'Pamela Anderson didn't get to where she is today by thinking small. When it comes to horsepower, the only way to make it big is to put fuel and air into the engine in vast quantities. While superchargers and nitrous will help on the oxygen side of things, you gotta think big when it comes to fuel delivery as well. A 51/416-inch fuel pickup in your '65 Mustang fuel tank will not feed even a 400hp small-block no matter how much pump you bolt on. We'll hit on several important points in this story, but the essential element is that a good fuel delivery system is exactly that-an integrated system where all the components work together to move fuel in the most efficient manner possible. Whether you're feeding a single four-barrel carburetor that fuels a 1,000hp Rat motor, or an EFI system with 60 psi of line pressure, there are certain fuel-system goals you must achieve to be successful. Get that done, and you can amaze your friends with how much quicker your car runs. Amazing Pamela may take a bit more effort. Call us if you're successful.
Fuel Pumps Let's start where everyone wants to use a bigger hammer. The aftermarket is crammed with pumps that can make a fire hose look like a lawn sprinkler. But do you need all that capacity? Let's find out.
This section will deal with electric fuel pumps since that's where all the high-performance pumps live. A typical 300hp street engine can survive nicely on a typical mechanical pump. But when you get up into the 500-plus horsepower range, fuel delivery becomes an essential consideration. Get into power adders with nitrous or blow-through superchargers, and a good electric fuel pump is an absolute necessity.
While those big electric fuel pumps may look glamorous, the first thing to consider is your application. If you have an 8-second drag-race car that never sees the street, then a high-volume upright pump like a BG 400 will work fine. For a streeter, an electric pump must be rated for continuous duty. For a high-volume, continuous-duty pump, the horizontal-style pumps are a better choice, since fuel runs through the pump. This is especially important in EFI applications, because more current is required to create those higher pressures, and more current also equals more heat. A big pump like the Aeromotive A1000 will require 10 amps to feed a 45-60-psi EFI application, but at 10 psi for a carburetor it may pull as little as 5 to 6 amps.
In all applications, the best approach is to match the pump to the engine's requirements. Often pump ratings in gallons per hour are at zero pressure. This rating is meaningless except for advertising hype since all fuel systems operate under pressure. A pump that's rated at 100 gallons per hour (gph) free-flow may only produce 60 gph at 5 psi. Look for pump ratings at the pressure the pump will operate. If you need 20 psi up to the regulator, for example, many pumps are far less efficient at that pressure than they are at 10 psi. Conversely, 5 psi is a reasonable fuel pressure for a mild street car as long as that pressure can be maintained under load. Let's look at what 60 gph will deliver. Most street engines operate at a brake-specific fuel consumption (BSFC) of 0.5 pounds of fuel per horsepower per hour (lb/hp/hr). Assuming fuel weighs 6.2 pounds per gallon, a 60-gph pump delivers (60 gph x 6.2) 372 pounds per hour of fuel. With a BSFC of 0.5, this means 372 lb/hr will support roughly (2 x 372) 744 hp. This is a stout number but doesn't take into account pressure losses within the system. Chances are a pump that can only deliver 5 psi will not be able to sustain that pressure as demand increases under load.
A decent rule of thumb is a factory-style fuel delivery system will support up to around 500 hp. After that, a 550 to 600-plus horsepower engine will require a fuel delivery system capable of flowing a significant volume of fuel to ensure the carburetor or injectors have adequate fuel to use. Many manufacturers offer flow graphs in their catalog that rate fuel flow at various pressures. These graphs are very helpful in determining fuel flow. If you're still unsure as to what you need, contact your favorite manufacturer's tech line and get its recommendation. Most companies also offer complete fuel delivery system kits like Aeromotive's Fox-body Mustang kit that includes a fuel tank, lines, pump, regulators, and all the lines and fittings designed for that car. The kit is very complete, but also expensive.
Pressure Vs. VolumeIt doesn't require an engineering degree to see that it takes pressure to move gasoline from the tank to the engine. Add the g-force load of a fast car on the dragstrip and that same acceleration that pushes you back in the driver's seat also makes the fuel pump's job of pushing fuel forward doubly difficult. Buried in some obscure engineering handbook somewhere is the concept that as pressure increases in a fuel system, the volume delivered decreases, or, volume is inversely proportional to pressure. This is clearly evident when you look at a flowchart for any pump. As pump pressure increases, this reduces the amount of fuel the pump can deliver because it is squeezed through a smaller orifice.
So it makes sense that you would want to design your fuel delivery system-the lines, fittings, filters, and pressure regulator-to be carefully matched to the style of pump you will be using. One of the most common misconceptions about fuel delivery systems is that a too large fuel line (such as -10 or 51/48-inch id) creates a huge pressure drop in a drag car because g-forces work against a large surface area. This is not true. While the volume and therefore the weight of fuel in a larger-id fuel line is greater, the real variable in this situation is the length of the fuel line from the front of the car to the rear.
Try this-imagine a column of fuel in a vertical tube 31/48-inch id and 15 feet tall. If we increase the diameter of that tube to 11/42 inch, does the pressure (in psi) measured at the bottom of the larger tube change? The answer is no. While the weight of that column of fuel increases with the larger diameter, the pressure per square inch does not. Think of the standing height of a column of water behind a dam. It doesn't matter whether you slide a long 11/42-inch tube or an equal-length 1-inch tube to the bottom of the dam and measure the pressure at the bottom-both tubes will measure the same. Now, if we shorten the height of the vertical column, the pressure at the bottom of that column will be reduced. Conversely, increase the height of that column and the pressure at the bottom will increase.
Now let's lay that vertical column horizontally and place it in a drag car. A vertical tube works against the force of gravity-or 1 g. Now, let's say our car launches very hard and can generate an initial acceleration rate of 2 g's. Increasing the distance from the fuel tank to the regulator will create a greater pressure drop in the system. That's why Top Fuel cars place the fuel tank ahead of the engine-to make it easier for the pump to maintain fuel delivery pressure under a sustained 4 g's. This is why high-g launch cars (and cars that commonly reach for the sky) need higher fuel-system pressure to overcome the initial pressure drop as the car accelerates. Isn't physics cool?
The Electrical SideFew enthusiasts put much thought into powering up their electric fuel pump, but fumble here and even the best fuel pump will never perform up to its capability. All automotive fuel pumps are designed and tested to operate at maximum efficiency, usually between 13.5 and 14.2 volts DC. Aeromotive publishes a graph for its A2000 drag-race pump that shows roughly a 10 percent drop in flow when voltage drops from 13.5 to 12 volts. Another critical point is that as fuel pressure increases, so does the current load on the pump. Why spend big money for a high-volume pump and cripple it with insufficient electrical power?
If you think of voltage as electrical pressure, you're on the right track. But sufficient current (amperage) is also important. That starts by always using a relay to do the switching as well as using larger, high-quality electrical wire to feed the pump. A pump that runs hot all the time may be suffering from low operating voltage. This runs the pump less efficiently and can cause eventual electrical damage. This also demands high-quality electrical connections that are soldered for less resistance. Perform a quick voltage test on your fuel pump by using a multimeter to compare voltage at the back of the alternator with the engine and pump running to voltage measured at the pump. If there is more than a 1-volt drop between these two points, look for resistance in the circuit. Pay particular attention to the ground circuit. Did you use an equal-sized wire for the ground circuit? Remember that the current load is the same for both sides of the circuit.
Dead Heads vs. Return StyleNo, we're not talking about geriatric hippies stuck on the Grateful Dead. This is about full-flow, or return-style fuel delivery systems. In the old days when all a car crafter had to feed was a 300hp small-block, a mechanical fuel pump and a length of rubber hose was all you needed. But with mega-power, normally aspirated engines,and monster supercharged and nitrous'd powerplants, those old ways just won't cut it anymore. If your engine is making more than 500 hp, it's time for a return-style fuel delivery system.
A dead-head system is defined as any system where fuel makes a one-way trip from the tank to the carburetor. When the fuel level drops in the float bowls, fuel that has been hammering up against the regulator begins to move to refill the bowls. When the float first drops, this initial surge of fuel into the bowl creates a pressure drop until the system recovers as fuel flow continues. Once the floats cut off the flow of fuel, the fuel again begins to hammer against the regulator. As you can see, this requires the fuel column to constantly start and stop in an attempt to feed the engine.
In a full-flow system, the pump still pushes fuel up to the regulator, but it is designed to bypass all fuel not used back to the tank by way of a separate fuel line routed back to the tank. This system offers several advantages including a more dynamic versus a static system that reduces unwanted pressure drops. In a dead-head system, the check valve (regulator) is located between the fuel pump and the carburetor. This means line pressure is generally higher up to the regulator and reduced going into the carburetor. This higher system pressure is necessary to overcome g-forces during the launch and to push the fuel through a restrictive regulator because the pump must start that column of fuel moving from a static position. In a full-flow system, the regulator is downstream of the carburetor and is configured so that the pressure to the carburetor is the same as line pressure. The full-flow system still has to overcome g-forces, but the pressure required to do this is less because the fuel is already moving in the system. Another advantage of a full-flow system is that with the engine at idle and the charging system at full efficiency, you can also set the fuel pressure more accurately and expect it to remain constant regardless of load. That doesn't happen with a dead-head system.
The downside to a full-flow system is more lines and fittings and increased expense, since this requires a more complex regulator as well as a separate line back to the tank where a return must be built into the fuel tank. Another critical point is that high-volume carbureted fuel-flow systems demand a large return line. For a big pump like a BG King Demon, Aeromotive A-1000, or any large pumps for a carbureted application, the recommendation is no less than one size smaller return line than the feed line. This is due to the large volume of fuel returned to the tank at idle at carbureted fuel pressures. If you cannot set the line pressure low enough to suit your needs with the engine not running (and sufficient voltage to the pump), this is usually an indication of a restrictive return line. EFI systems do not require as large a return line since these systems generally operate at 43 to 65 psi and a slight 4-5-psi return-line pressure will not affect fuel flow.
Flow TestingIf there is one point worth emphasizing with any fuel delivery system it should be to eliminate the 90-degree fittings. Anytime you pressurize a liquid-transfer system, a tight-radius 90-degree change in direction will cause a flow loss. The worst of these fittings are those cheap, brass fittings you can buy at the auto parts or hardware store. Since pumps push better than they pull, it's also best to improve the inlet side as much as possible, usually with a larger inlet line. To test these ideas, we built a bench-top 31/48-inch fuel system with a Mallory 140 pump with six nasty brass 90-degree fittings and measured the time it took to pump four gallons of solvent from one tank to another. Then we tossed all the ugly fittings and replaced them with straight -6 AN fittings. The time difference between the two systems was not dramatic, but we did see a 10 percent improvement in flow. This was not a true fuel-delivery test because we were not running the pump against 5 or 6 psi of pressure, but it does point to the importance of minimizing flow restrictions.
|PARTS LIST |
|DESCRIPTION ||PN ||SOURCE ||PRICE |
|BG electric pump ||BG220RR ||Jegs.com ||$269.99 |
|BG radius fuel fitting ||140012 ||Jegs.com ||11.99 |
|BG Mighty Enduro pump ||170043 ||Jegs.com ||459.99 |
|Aeromotive A1000 pump ||11101 ||Summitracing.com ||295.88 |
|Aeromotive 100-micron filter ||12304 ||Summitracing.com ||94.95 |
|Aeromotive 10-micron filter ||12301 ||Summitracing.com ||79.95 |
|Aeromotive bypass reg. ||13202 ||Summitracing.com ||145.95 |
|Aeromotive carb reg. ||13205 ||Summitracing.com ||81.95 |
|Holley 140 pump ||12-815-1 ||Summitracing.com ||145.88 |
|Holley EFI pump ||12-920 ||Summitracing.com ||N/A |
|Holley bypass carb reg. ||12-803BP ||Summitracing.com ||54.39 |
|ACCEL electric pump ||74702 ||Summitracing.com ||299.95 |
Barry Grant Fuel Systems
Mr. Gasket Company (Mallory)
Holley Performance Products
1801 Russellville Rd.
Bowling Green, KY 42101