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How to Upgrade Your Foxbody Mustang’s Fuel System

Written By: Josh Honeycutt

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Upgrading your Foxbody's fuel system is cheap insurance against motor damage and other high cost repairs. Before you add performance mods, make sure your fuel system is up to handle the added need for fuel.

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Want a higher horsepower and more reliable Foxbody, but unsure of what fuel delivery upgrades are needed? By replacing your fuel injectors and other parts you can increase your power potential and driveability while safely upping the horsepower.

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Upgrading Foxbody Fuel Systems - Basics 

  • Foxbody stock fuel systems are simple and easily modified to handle more power
  • One of the first modifications should be the fuel regulator
  • Upgrading to aftermarket fuel rails can provide great insurance and could save a lot of money if something goes wrong with the stock fuel rails
  • Stock fuel pumps can handle mild bolt-ons, but with little difference in prices, most Mustang owners upgrade to a 255lph pump
  • Upgraded fuel injectors are necessary to support enhanced horsepower

The Foxbody Fuel System - An Overview

Big horsepower demands more fuel. A gasoline engine is essentially a high volume air pump. The more air pumped through the engine, the more horsepower we create. Of course, something has to keep that beautiful air pump moving, and that my friends is fuel. For the majority of us that fuel is gasoline in the form of a 13:1 air to fuel ratio. As many of us strive to force more air through our Mustang power plants via forced air, high flow intake, heads, etc., we find the stock fuel systems on these Foxbody Mustangs have limitations.

One beauty of the Foxbody fuel system is the simplicity in design. The return style fuel system makes it so easy to modify these cars to handle big power. It is nearly impossible to supply these engines with too much fuel as any excess will be sent back to the fuel tank. This makes the Foxbody fuel system pretty much fail proof.

Although it’s not capable of supporting high horsepower in stock form, there is a lot of room to upgrade your Foxbody’s fuel system. With simple upgrades like fuel injectors, fuel pump and fuel rails, your 'stang can support some serious power. Granted, if the engine is set up to produce serious power.

1986-1995 Foxbody Cobra or GT Fuel Rail Kit
Aftermarket Fuel Rails and Regulator with Braided Steel Lines

Do I Need to Upgrade My Stock Foxbody Regulator?

If you’re still relying on your stock fuel regulator, this is definitely an area for an upgrade. Often times this piece is overlooked and malfunctions. The stock unit is non adjustable. At idle, it should be set to 39psi fuel pressure. This is set with the vacuum line disconnected. Often times problems with idling or hesitation off the line indicate a defective fuel regulator. There are several aftermarket fuel regulators on the market. BBK makes a nice billet fuel pressure regulator for around $100. It is also adjustable.

1986-1993 Foxbody Mustang Adjustable Fuel Regulator
Adjustable Fuel Regulator Installed

Do I Need to Upgrade My Stock Foxbody Fuel Rails?

There are reports of people making 450 horsepower on the stock fuel rails. However, most Mustang owners will agree upgrading to some aftermarket fuel rails proves to be great insurance and could save you big bucks if something does go wrong with the stock rails. It also provides ample headroom for future upgrades. There are some nice aftermarket options out there usually constructed of anodized aluminum. BBK and Aeromotive all make pretty durable fuel rails. Usually by upgrading the fuel rails to begin with you allow yourself the option of forced induction or other major changes in the future. So this is a one-time purchase. Buy now, save later.

1986-1993 5.0L Foxbody Aluminum, High Flow Fuel Rail Kit
Aluminum, High Flow Fuel Rail Kit

Is the Stock Fuel Pump Enough to Handle Performance Modifications?

The Foxbody utilizes an in tank return style fuel pump. The pump is rated in liters per hour (lph) with the stock piece pushing 88lph. The stock pump will do for mild bolt-ons. Most Mustang owners just bite the bullet and upgrade to a 255lph fuel pump. Although there are smaller volume pumps on the market, the price difference is so small that most opt for the 255lph pump. This pump can handle a good bit of horsepower and modifications.

340lph Fuel Pump for 1986-1993 V8 Foxbodies
340lph Fuel Pump

Should I Upgrade My Stock Foxbody Fuel Injectors?

The stock fuel injectors on the foxes were 19lb/hr, which can handle about 285 hp. A good upgrade is the 24lb/hr injectors, which will support roughly 358 hp at the supplied 39psi fuel pressure. These injectors are enough to handle your typical bolt-on upgrades. Basically, the more air pumped into the engine, the more fuel needed to keep up. The following chart provides a general injector/ horsepower capability. Keep in mind, upgrading the injectors will require a mass air flow change. The transfer function for the MAF and injector size will have to be changed.

30lb/hr Fuel Injectors Installed on a 1987-1993 V8 Foxbody
30lb/hr Fuel Injectors Installed

Injector to Horsepower Ratio

Injector size

Horsepower capability











Plan Your Engine, And Build For Your Plan

As the saying goes, plan your work, and work your plan. Decide what kind of mustang you want to build. The sky is the limit for the foxbody, from supercharged or turbocharged builds to high powered N/A builds. There’s a reason the fox is so coveted by mustang lovers and drag racers alike. Lightweight from Ford, with a return style fuel system makes the fox the racer’s dream.

Fitment includes: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, GT, Cobra, LX, SVO