Fuel Delivery

Any forced-induction retrofit has to consider the capacity of the fuel delivery system. One leading cause of engine failures during these projects is a lean AFR caused by insufficient fuel delivery.

The car had a TBI engine from the factory. It had a 15 PSI fuel pump in the tank. I replaced this with a new TBI pump. This is the transfer pump. It helps to prime the main pump which is an inline pump mounted below the trunk near the gas tank. The main pump is a Motorcraft unit for a Ford pickup. It is rated at 95 PSI and seems to haver ample flow capacity, especially with the transfer pump helping it.

All injectors have a somewhat limited range of fuel flow rates with a fixed fuel pressure. This is caused by the inaccurate metering at very small pulse widths. The larger the injector, the worse it works at idle. An injector large enough to handle the upper flow rate at high power operation, may not be consistent enough for the engine to have a stable idle quality.

There are ways to work around this. The injectors need to be sized correctly to start with. Combine this with a rising-rate fuel pressure regulator and the system can accurately deliver proper fuel over the entore engine operating range. The rising rate FPR reduces fuel pressure at high-vacuum (idle) conditions. The ECM can then provide a longer pulse-width to the injectors at idle. This keeps the injectors in their more accurate duty-cycle range without any excessively small pulse-widths. At high-boost conditions, the rising-rate FPR increases fuel pressure above the level that a standard FPR will. This allows the ECM to use a lower pulse-width at high power conditions. Thus it allows the same injector to flow at a higher rate than it otherwise could, before reaching 100% duty-cycle.

To take full advantage of this, the ECM needs to be tuned to add duty cycle at low MAP conditions and reduce duty cycle at high MAP conditions.

This picture shows the factory fuel pressure regulator which is built into the fuel rail. It has been gutted and the vacuum fitting welded closed to prevent fuel leaks. The silver bolt in the fuel rail to the right of the old FPR is a plug where the original cold-start injector tube connected. The new ECM doesn't need a cold-start injector.

The Malpassi regulator shown above is in the return line before the fuel returns to tank. It has an adjustable base pressure and is referenced to engine manifold pressure.

This is the Motorcraft main pump viewed under the car near the factory fuel filter. It was installed with the original turbo system and has been there for about 8 years! The crud has accumulated slowly over 100,000 miles!