The Lifeblood Of A Successful Program
One of the most ignored areas of building and maintaining a race car is the fuel system. The fuel tank, filters, vent system, and fuel line size all must be designed to work together to supply the carburetor or fuel injectors with the correct fuel volume and pressure for the engine demands.
Fuel Pressure The fuel pump must be able to supply the proper fuel pressure and enough volume to the carburetor in order to supply the correct air/fuel mixture for the engine’s demands. Based on my experience, the fuel pressure needed for a race vehicle equipped with a carbureted engine is 5.5 to 6.5 pounds under all load conditions.
Always measure the fuel pressure at the carburetor inlet. If the fuel pressure goes lower, the fuel level in the carburetor’s float bowl will drop too low, causing the air/fuel mixture to go lean. The lean fuel mixture can cause engine damage. If the fuel pressure increases, the float, needle, and seat will not be able to properly control the float level. If the fuel level in the carburetor’s float bowl goes too high, the air/fuel mixture will go too rich and cause the engine to miss and foul the spark plugs.
We have done tests by using an exhaust gas analyzer for detecting changes in air/fuel mixture that occur as the fuel pressure changes. When the fuel pressure goes beyond 7 pounds, we can observe the fuel mixture going rich as the vehicle is driven over the normal imperfections that are in almost any track. When the fuel pressure drops below 4 pounds, we can observe the air/fuel mixture shifting lean, especially when we are driving at wide-open throttle/high-load conditions.
Fuel Pumps The two most common methods of supplying fuel to a carburetor are with a remote electric fuel pump or a mechanical fuel pump mounted on the engine. A mechanical pump can do a good job, but if there is a problem of space or heat in the engine compartment, an electric pump mounted away from the heat near the fuel tank may work better. No matter what type of fuel pump is used, it must be able to supply enough volume of fuel to keep the pressure at 5.5-6.5 pounds at all driving conditions. A high-volume or race-electric fuel pump must have some kind of bypass/return fuel pressure control system that keeps the fuel flowing through the fuel pump in order to cool it, or the fuel pump will fail. Many electric fuel pumps have failed due to the pump overheating because a standard non-return fuel pressure regulator was used. A standard pressure/volume electric fuel pump may not overheat, but it may not supply enough fuel volume/pressure for the engine’s needs at full throttle.
Both mechanical and electric high-performance fuel pumps are rated for flow of a certain number of gallons per hour of free flow (zero pressure). The flow of that same fuel pump will be less when the fuel pump is maintaining proper fuel pressure. The important thing is that the fuel pump can deliver enough fuel to the carburetor while maintaining proper fuel pressure.
One of the performance aftermarket electric fuel pumps is rated at 97 gph free flow (zero pressure), but it only flows about 25 gph at 6 pounds of fuel pressure (this is about enough fuel for a 300hp engine).
The most common way of figuring fuel volume needed from a fuel pump is in this manner. A typical engine requires 0.5 pounds of fuel per horsepower, per hour. A gallon of fuel weighs about 6 pounds, so an engine producing 300 hp will require 25 gph of fuel delivered at 6 pounds of fuel pressure. (300 hp x 0.5 pound = 150 pounds of fuel; 150 pounds 6 pounds = 25 gallons of fuel per hour.)
VaporLock and Return-Style Fuel Systems Vaporlock is created when fuel in the fuel lines, fuel pump, or the carburetor fuel bowl overheats and then boils. When this happens, the flow of fuel to the engine will cease. If you have a vaporlock problem, about all you can do is open the hood and allow the heat to dissipate so the fuel can again be supplied to the engine.
The best way to prevent this problem is to try to route all the fuel lines away from heat sources such as radiator hoses, transmission cooler lines, and the exhaust system. Most engines have their mechanical fuel pump mounted in a high heat area, at the front of the engine, very near the radiator hose and right where the radiator fan is blowing the hot air from the radiator. A partial cure could be to use insulation or place shielding around the fuel pump to keep the heat away. A mechanical fuel pump has to suck the fuel from the fuel tank in the rear of the vehicle and pump it up to the carburetor under pressure. If the fuel in the lines or fuel pump gets too hot, the fuel will boil, causing the vaporlock problem.
A way to keep the fuel cool is by using an electric fuel pump mounted near the fuel tank that will pump the fuel through lines routed away from any heat sources to a bypass fuel pressure regulator mounted at the carburetor. The fuel is returned to the fuel tank through fuel lines again routed away from all heat sources. This type of system uses a high-flow, high-pressure electric fuel pump that routes the fuel up to the carburetor and fuel pressure regulator, at which time the pressure is reduced to the 5.5-6.5 pounds of pressure needed by the carburetor. The unused fuel is returned to the fuel tank. However, many racing sanctions do not allow electric fuel pumps.
The use of a high-pressure/high-volume mechanical fuel pump can also work well with a bypass/return fuel system as long as the mechanical fuel pump is not mounted in an area that has too much heat. If the mechanical fuel pump is in a high heat area, the use of an electric fuel pump mounted in a cooler area near the fuel tank may be necessary.
A bypass/return fuel system allows the cooler fuel from the fuel tank to keep flowing through the fuel lines, just like a factory fuel injection system. This avoids fuel boiling due to the fuel sitting in the fuel line in the hot engine compartment, as a standard non-return fuel system tends to do. The best fuel line routing keeps the fuel lines away from heat sources such as the radiator hoses and exhaust heat.
Using proper fuel line size and low restriction fittings is a very important part of proper fuel system design. The minimum recommendation for fuel lines is 31/48 inch. You should avoid 90-degree fittings because they restrict fuel flow. If the fuel lines are too small, the engine may starve for fuel. If a high-performance electric fuel pump is used, the pump can be damaged if the fuel pump supply system allows the fuel pump to starve for fuel. If the fuel lines are too big, you may lose fuel pressure during hard acceleration.
The fuel pump tries to overcome the g-forces created as the car accelerates. In most cases, steel tubing fuel lines work quite well; other options are aluminum tubing or steel braded fuel lines. Rubber fuel hose should only be used in areas where its flexibility is needed. When designing a fuel system, always follow the fuel pump supplier’s recommendations for fuel line size, fuel tank supply, and venting lines.
Fuel Tank Venting
Always check and set the fuel pressure at the carburetor, and pay attention to fuel tank venting. For every gallon of fuel pumped out of the fuel tank, a gallon of air is drawn in through the fuel tank’s vent system. Always use some type of filter in the fuel tank vent system in order to avoid dirt and debris coming into the fuel tank. Place the vent inlet in a “clean” and dry area, not in the wheelwell where water, rubber, and debris from the tires is always flying around. Often when taking a carburetor apart for a jet change, dirt and rubber are found in the fuel bowl. This debris may have gotten into the fuel system through the fuel tank vent.
A filter in the fuel tank vent system can help prevent a carburetor jet from becoming plugged by this debris, but always keep fuel safety in mind. Use a check valve in the vent line to avoid fuel spillage in the event of an accident. We have also seen many vehicles in which the vented fuel cap does not allow air into the fuel tank as quickly as the fuel is pumped out. Consequently, a vacuum was created in the fuel tank, causing the fuel pump to starve for fuel. The fuel pressure would then drop to less than 2 pounds under sustained high-load driving conditions. The cure for this problem was to add extra fuel tank venting.
Filters for the fuel system are important and often neglected. The apparatus must filter the fuel in order to keep any debris in the fuel tank out of the carburetor, yet not present any restriction to proper fuel flow. A dirty or neglected filter can restrict the fuel flow, causing the engine to run lean due to a lack of fuel volume.
All fuel filters are not the same; some can filter finer particles than others. Filters are also designed to move a certain number of gallons per hour of flow. If the filter offers too much resistance to the flow of fuel, the fuel pressure will drop at high fuel flow conditions, and the carburetor or fuel injectors will starve for fuel, causing the engine to lean out and create possible engine damage.
A Problem-Free Fuel System
A well-designed fuel supply system with the fuel tank, vent system, fuel filters, fuel line size and routing, fuel pump, and fuel pressure all matched to each other and the engine’s fuel demands will help make tuning a race engine easier and avoid fuel system problems on race day.