BMW S58 Auxiliary Split Fuel System

 

The Spool Performance S58 Auxiliary Fuel System is designed for S58 owners pushing beyond the limits of the factory fuel system who want a solution that’s properly engineered, easy to install, and built for long-term reliability.

From the start, our focus was on correct fuel architecture and real-world usability. Every component and layout choice was made to deliver stable, predictable fueling at higher power levels while keeping installation clean and OEM-friendly.

Why a Properly Split DI + PI Fuel System Matters

As power levels increase on the S58 platform, fueling challenges typically come from supplying direct injection (DI) and port injection (PI) from the same fuel feed.

When DI and PI share a feed:

• The pulsed nature of DI can introduce pressure fluctuations that disrupt fuel flow to the PI injectors

• Port injection requires boost-referenced fuel pressure to maintain consistent injector differential under load

• At higher boost and airflow, maintaining precise and repeatable fueling becomes more difficult

By separating the DI and PI supply, each system operates independently and exactly as intended. The result is smoother fuel delivery, improved consistency, and greater confidence as power increases.

Designed to Install Cleanly

A significant amount of development time went into making this system straightforward to install without unnecessary modifications.

• The OEM fuel pump top hat is retained using additional precision-machined aluminum fuel fittings that integrate directly into the factory top hat, eliminating the need for repinning or modifying factory wiring

• Optional billet top hats are available separately for those who prefer them

• A plug-and-play wiring harness is supplied for direct compatibility with the Reflex controller, simplifying installation and control

These design choices keep the install clean, reduce complexity, and maintain a factory-like fit and finish.

Scalable Pump and Control Options

The system is built to grow with your setup:

• Dual Walbro 450 fuel pumps included standard

• Optional Walbro 525 upgrades available for higher horsepower goals

• Choice of:

  • Standard relay configuration

  • Solid-state relay option for full PWM pump control

This flexibility allows you to match the fuel system to your current build while leaving room to expand.

High-Quality Fuel Plumbing

All fuel lines included in the kit feature triple-layer PTFE construction:

• Inner PTFE core for full fuel and ethanol compatibility

• Middle stainless steel reinforcement for pressure strength

• Outer nylon protective layer for abrasion resistance and durability

This ensures long-term reliability in high-heat, high-vibration environments.

Compact, Purpose-Built Fuel Pressure Regulation

The system includes Spool Performance’s in-house designed compact fuel pressure regulator, developed specifically for auxiliary fuel applications:

• Boost-referenced for accurate pressure control under load

• Stable regulation at high flow rates

• Compact design for clean engine bay packaging

• Built to withstand demanding conditions

Built to Support Serious S58 Builds

Whether you’re building a strong street car or planning a higher-power setup, the Spool Performance S58 Auxiliary Fuel System delivers the fueling control and capacity needed to push the S58 platform with confidence.

With properly separated DI and PI supply, OEM-friendly integration using precision aluminum fittings, Reflex compatibility, high-quality plumbing, and scalable pump options, this system is designed to perform reliably as your build evolves.

If you’re ready to upgrade your S58 fueling with a system that’s thoughtfully engineered and genuinely easy to live with, this is the solution built to get you there.

 

Fitment

This auxiliary fuel system is designed for BMW vehicles equipped with the S58 engine, including:

• BMW M3 (G80) – 2021+

• BMW M3 Touring (G81) – 2023+

• BMW M4 (G82 / G83) – 2021+

• BMW M2 (G87) – 2023+

• BMW X3 M (F97) – 2020–2024

• BMW X4 M (F98) – 2020–2024