Fuel System Technical Notes and Performance Tips

Why do you need a high-output, high-pressure fuel system? How can you plan your racing fuel system? Should you use a fuel filter?
How much fuel flow is enough? Is it necessary to plumb the system for a return line to the fuel tank? How can you check a system for flow and pressure?
How much fuel pressure is necessary? How important is the size of the tank vent? Testing and Calibration of the Fuel System

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Why do you need a high-output, high-pressure fuel system?

Today's racecars need more fuel system than ever before because of improvements in torque, RPM and horsepower. As carburetors ' manifolds and cylinder heads improved in airflow capacity, the NEED for more efficient racing fuel systems grew substantially. More efficient chassis and tires also created the need for more fuel in order to maintain the maximum output power of the engine. Simply put, the harder the racecar launches, the higher the system pressure must be to overcome the effects of gravity that cause restriction to flow. Firemen plan for this restriction to flow by adding 5 psi to the fire hose pressure per floor above street level. Pilots flying in high performance fighters must control their blood supply with exercises and special flight suits when in high g maneuvers or they suffer "redouts" or "blackouts" because they can't control their blood pressure. Aircraft launched with catapults from aircraft carriers must take off with fuel systems in high boost or the engine will starve for fuel. High g launches coupled with wheel stands increase the demands on fuel systems whether the application is for Pro Street, Stock, Bracket, or Pro Stock.

 

How much fuel flow is enough?

The correct volume of fuel is that which is required to support the amount of horsepower that the engine can produce. Most engines that are using gasoline burn approximately .5 pounds of per horsepower-hour. This is sometimes called BSFC (Brake Specific Fuel Consumption). What this means is that for each horsepower (HP) produced, it takes 112 pound of fuel. This is a general statement and sometimes engines can be a little more efficient than .5lb/hp-hr., but it is a good practice to plan and measure fuel system operation using this number. Carburetors must have a stable supply of fuel in order to maintain the correct liquid fuel height. This is most difficult with drag racing vehicles that sometimes have forward acceleration and wheel stand at the same time. Each time that a nitrous system is engaged, additional fuel supply demands must be met or melted parts can be the result of "system lean-out", because the fuel required is in excess of the .5 lb/ hp-hr. for normally aspirated conditions. The additional fuel requirements for nitrous system planning is about .7 lb/hp-hr.

 

How much fuel pressure is necessary?

First, the fuel system pressure (provided by the fuel pump) must be enough to oppose the effects of gravity during the launch and during the run for drag racers. The system pressure of at least 8 to 10 psi per g is generally adequate. MagnaFuelPro Star 500 series pumps are factory set to 28 to 30 psi. They are field adjustable from 24 to 36 psi. The MagnaFuel Quick Star 300 series pumps are factory set to 25 psi and field adjustable from 25 to 36 psi. The Quick Star 275 series pumps are factory preset to 18 psi (these units are not field adjustable). MagnauelPressure Control (not just another regulator) units need to be adjusted to 6.5 to 7.5 psi WITH FUEL FLOWING at a rate of about 112 cc per second (that's about 10 drops per second). Higher fuel pressure will generate more foam in the float bowl.

 

How can you plan your racing fuel system?

Use a hand held calculator and plan on .5 lbs/hp-hr (gasoline).  Methanol Alcohol requires about 1.0 lbs/hp-hr. Use .7 lbs/hp-hr when planning a gasoline system for nitrous assist.

EXAMPLE: You have a 650 hp engine. 650hp x .5 = 325 lbs/hr (gasoline). Although you need to know how much your fuel weighs, assume for this example that it weighs 6.2 lbs/gal. 325 lbs/hr 6.2 = 52.42 gal/hr. Dividing by 60 (minutes per hour) yields .847 gal/ min (GPM).

 

Is it necessary to plumb the system for a return line to the fuel tank?

Yes, because all MagnaFuel pumps are equipped with external by-pass system.

We don't think that any well engineered racing fuel system should use internal by-passes because all they do is heat up and add foam (bubbles) to the fuel.  Yes, it is a little more difficult to plumb, but it provides a better and more efficient system.

 

How important is the size of the tank vent?

Attention to this detail may make the difference between winning and losing. It should be equipped with a filter so that trash and dirt cannot enter the fuel system. Absolute minimum size vent is -6 AN, but -8 AN is preferred for any application over 600 HP. Some specialty applications actually need a -10 AN. MagnaFuelrollover/vent (MP-3125) is -8 AN and provides some safety benefits if the vehicle gets upside down.

 

Should you use a fuel filter?

All fuel systems are dirty and need to use a fitter in the system. The filter should be located on the suction side (between tank and pump) of the pump. MagnaFuel offers a stand-alone filter (fitter MP-7005) for use with the Pro Star (MP-4401) or Quick Star 275 (MP4501) and 300 series (MP-4601) pumps. The fitter cartridge is washable. Replacements are available.

 

How can you check a system for flow and pressure?

Free-flow ratings of racing fuel systems are a joke so MagnaFuel stresses that the only way to test a system is AT RATED PRESSURE. MagnaFuel rates all its systems at FLOW vs PRESSURE.

Have a fire extinguisher handy. Observe safe practices when dealing with fuel. NO Smoking. You will need an accurately scaled jug (semi-clear polypropylene is ideal) of at least one gallon capacity. One gallon = 128 fluid ounces. One gallon = 4 quarts. One gallon = 231 cubic inches. You need to test the complete fuel supply system, so this measurement will be after regulated control. You wilt need a stopwatch or a watch with a second counter. You will need to provide a variable orifice (brass draincock or petcock works well) for attachment to the end of the fuel line, or use MagnaFuel's Flow Calibration Kit.

Run the test at various fuel pressures (regulated flow) and you will learn what your system can do. This is particularly important for nitrous enrichment fuel applications so that you wilt know the result of changing the fuel pressure 1/4 psi at a time.

Use the previous example under fuel system planning.

 

Testing and Calibration of the Fuel System

The correct process for testing and calibrating the fuel system is easily applied by using the MagnaFuel Flow Check Kit (MP-0100). It is important to use an accurate pressure gauge that has good resolution (MP-0101 or MP-0102). The use of small gauges will not provide sufficient accuracy for the precise job of calibration.

 

Pump/Filter combo for Single 4-bbl Carburetor
   

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