Fox Body Fuel Delivery System Overview

• 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

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.

• Installing a 255lph fuel pump will accommodate any power modifications while being only slightly more expensive than the 190lph
• 19lbh fuel injectors are stock grade and can handle around 285hp; some of the larger fuel injectors are 44lbh and can handle ~627hp. Other common fuel injectors come in sizes of 24lbh, 30lbh, and 36lbh
• Aftermarket fuel pressure regulators are billet aluminum and are externally adjustable
• Stock fuel rails can handle up to 500hp in some cases, but more power requires a fuel rail upgrade

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 us 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.

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. 

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 above chart provides a general injector/ horsepower capability.

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.

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.

As we know, more fuel means more power. Although your fuel pump can flow at a high rate, that doesn’t mean it will always be precise. Sporadic fuel flow can be detrimental to the gains added fuel can provide. Too little fuel means the engine will be starved of the precious go juice. Too much and suddenly, it’s running rich and power can be killed just the same. An aftermarket fuel pressure regulator gives the builder the capability to tap into the flow rate, and make sure the amount of fuel being sent to the engine is as consistent as possible. 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. 

A higher fuel pressure can allow for smoother fuel flow in higher horsepower applications. When more fuel is called on, pressure is lowered until it can be properly compensated for. In a high horsepower application, the slight increase in pressure allows for an easier compensation when fuel pressure is lowered.

If a fuel pressure regulator isn’t present, any time is a great time to add one. If you're in the process of upgrading the fuel pump, they might as well add one at the same time. Installation is simple enough and should take well under an hour to do. The only things to keep in mind are where it will be mounted, and that you set the regulator up to allow the fuel to flow in the right direction.

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.

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!

Larger fuel lines will not be necessary in order to accommodate those mods. While there is a higher demand for fuel, it is not too demanding where the diameter of the line is too short to support the demand. If you changed to larger lines, there would be less fuel pressure and would have fuel delivery issues.

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.

There is great myth and hyperbole surrounding octane ratings and cars. Such that you'd be hardpressed to find an online automotive community that doesn't face this question at least once a week. The case is no different with Fox Mustangs. In fact, perhaps octane questions are more common due to the Foxbody's performance nature, as who doesn't want to squeeze out as much performance as possible? The guide below is written to address the many common questions, myths and facts floating around the internet regarding automotive fuel and octane rating. For this article, the use of the word octane is synonymous with octane rating, unless otherwise noted.

If you haven't already had this happen, at some point during your Mustang years, you will be conversing with a fellow Mustang enthusiast when all of a sudden said other enthusiast will say quite smugly: "I ONLY run 93 in my 'Stang". Oh no, alarm bells are suddenly ringing! You only put in 87! Sweat starts dripping down your face, you get the cold shivers. You need to get out of there fast. This guy has you beat!

Well actually, this probably isn't the case. As already mentioned, higher octane fuel does NOT mean it has more energy to burn or make more power, but rather requires a higher activation energy to spontaneously combust.

Also recall, at the end of the last section, I mentioned that compresssion, octane level and ignition timing are all related. Let's look into this triangle affair a little further.

Octane level, engine compression, ignition timing: A tangled web of love

Compressing a gas causes the ambient temperature to rise. Greater compression creates greater heat. Greater heat increases the chance of fuel to self-ignite. Self-ignition creates a much greater risk of engine knock. A higher octane rating has an increased ignition temperature. See where I am going here?

A lower compression engine will receive no added benefit of running a higher octane than what is needed to prevent detonation. Because of the lesser temperatures seen in a lower compression cycle, it is entirely possible to have unburned gasoline left after the combustion cycle if using too high an octane (recall: higher octane has a higher temperature threshold).

Conversely, a high compression motor running a low octane fuel will not fair so well, as their is a much greater chance the low octane fuel's temperature threshold will be surpassed during compression, causing the fuel to ignite prematurely.

Ignition timing further ties into this cycle. If the spark is left too late, there is an increased chance of unwanted detonation. The same goes with advanced timing.

So, to finally answer the question. Running a higher octane fuel will not increase an engines performance unless the engine has the proper ability to take advantage of the higher activation threshold. What is the 'proper ability'? Increased compression and/or advanced ignition timing.

A stock motor will not benefit from an octane increase than what was specified by the manufacturer, unless the compression has been upped or the timing advanced. The way I look at it, octane rating is the last piece of the puzzle.

First figure out compression and ignition timing, as playing with these two aspects is where the extra power come from. THEN look at what octane is needed to satisfy those first two variables.

In the case of antiquated pushrod 5.0L Foxbody Mustangs, no, premium fuel will not increase MPG. That smug guy at the gas station smirking while you pump 87 and he pumps 93 into stock vehicles is just wasting money. You'll both go just as far on a tank, you'll just have spent less.

However, on a modern car, it just might. It also might also just increase performance a little bit too. How can I say this, you ask, after quite definitely saying higher octane will result in no gain in performance or MPG? I just finished explaining that the only way to necessitate higher octane is to increase compression or engine timing.

Increasing compression isn't exactly something that can be done on the fly. However, in modern cars, the latter can. Due to ever more powerful ECU's and ever more many sensors, a modern car can monitor its own combustion cycle and increase or decrease timing based on it's findings. Thus, many modern cars today will recognize that the gas you've put in has higher octane, and will advance the timing accordingly.

As previously mentioned, no, there will be no gain whatsoever in either the performance or fuel economy departments. Compression for a stock 5.0 is 9.0:1 and inital timing is usually at 10°. Ford recommended 87 will do just fine (as is indicated in the owners manual). However, if you bump either of these factors up, then you ought to consider using a greater octane fuel.

For street driven cars, use the minimum octane needed to prevent any knocking or pinging. Whether the minium is 87, 91 or 93 is entirely dependent on your motor setup. No need to spend any extra dough on something that is not needed.

Tuning an engine with an onboard computer is a great way to get more power or fuel economy. To do so, one can use computer programmers or hire a skilled tech to tap into the system and manipulate it to performance desires. 

The onboard computer hosts fuel and spark maps which dictate the timing and fuel information that’s needed to create more or less power and even improve fuel economy. The old systems on a Foxbody do not have the advanced security barriers that newer cars have, and one will easily be able to get into the system. Though, proper equipment will be needed in order to link the program to a laptop considering you'll be working with pre-OBD2, plug-and-play connections. Preliminary knowledge on how to manipulate the curves is also a must. 

This is why hand-held tuners are often used. New EFI systems with the means to tap into the system out of the box are also a viable solution.