Wes Duenkel
December 7, 2017

Contemporary thoughts of EFI tuning usually conjure up images of a guy sitting in the passenger’s seat tapping a way on a laptop while a car roars on a chassis dynamometer. But cars with early forms of EFI can often be tuned without rearranging any ones or zeros. With many cars, a simple fuel pressure adjustment can accomplish the same thing as sophisticated tuning of car’s electronics.

Measuring Air/Fuel Ratio
The air/fuel ratio is usually measured using a wide-band oxygen sensor, and the signal from the sensor is translated to an air/fuel ratio display. Sensors can be installed directly in the exhaust pipe via a threaded bung, or sampled from the end of the tailpipe.

While the tailpipe sampling tube method is much easier to install, the measurements are only valid during full throttle, as outside air can contaminates the readings during idle and part-throttle. As Shannon Taylor of Boost Addicts explains, “Even at wide open throttle, we subtract five tenths from the air fuel ratio readings if it’s sampled at the tailpipe.”

Most dynamometer shops have a means to measure an engine’s air/fuel ratio in real time. Here, a technician at Boost Addicts in Madison, Tennessee places a wide-band oxygen sensor sample tube inside the tailpipe of a Mustang.
This adjustable fuel pressure regulator on this 1995 Mustang is a direct replacement for the OEM regulator, but has an adjustment screw that alters the fuel pressure setting. We found that by reducing the fuel pressure from 43 psi to 39 psi changed our air/fuel ratio at full throttle from a very rich 11.5:1 to a power-friendly 12.5:1.

Making Adjustments
Most OEM electronic fuel injection systems are programmed to maintain a stoichiometric, or 14.7:1 air/fuel ratio during normal idling and driving conditions. This is the mixture where catalytic converters are most efficient and effective in scrubbing the exhaust of nasty carbon monoxide and nitrogen oxide. The OEM narrow band oxygen sensors provide feedback to the car’s powertrain control module (PCM) on whether the mixture is over or under the 14.7:1 target. This is called, closed loop operation.

However, under wide-open throttle, most EFI systems enter open loop, where the PCM ignores the oxygen sensor readings and refers to a pre-programmed tune. The PCM injects a predetermined amount of fuel based upon readings from the engine’s sensors.

The amount of fuel injected at wide-open throttle is based upon three factors: fuel injector flow rate, injection time, and fuel pressure.

Because most EFI systems are ignore the oxygen sensor readings at wide-open throttle, altering fuel pressure (within reason) changes the air/fuel ratio at wide open throttle without affecting the air/fuel ratio during part throttle.

Most OEM fuel pressure regulators aren’t adjustable. Luckily, adjustable fuel pressure regulators that are direct replacements for their non-adjustable counterparts are available for many popular applications.

Boost Addicts