peterson fluid pump is recommended to guarantee a minimum flow rate of 1.5 liters per minute at peak shaft speeds. Furthermore, the installation of a 10-micron filtration system is non-negotiable, as metallic particulate generated during the break-in phase of the custom combustion hardware will rapidly degrade the bearing journals. To prevent post-shutdown heat soak—the primary cause of carbon buildup and oil coking in the bearing cavity—an electric oil circulation pump must remain active for a minimum of 180 seconds following fuel cut. This thermal management protocol preserves the integrity of the dynamic piston-ring seals and prevents the degradation of the center housing's internal tolerances, which are calibrated to a precise 0.038mm - 0.051mm journal clearance.