Fox Body Fuel Delivery System Overview
Perhaps now you have got whiff of the idea that if you can add more fuel, you can get a bigger boom. So, why not just dump 500 gallons per hour through the engine and along the way make a couple million horsepower? That would be nice (albeit expensive) but unfortunately this is not the case. Remember, the fuel has to mix with the air, and if there isn’t enough oxygen to mix with all the fuel, then unburned gasoline will be left over. Here’s a quick high school chemistry review. The ideal ration between air to fuel, for a complete and optimal combustions, is 13 parts air to one part fuel. Thus 1 gallon of fuel needs to mix with 14 gallons of oxygen to ignite and provide maximum power. So for the case of an internal combustion engine, air is the limiting factor in making horsepower.
OK, enough science talk. What does this actually mean in terms of your Mustangs fuel system? Let’s go over the stock setup real quick. Out of the factory, 1987-1993 5.0 Foxbodies all came with an in-tank electric fuel pump rated at 88 lph (litres per hour) that would send fuel through the lines up through the regulator and into the fuel rails, where it would be dispensed into the proper combustion chamber via the 19 lb/hr (pound per hour) fuel injectors. The tank itself has a capacity of 15.4 gallons (60 litres). All in all, for the stock 225HP engine, this system performs very well, but does not leave much room to grow.
Now, say you are going to be pushing more than the factory 225 ponies, you will need to make a few adjustments to the fuel system to enhance its capacity.
Fox Body Fuel Pumps
As mentioned, the stock pump is an electric in-tank pump, flowing 88 lph. Surprisingly durable, these pumps fuel the stock motor without much fuss. Now, change the motor a bit and a larger capacity pump is in order. What size, you ask? Let’s take a gander at the three most common sizes.
88-110 lph: There are both factory and aftermarket pumps out there that flow between 88 and 110 lph. Despite the latter being able to flow an additional 22 litres per hour over the stock pump, it is considered a stock replacement pump as well. If you intend to keep the motor stock, either of these sizes will do the trick. But then again, how many of use say to keep it stock and actually do…
190 lph: The next step up is a 190 litre per hour rated electric fuel pump. Definitely a huge upgrade from the stock pump, a 190 lph will do the trick for any naturally aspirated 302 engine out there.
255 lph: The last typical size to find in the Mustang aftermarket is a 255 lph pump. Just slightly more expensive than the 190 lph variety, the 255 is the most popular amongst Mustang enthusiasts.
Now, although the pumps are rated in litres per hour, it is important to know that you can install a 255 lph pump on a stock engine and not see any adverse effects. Foxboy Mustangs utilize a return fuel system, so there is no need to worry about the fuel pump sending too much, as whatever is not needed will simply be sent back to the tank via a return-line. Basically, going big can’t hurt. One last piece of advice: when a selecting a fuel pump, it is always better to go with a quality, known brand. There are some cheapies out there, but it definitely would not be fun to drop the tank a second time due to a premature failure.
Mustang Fuel Injectors
The next aspect to look at is the fuel injectors. The stock 19 lbers are pretty much at their limit as is, so if increasing power you will certainly need larger injectors. Like fuel pumps, there are many different sized fuel injectors and flow rates for Mustangs. Below is a guideline chart (with the most common sizes) in selecting them.
|Size (lb/hr)||Capacity (HP)|
Now, there are many factors in determining injector capacity (naturally aspirated or forced induction, pressure the system is running at, duty cycle etc.) so take those numbers with a grain of salt. Ideally, these numbers reflect a duty cycle between 85-100% at a pressure of 40 PSI and are best suited for a naturally aspirated motor.
Also keep in mind when swapping injectors, you will either need to change the MAF or have it re-calibrated for the new injectors.
Fox Mustang Fuel Pressure Regulators & Fuel Rails
Another area to upgrade is the fuel pressure regulator. The stock piece is non-adjustable and set to 39 PSI when the vehicle is at idle (with vacuum line disconnected). As it ages, the stock unit can slowly begin leaking pressure or unable to hold it at all, possibly making the car hard or impossible to start and stumble or die under throttle. Typical aftermarket replacements come in the form of billet aluminum, and are externally adjustable, a major plus for those seeking every last pony out of their five oh. Furthermore, an adjustable regulator is a must-have for anyone building a forced induction motor. At higher RPM with a turbo or supercharger installed, the stock unit is simply incapable of sufficient fuel delivery.
The last element to look at is the fuel rail. All in all, the stock unit is a pretty good piece. It certainly isn’t fancy, but it will get the job done up to 500HP, no worries. If you are going to surpass this level or just wish for a change anyway, BBK makes a nice anodized-blue aluminum rail that fits right in place and with a greater flow capacity.
Have a Plan
If you want to make some power, the stock fuel system will have to go – there is no doubt about that. However, what is needed as replacement is entirely subjective to your engine and your goals, uniquely. The more power you intend to make, the bigger your supporting fuel system will need to be. Just remember, the limiting reagent is oxygen. You can always add more fuel!