The Stand-Alone Port Injection Fuel System
Precision Raceworks LLC is the first company to bring a fully engineered dedicated secondary fuel system to market for a GDI (Gasoline Direct Injection) vehicle. With over a decade of experience specializing in fueling solutions, primarily in GDI platforms, we are the first to recognize the traditional methods are approaching their practical limits and offer a solution.
As the capabilities of engines like the BMW B58 and S58 continue to climb, the fueling solutions that have worked for years are no longer enough. Power levels once considered extreme are becoming increasingly common, and a new definition of extreme is quickly beginning to form.
So why design a dedicated fuel system?
Since the early days of GDI performance tuning, adding port injection has been a common solution for increasing fuel capacity. However, when it came to the low-pressure side of the fuel system, most approaches continued to follow the same traditional path: simply adding more pump capacity through larger pumps, multiple pumps, staged pumps, or sometimes combinations of all three. While this approach of splitting the feed to supply the high-pressure pump and port injection does work and can work well, the limitations of a less-than-ideal fuel system became rapidly more apparent as power levels increased.
The Non-Issue Issue Low-Pressure Oscillation from the GDI Pump
One of the commonly discussed concerns in shared low-pressure fuel systems is pressure oscillation caused by the mechanical high-pressure fuel pump. The high-pressure pump’s inlet metering and pumping events naturally create harmonics in the low-pressure supply (Oscillation). When only focusing on low pressure in logs, this appears to be a real problem, and for a car with port fueling only oscillations like these would be.
However, for supplemental fueling, while oscillations are not ideal, they are rarely a problem worth addressing. Modern OEM GDI systems use extremely precise, closed-loop fueling control with wide-band oxygen sensor feedback, fuel trims, and highly accurate direct injection control strategies. This allows the ECU to continuously correct measured lambda error and maintain target air-fuel ratios with remarkable accuracy even when supplemental port injection isn’t overly precise.
A good historical example is the BMW N54 platform and the early aftermarket batch-fire port injection systems (example: split second AIC). Those systems were far from elegant by modern standards, often delivering fuel well before the intake valves opened and relying heavily on the factory ECU to compensate for inconsistencies. Yet despite the crude nature of those early supplemental port injection systems, the factory GDI control strategy managed AFRs accurately, safely, and reliably. So even with crude injection control and oscillations holding accurate, lambda wasn’t a problem, and this has only improved with the more modern sequential fired systems we have today.
Further evidence of this is seen in OEM applications. There are many engines out there that come with port injection and direct injection from the factory. In these vehicles both share a single low pressure fuel supply and the port injection is subject to the same oscillations we see when adding port injection in the performance world. Based on these examples you can see why we are clear that this is not a real issue. It is just a less then perfect condition that has no real impact in application.
The Real Issue Fuel Pressure Scaling
Traditional 1:1 fuel pressure regulation has existed for decades and was once standard practice in both OEM and performance fuel systems. In that design, fuel pressure increases in direct relation to boost pressure, maintaining a constant injector differential pressure.
As OEM fuel systems evolved, higher baseline fuel pressures and electronically controlled pump strategies reduced the need for traditional boost-referenced 1:1 regulation. For factory applications, this works very well.
Problems begin when adding port injection to a shared low-pressure fuel system without boost-referenced, fuel pressure scaling. Every PSI of boost reduces injector differential pressure by the same amount. As differential pressure drops, injector flow at a given duty cycle also drops. At moderate power levels, this can be managed by increasing injector size and adding additional pump capacity.
At some point, however, injector sizing becomes excessive, tuning becomes less ideal, and the system increasingly relies on forcing more fuel through an architecture that was never designed to support both the high-pressure direct injection system and supplemental port injection at extreme demand levels.
Many companies, including ourselves, have tried to redesign the original fuel system, adding 1:1 regulation and deleting the factory regulators (often inside the fuel tank). However, the sophisticated factory control system prevents 1:1 scaling. By monitoring pump voltage, pump current, and sometimes fuel pressure & pump speed, the factory control system knows exactly how much fuel should be supplied and what the fuel pressure should be (even without a sensor). Based on this data, the vehicle electronics will reduce duty cycle to the pump as you try to increase pressure so that pressure does not increase and the factory-programmed pressure target is maintained.
The Solution
The right way was never simply adding more fuel pumps and hoping the rest of the system could keep up as is done in a common split feed system. The real solution was dedication... literally.
Instead of forcing the factory low-pressure system to serve two completely different jobs, we engineered a truly dedicated secondary fuel system purpose-built for supplemental fueling. By separating the fuel supply for the mechanical high-pressure direct injection system from the port injection system, each can operate in the environment it is designed for.
The factory direct injection system retains a stable and reliable low-pressure feed for the high-pressure pump (as the auto manufacturer intended), while the secondary system operates with its own dedicated pump(s), filtration, fuel lines, and boost-referenced 1:1 pressure regulation to maintain consistent injector differential pressure regardless of boost level.
This approach does more than solve fuel pressure scaling. By separating the fuel demand into completely independent supply paths, the system eliminates the compromises of a split feed low-pressure architecture while significantly increasing total fuel delivery capability. Instead of forcing all fuel volume through a single feed path, the workload is distributed across dedicated systems optimized for their specific purpose. The result is improved fuel stability, more predictable tuning behavior, and a much more scalable solution for serious power applications.
Now to be clear, that does not mean a dedicated secondary fuel system is the only path for every build. At more moderate power levels, traditional solutions like upgraded or staged split feed systems can absolutely work and may be a practical option depending on the application and performance goals. But as power levels continue to climb, the compromises of a shared low-pressure fuel system become increasingly difficult to ignore. What begins as an acceptable workaround at lower power quickly becomes a limiting factor at the top end.
That is where dedication becomes more than a premium option; it becomes the correct engineering solution. Because serious power demands a serious fuel system. Dedication truly is the key to success.