Last Update: July 28, 2008
When I first approached the installation of a supercharger I hadn't planned to upgrade the fuel system beyond the injectors. It didn't seem to be required. The fuel pump which comes stock on the '99 Mustang (Pump P/N: F8CU-9350-AC, Bucket/Float/Pump Assembly P/N: XR3Z-9H307-AD) is the same pump which can be found on the GT. Exactly what that pump's capacity might be is highly debated in the modification community. Generally estimates run between 110 and 130 liters per hour.
So, I did some digging and came up with a whole slew of rules of thumb and tables to help size fuel pumps. The one I found on Motorsports Digest indicates that the stock pump should probably be able to support close to 280+ HP running off the alternator between 12 and 14 volts. Another common rule of thumb is to plan for 0.6 pounds per hour for each horsepower delivered (0.1 gal/hr-hp). Converting that to liters you get 0.38 L/hr-hp. So delivering 280 hp requires 107 L/hr which gels with the Motorsports Digest table. Not being one to trust horsepower capacity tables or rules of thumb blindly I decided to calculate the fuel demand more directly.
We can make a good estimate of fuel demand based on the amount of air the engine is processing. This is pretty involved, so you can skip down if you're not really interested. We'll start with some facts.
(1) Standard air pressure at sea level is 14.7 PSI.
(2) Standard air density at sea level is 0.076510 pounds per cubic foot.
(3) The cylinder displacement of the 1999 V6 is 3.8L = 232 cubic inches = 0.1342 cubic feet.
(4) The stoichiometric ratio is 14.7:1 so 14.7 lbs air burns 1 lb gasoline.
(5) Gasoline has a density of 739 g/L = 1.629 lb/L.
Sorry about all the mixed units here, but my references are all over the place, so I'll just sort it out as we go.
As shown above, the supercharger delivers about 9 PSI boost at 5600 RPM. So, assuming that the amount of air delivered varies linearly with the intake manifold pressure (it doesn't by the way, it will be somewhat less due to increased energy loss to friction), we find:
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about 61.2% more fuel and air being forced through the engine under boost. |
So, the engine processes:
Where (and this is important) one cycle is two engine revolutions. That leaves us with 2800 cycles per minute (5600 RPM/2). It then follows that the weight of air consumed in a revolution is:
And the weight of gasoline burned with this air is determined by the stoichiometric ratio of 14.7:1 thus:
Thus, we can determine the fuel demand in liters per hour:
So, running 9 PSI boost, at 5600 RPM, we're drawing around 115 L/hr off the stock fuel pump by this admittedly conservative methodology. Once again, this seems to match up with the rules of thumb since my dyno's show 230 RWHP, which could be estimated as 270-280 or so at the flywheel. The pump is being taxed heavily, but how close are we to the limit?. I don't like to run any critical component too close to it's limit, so a new fuel pump seems like inexpensive insurance. Finding a suitable replacement wasn't as easy, but a lot of digging and I found one.
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The replacement pump and bucket assembly is Ford part number 3S4Z-9H307-BA and is rated at 300 L/hr. (This is the pump, bucket, float and tank access plate for some other Ford vehicle. The pump itself is inside.) This pump can deliver WAY more than I'll ever be able to push through the 3.8L engine. I ordered mine from www.fordparts.com for $98 including the shipping. When I did this mod it never occured to me to simply swap the pump, which is what you should do. I chose to exchange the tank cover/access panels, which was a lot more work and also has hidden consequences. For example, your fuel gauge won't read correctly with the wrong float and bucket assembly. Thanks to Justin at Velocity Mustang Performace for pointing out my misguided approach. I have since undone my error and put the original float and bucket assembly back in the car with the new pump. |
The only special tools you need to replace the fuel pump are a set of spring-lok disconnect tools. Ford uses spring-lok connections for high pressure lines in the fuel system and air conditioning on the modern Mustangs. Otherwise this is purely a job for a good socket set.
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As with most modifications to the car, step one is DISCONNECT THE BATTERY at the negative terminal. The last thing we need is sparking wires when we're playing with a tank full of gasoline. I'd also recommend waiting until the tank reads 1/4 or less before doing this mod. I dropped my tank with the fuel gauge reading slightly more than 1/2 full, and it was almost high enough to flood out when I removed the access panel. It certainly made the tank heavier than it needed to be. |
After getting the car up in the air and the battery disconnected, we have to remove a wide array of hoses and one electrical connection from the fuel tank. The only place you're likely to find gasoline waiting is in the fuel filter, so be prepared with a drip pan and newspapers to avoid a huge mess.
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near differential |
There are two connections on the drivers side (see detail below). One is just a rubber hose on a barb and can be pulled apart. The other has a yellow locking ring. Remove the yellow locking ring by gently prying up the little tabs with a small screw driver and then sliding the ring off the connector. The hose can then be pulled apart.
The passenger side has three connections from the fuel fill and gas cap (see detail below). One of these can be pulled apart near the gas cap. One requires loosening a worm clamp in a rather tight space, but isn't too terrible. The main fuel fill pipe isn't actually attached to the tank at all, but you need to remove the bolt holding it's retaining ring to the edge of the tank. Also disconnect the fuel pump and tank level sensor connector located near the center of the tank at the back of the car.
Lastly at the forward edge of the tank there are two more connections to seperate (see detail below). One is held together by a little white locking ring. Press the thumb tabs together and it will just pull off of it's barb. The other line is the main fuel feed and you'll need the spring-lok tool to get it off the fuel filter. Unclamping the fuel filter helps considerably, so undo the worm clamp holding it in place and pull the filter out of it's bracket first. Then slide the spring-lok tool into the tank side connection of the filter from the fuel filter side and pull the line apart. Whatever fuel is in the filter will come pouring out. It shouldn't be more than a cup or so, but it's messy regardless.
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near differential |
| After completing the disconnection of the tank's hoses and wires, it is held in place by two straps under the tank and the fuel fill pipe. A jack should be placed under the tank to support it before removing the three bolts that attach the support straps to the car. I used a regular automotive jack with a board to help spread out the pressue. A transmission jack would also work well if you have one. |
| After removing the three bolts, two on the passenger side strap and only one on the driver side strap, the tank is free to move. Lower the tank 3-4 inches and then slide it 5-6 inches towards the drivers side to get the tank off of the fuel fill pipe. If you don't lower the tank far enough it will bind against a deformation in the car on the passenger side, so pay attention to what is resisting you. It's best to have some help with this. The tank is unweildy to begin with, and when full of gasoline it's really a two man job to get it off the car undamaged. |
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Once you have the tank off the car, you need to disconnect another connector laying on the top of the tank, and remove the fuel pump access panel by taking out 6 bolts. The cover might be stuck down pretty tight since there is an adhesive gasket underneath, but it will just pry up after the bolts are removed. The pump itself is sitting inside a plastic holder inside the tank. there are two plastic tabs that need to be pressed in slightly to release the pump. After that you just need to lift it out and rotate it as necessary to get the float arm out of the tank.
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After you have the pump out it is full of evaporating gasoline and should be treated as extremely flammable. Now we need to exchange the pumps.
| To remove the pump from the bucket assembly, you need to take out 4 hex head screws. Three around the perimeter on top of the bucket, and one holding on the float assembly and its ground wire. (See photo for locations) |
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Once the bucket is off, just disconnect the pump from the electrical harness and pull it out. Extract the 300 L/hr replacement pump from the new bucket assembly the same way and make the switch. The stock fuel pump totally removed from the bucket and float is shown in the photo to the right. |
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After exchanging the pumps inside the bucket/float assembly, the unit is reinstalled in the tank. Bolt the cover back on, check that all the seals are tight and the lines run the way they did when you took the tank off. Reconnect the electrical harness on top of the tank and you're ready to put the tank back on the car. |
When reinstalling the tank, make sure you reconnect all the lines you took off when removing the tank and reinstall all the clamps and other apparatus BEFORE attempting to start the car. When all the lines are connected properly, reconnect the battery and start the car. Let it idle for a minute and then turn it off to check for leaks. If everything looks fine you can then restart the car and let it idle for 3-5 minutes to let the computer relearn the idle settings. Check for leaks again before driving any distance. That's it. We can now demand much more of our fuel system.