Subiesport Magazine - April/May 2008 - (Page 92)

SUBIE TECH Fueling up for more power By Tim Bailey AMONG THE LIST of go-fast parts and engine control systems that make your Subaru a joy to drive, perhaps the least appreciated is the fuel system. While turbochargers, intakes, blow off valves, and intercoolers shine like beacons of power under the hood, the fuel system components lay unnoticed and out of sight. However unseen and unobtrusive, the fuel system delivers the critical hydrocarbon substrate to support the exothermic chemical reaction which creates cylinder pressure and, ultimately, mechanical force that we enthusiasts affectionately understand as torque and horsepower. The precise and proportionate delivery of fuel and air into each combustion cycle is the first critical step toward converting chemical bonds into mechanical energy in an internal combustion engine. The focus of this article is a tutorial to understanding the Subaru fuel system and its integrated components. Once we understand how the stock system works and what its limitations are, I’ll outline some basic approaches for increasing the power-handling capacity of your Subaru to best match your desired power levels. stock, it’s generally a very good idea to upgrade the fuel pump. The most popular upgrade pumps are those produced by Walbro. These higher-pressure and higher-flow capacity pumps fit the stock location and are simply a drop-in replacement. These pumps are capable of sustaining both higher-pressure and higher-flow and are usually a sufficient fuel pump upgrade for most stock location turbo upgrades. Sustaining high flow under higher pressure is critical when boost pressure is raised. This is because when boost pressure increases, the fuel pressure must also rise to compensate. Simply put: the positive pressure inside the manifold works against the pressure in the injector. Working Fuel Pressure = Fuel pressure at the injectors – boost pressure For example, base fuel pressure is 43psi. At 10psi of boost, the fuel pressure must increase to 53psi in order to maintain 43psi of working fuel pressure. Most pumps provide the highest amount of flow at lower pressures, and flow drops off rapidly as overall pressure increases. So, raising the boost, and thereby increasing fuel pressure, puts an increasing strain on the fuel system. The ultimate in-tank fuel pump upgrade that is still suitable for daily driving is the addition of a second Walbro fuel pump. This so-called “double pumper” can deliver sufficient fuel for continued high boost and high horsepower applications. Deatschwerks offers a full-size assortment of drop in replacement fuel injectors for Subaru applications. To determine the size of fuel injector you need, you can use their handy on-line fuel injector calculator to the engine bay. At the engine bay the fuel continues through a series of lines that provides fuel to each of the fuel injectors. Precise quantities of fuel are delivered to each cylinder by a single fuel injector. Fuel injectors are simply valves that open when energized by an electric current and close under a deactivated state. The excess fuel not used is returned back to the fuel tank via a return line. Before exiting the engine bay, the returning fuel is sent through a fuel pressure regulator. This device restricts the returning flow of fuel in a dynamic manner to maintain a constant pressure at the injectors. Fuel system overview The basic components of the fuel system, in both stock and modified cars, are very similar. Fuel is stored in the fuel tank at the rear of the car just below the rear passenger seats. An in-tank fuel pump moves fuel through hard lines that run inside the car Fuel Pump The factory fuel pump is located inside the fuel tank at the rear of the vehicle. The stock fuel pump is sufficient to supply more than enough fuel under stock conditions. When making more power than 92 MAY 2008

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Subiesport Magazine - April/May 2008 Contents Travis Geny Letters Readers' Rides Subaru News Wicked in White Pink Rules Harman Motive Nouveau Tuning the 2008 STI 2008 Rally Guide Alcan 5000 Rally Alcan Unleashed WRC Rally Monte Carlo WRC Rally Sweden WRC Rally Mexico 100 Acre Wood Kumakubo! D1 Drifting Where to Buy Subiesport 2008 Impreza WRX 2007 Legacy GT 1993 Impreza Wagon Fueled Up Ad Directory Spotted!

Subiesport Magazine - April/May 2008

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