Fuel What Size Injectors & Fuel Pump For My Combo?

Discussion in 'Fox 5.0 Mustang Tech' started by Exile, Oct 8, 2012.

  1. So I am starting to round up all the parts to complete a Trick Flow top end kit. I am also wanting to add say a 100 shot of nitrous on top of the kit (if the stock bottom end will handle it?), and am wondering what size injectors, and fuel pump I should go with?

    I would say with just the Trick Flow top end kit that 24lb injectors, and a 190lph fuel pump would be fine, but if adding a 100 shot of nitrous on top of it should I go with 30lb injectors, and a 255lph fuel pump? Or would 24lb injectors, and a 190lph fuel pump still be ok for this combo? Trick Flow top end kit + 100 shot nitrous

    1991 5.0 Mustang LX-Hatch - Stock Bottom End, No A/C, No Smog
    Full Suspension/Control Arms/Sub-Frame/Trac/Gears/Springs/Shocks
    MAC Long Tube Headers, X-Pipe with MAC Dumps
    Built 5-Speed with King Cobra Clutch & Steeda Tri-Ax
    Aluminum Radiator with Taurus Electric Fan
    Aluminum Driveshaft

    Trick Flow Street Burner Top End Kit - Heads/Intake/Cam
    Trick Flow 75mm Throttle Body
    Pro-M 75mm MAF (calibrated)
    100 Shot Nitrous
    Injectors 24lb or 30lb?
    Walbro Fuel Pump 190lph or 255lph?

    Any feedback on this combo is appreciated...
  2. Yes, and have the whole set-up properly tuned.
  3. Are you using a wet nitrous kit that has single fuel injector nozzle or a dry kit that depends on the 8 individual fuel injectors for both normal driving and using nitrous?
  4. I will more than likely be using a wet nitrous system.

    And in case it matters this will be a daily driver that might see the track/nitrous maybe once every few months.
  5. how often you use the bottle means nothing... id go 255LPH walbro / focus pump/ trex inline... something to get a bunch of fuel to the motor. having to little fuel just 1 time will be BAD!
  6. are you sure you are gonna make enough hp to run 30s when NA, could be a big headache getting it to run right and wash out the cylinders
  7. A wet nitrous system will require a large fuel pump but smaller injectors. The injectors need to match the engine HP output without the nitrous. This is because most wet nitrous systems supply matching jets for the fuel & nitrous. The computer does not have to account for the fuel provided by the wet nitrous system, therefore you don't need monster injectors.

    Copied from the FORD RACING PERFORMANCE PARTS catalog:


    Fuel Pumps
    The following information is presented assuming the above information has been taken into consideration regarding BSFC, fuel pressure and specific gravity of the fuel being used. Most fuel pumps for electronic fuel injection are rated for flow at 12 volts @ 40 PSI. Most vehicle charging systems operate anywhere from 13.2v to 14.4v. The more voltage you feed a pump, the faster it spins which, obviously, will put out more fuel. Rating a fuel pump at 12 volts then, should offer a fairly conservative fuel flow rating allowing you to safely determine the pump’s ability to supply an adequate amount of fuel for a particular application.

    As previously mentioned, engines actually require a certain WEIGHT of fuel, NOT a certain VOLUME of fuel per horsepower. This can offer a bit of confusion since most fuel pumps are rated by volume, and not by weight. To determine the proper fuel pump required, a few mathematical conversions will need to be performed using the following information. There are 3.785 liters in 1 US Gallon. 1 gallon of gasoline (.72 specific gravity @ 65° F) weighs 6.009 LBS.

    To be certain that the fuel pump is not run to its very limit, which could potentially be dangerous to the engine, multiply the final output of the fuel pump by 0.9 to determine the capacity of the fuel pump at 90% output. This should offer plenty of ‘cushion’ as to the overall “horsepower capacity” of the fuel pump.

    To determine the overall capacity of a fuel pump rated in liters, use the additional following conversions:
    (Liters per Hour) / 3.785 = Gallons
    Multiply by 6.009 = LBS/HR
    Multiply by 0.9 = Capacity at 90%
    Divide by BSFC = Horsepower Capacity
    So for a 110 LPH fuel pump:
    110 / 3.785 = 29.06 Gallons
    29.06 x 6.009 = 174.62 LBS/HR
    174.62 x 0.9 = 157 LBS/HR @ 90% Capacity
    157 / 0.5 = 314 HP safe naturally aspirated “Horsepower Capacity”
    Safe “Horsepower Capacity” @ 40 PSI with 12 Volts
    60 Liter Pump = 95 LB/HR X .9 = 86 LB/HR, Safe for 170 naturally aspirated Horsepower
    88 Liter Pump = 140 LB/HR X .9 = 126 LB/HR, Safe for 250 naturally aspirated Horsepower
    110 Liter Pump = 175 LB/HR X .9 = 157 LB/HR, Safe for 315 naturally aspirated Horsepower
    155 Liter Pump = 246 LB/HR X .9 = 221 LB/HR, Safe for 440 naturally aspirated Horsepower
    190 Liter Pump = 302 LB/HR X .9 = 271 LB/HR, Safe for 540 naturally aspirated Horsepower
    255 Liter Pump = 405 LB/HR X .9 = 364 LB/HR, Safe for 700 naturally aspirated Horsepower

    Note: For forced induction engines, the above power levels will be reduced because as the pressure required by the pump increases, the flow decreases. In order to do proper fuel pump sizing, a fuel pump map is required, which shows flow rate versus delivery pressure.

    That is, a 255 liter per hour pump at 40 PSI may only supply 200 liters per hour at 58 PSI (40 PSI plus 18 lbs of boost). Additionally, if you use a fuel line that is not large enough, this can result in decreased fuel volume due to the pressure drop across the fuel feed line: 255 LPH at the pump may only result in 225 LPH at the fuel rail.

    My Comments:

    A lot of people oversize the fuel pump by buying a 255LPH pump thinking that the fuel pump regulator will just pass the excess gas back to the tank. It does, but… Did you ever consider that circulating the fuel around as a 255 LPH pump does will cause the gas to pickup engine heat? What happens to hot gasoline? It boils off! With most of the 5.0 Mustangs having the carbon canister removed or disabled, the car stinks like gas, and the gas mileage drops since the hot fuel evaporates away into the air.

    Fuel injector sizing & injector photos

    Revised 11-Dec-2011 to add larger injector sizes to injector table

    Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines at the flywheel.

    19/.5 = 38, 38 x 8 = 304 HP
    24/.5 = 48, 48 x 8 = 384 HP
    30/.5 = 60, 60 x 8 = 480 HP
    36/.5 = 72, 72 x 8 = 576 HP
    42/.5 = 84, 84 x 8 = 672 HP

    The preferred duty cycle is about 85% maximum, so for a safety factor multiply the final figure times .85.

    19/.5 = 38, 38 x 8 = 304 HP x .85 = 258 HP
    24/.5 = 48, 48 x 8 = 384 HP x .85 = 326 HP
    30/.5 = 60, 60 x 8 = 480 HP x .85 = 408 HP
    36/.5 = 72, 72 x 8 = 576 HP x .85 = 490 HP
    42/.5 = 84, 84 x 8 = 672 HP x .85 = 571 HP

    Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.

    See http://users.erols.com/srweiss/ to get the calculators used in these examples.

    Here's the duty cycle explanation. Duty cycle is how much of the time the intake is open the injectors are turned on. The 85% figure means that for 85% of the time the intake valve is open, the injectors are spraying. The idea is that you want some percentage of the duty cycle left over so that you have some room to grow the process.

    If you are at 100% and you need more fuel, all you can do is turn up the fuel pressure. That means the whole fuel curve from idle to WOT is affected. Maybe you are already too rich at idle, and turning up the fuel pressure makes it worse. If you had some injector duty cycle left to play with, a custom tune could use that where it is needed. That would not over richen the whole range from idle to WOT.

    If you did turn up the fuel pressure, you might be able to change the injector duty cycle to get the air/fuel mixture ratio you want since the injectors will have extra fuel delivery capability.

    With larger than stock injectors or higher that stock fuel pressure, you will need an aftermarket MAF that matches the injector size. The MAF “lies” to the computer to get a fuel delivery schedule that meets the engine’s needs and isn’t too rich or too lean. The best strategy is an aftermarket MAF and a custom tune to insure the best air/fuel ratio over all the RPM range.

    Don't forget to increase the fuel pump size when you increase injector size or significantly increase the fuel pressure

    Diagram courtesy of Tmoss & Stang&2birds

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

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