K.J. Jones
January 23, 2006
Photos By: Michael Johnson

Horse Sense: The stoichiometric mixture is the point where there are exactly enough atoms of oxygen to burn 100 percent of the fuel, leaving no unburned hydrocarbons, which thus produces good emissions. This point is at an air/fuel ratio of 14.7:1. When air/fuel is evaluated from the stoichiometric standpoint, "lean" would be any ratio greater than 14.7:1 and less air than the stoichiometric point would indicate a rich mixture. The EEC system in a stock 5.0 Mustang is always trying to achieve this ratio, at idle and during normal driving.

Many of you may recall the in-tank fuel pump (Holley's 255-lph, high-volume unit) upgrade we performed on Associate Editor Mike Johnson's '89 LX ("Get Pumped" p. 178) in our May '04 issue. While we didn't take our own advice at the time, we recommended adding an adjustable pressure regulator to your fuel system's schematics whenever you step up to a bigger pump for your 'Stang.

In the never-ending saga of our man Johnson's ride, the factory regulator is still in place after nearly two years of daily driving with the high-volume pump, and laughing in the face of a nitrous shot that's always at the ready. The point is no longer arguable; it's time to make a serious effort to refine the coupe's fuel map. Once again, Holley gets the nod for helping us make things right for our resident playa's Screaming Yellow Zonker.

TJ began the project by removing the inlet tube, then disconnecting all the EGR hoses and TPS wiring.

With an adjustability range of 30 to 60 psi, Holley's billet, adjustable fuel-pressure regulator (PN 512-509; $245) is made specifically for '86-'94 EFI, street 5.0s. It's a direct replacement for the nonadjustable, factory unit and is a must for cars with fuel-pump/injector/rail upgrades, and, of course, any of the big three power-adders.

Typically, it's hit-and-miss with regard to how an engine responds to fuel pressure changes. Some motors require a ton of fuel and some are happier with less. Either way, having the ability to make changes is essential for dialing in the correct amount. As a rule of thumb, an engine can-but doesn't necessarily always-perform better with a lot of fuel. As a matter of fact, performance gains and fuel economy are achieved as fuel pressure is reduced. But there is a delicate balance when it comes to fuel pressure. Lean can be mean. Running inadequate fuel pressure-especially with power adders-can lead to detonation, blown head gaskets or worse.

The air/fuel ratio is one of the measurements of record when it comes to talking about combustion. In a nutshell, the air/fuel ratio is measured in terms of mass. For example, an air/fuel ratio of 10:1 means that for every 10 pounds of air, 1 pound of fuel is burned in the combustion process. When an NMRA Factory Stock racer says his engine is "running rich," whether he knows it or not, he's actually referring to an air/fuel ratio of approximately 10.5:1. If he were to say the motor is "running lean," he's really referring to the 12.5:1 range. Technically, running lean would be a value greater than the stoichiometric 14.7:1, so at 12.5:1, the racer is just running on the lean side of optimum power, not complete and total destruction. An optimum (naturally aspirated) air/fuel ratio for good street performance is said to be 12.5:1, but it does vary depending on the car's level of modification.

Once everything has been cleared away from the throttle body, it has to be removed for better access to the fuel regulator.

A good fuel-pressure gauge and some dragstrip laps or a few quick rips up the street will help you get a solid baseline for setting fuel pressure, but the chassis dyno is the best way to determine the results of your up-or-down fuel-pressure adjustments.

Installing Holley's regulator was a simple, wham-bam affair, and TJ Einkston and Sam Lippincott at Coastal Chassis Dyno took care of handling both the swap and running the LX on the rollers. After setting fuel pressure at 39 psi, Johnson's 12.78 air/fuel ratio was leaned out to 13.92, and the simple change showed us an additional 2.65 hp at 4,800 rpm and 3.47 more lb-ft of torque at 3,900 rpm.

Holley's adjustable fuel-pressure regulator helped make Mike's fuelish 5.0 a little more spark plug and environment friendly, with just a few turns of an Allen screw. The photos and captions give a closer look at how our project went down.

Well, you have your new adjustable fuel-pressure regulator installed, but you're lost on how/where to set the pressure for your 'Stang's 5.0. The procedure does involve a few more steps than simply turning an Allen screw clockwise or counterclockwise, so we wanted to make sure you know how to do it correctly.

TJ loosened the fuel rail bolts and pulled the rail away from the intake manifold. This allowed better access to the regulator mounting screws and the fuel feed and return lines that connect to the base of the regulator.

Basically, when adjusting fuel pressure, the vacuum line that attaches to the small nipple on the regulator must be removed, and the nipple should be left open to the atmosphere while you adjust the fuel pressure. That's how Coastal Chassis Dyno went about calibrating the fuel pressure on Johnson's ride, so a "zero-vacuum" setting (vacuum is eliminated at wide-open throttle) of 39 psi could be achieved. While the fuel pressure "drop" with the introduction of vacuum to the regulator varies from car to car, Sam at Coastal Chassis Dyno believes that Mike's LX may have lost roughly 4 or 5 pounds of fuel pressure once the vacuum line was reattached.

Setting fuel pressure with the vacuum hose connected can skew your actual WOT fuel pressure. While in most cases the EEC system has the ability to compensate for small changes in fuel pressure and will adjust the injector pulse width accordingly for the best air/fuel ratio at all times, the incorrect reference could result in damage to your engine (if you're using a power-adder), as the air/fuel mixture could end up dangerously lean.

For street-driven Mustangs, the typical pressure settings are between 37 and 39 psi at idle, with the vacuum line disconnected. Again, turning the adjustment screw clockwise will fatten up your mixture, and going the other way will make it lean.