Understanding High Performance Belt and Electric Fuel Pumps
While the old penchant of bigger is better is often true, there are situations where too much fuel pump can actually cause problems.
Diaphragm pumps use a membrane, usually a rubber composite, that moves in an up-down motion over a cavity. The cavity has an input and an output, each with a check valve in place to ensure a one-way motion of fluid. As the diaphragm moves up, it creates a vacuum, drawing fuel from the line to the cavity. As it moves down, the fuel is pushed out of the cavity under pressure. The check valves keep the fuel from being pushed out the wrong side. This is the design most commonly used in stock-style mechanical fuel pumps and cheap aftermarket low-volume electric pumps, and they last quite a long time.
One of the biggest benefits of a diaphragm pump is the vacuum created on the feed line as it actually draws fuel from the tank to the pump. They are also very good for dirty fuel systems, since the fuel does not flow through the membrane, debris and other contaminants are less likely to damage the pump. Diaphragm pumps are not as efficient as other designs; these are not commonly used in extreme performance applications.
Similar to the diaphragm pump is the piston pump, which mounts the engine like a standard diaphragm pump, but it uses a piston drive similar to a master cylinder. Like a diaphragm pump, the piston pump draws fuel into the pump and forces it out under pressure; the difference here is that there is no diaphragm to rupture and the piston action can create substantially more pressure and flow more volume.
Race Pumps mechanical pumps can generate 150 psi of fuel pressure using just 105 lbs of pushrod force. Piston pumps are more expensive than traditional pumps, but the fact that they are rebuildable for about $10 makes them a viable option. We talked with Race Pumps owner and pump designer Howard Stewart about the development of the piston pump and he told us, “The idea was to bring simplicity to fuel pumps. There is one moving part that displaces up to 450 gallons per hour.”
Because the pump is mounted to the block and runs off the camshaft, the pump only produces what the engine needs; there is no need for a return line. With that in mind, there is an option for a small return line for cars where vapor lock may be an issue, specifically for street and methanol cars.
The rotary vane design, which many brands use for high-volume high-pressure fuel pumps, have multiple sliding vanes (3) that are driven by a spindle (2). As they rotate, each vane slides out, sealing to the wall of the cavity. The fuel gets trapped between two vanes and pressurizes as the cavity gets smaller towards the outlet. The roller vane pump is the same except the square vanes are replaced with rollers.
Rotary vane pumps, such as the Holley Red and Blue fuel pump designs, operate with a paddle-wheel device inside a larger circular base. The wheel is offset to one side, creating a crescent shaped cavity. Paddles on the wheel slide in and out of the wheel as they spin inside the cavity, this draws fuel into the pump as the cavity opens up, then compresses it as it narrows again, finally pushing it out of the pump under pressure. The nature of the sliding vanes creates a lot of friction inside the pump. The slide must seal the pump, maintain pressure and slide in and out, resisting centrifugal force, all at the same time.
Sliding vane pumps are generally relegated to low pressure applications and are almost always T-style pumps (motor on top, inlet/outlet on the bottom), though there are a few inline sliding vane pumps. For high pressure applications, the roller vane design is used. Where the sliding vane has a lot of friction on the flat edges, the roller vane uses the same basic principle, but instead of a paddle, a roller bar is used. The roller still moves in and out of the inner wheel, but much of the friction is reduced, increasing the sealing and efficiency of the pump. This style of vane pump is more suitable for high pressure than the sliding vane. Rotary vane pumps are efficient, but they are loud. Contaminates in the fuel can create problems in the pump, but they are more tolerant than georotor pumps.
Gear-drive pumps use two spur gears that mesh together to pump fluid from one side to the other. This is the same type of pump that is used for external oil pumps on an engine. They are noisy, but they work well. The majority of belt-drive and hex-drive pumps use this design, with the exception of Aeromotive, which uses the georotor design.
Georotor pumps are the most common design for modern electric high-pressure, high-volume fuel pumps. A georotor pump operates by spinning spur gear that drives what is essentially an internal ring gear. This internal gear has teeth on the inside of the ring. As the spur gear spins, the ring gear rotates inside the cavity, creating a suction on the inlet and produces pressure on the outlet. These pumps are very efficient, quiet and can build very high pressures.
The drawback of a georotor design is that they are highly susceptible to damage from contaminants and overheating. When the fuel feed is reduced to the pump, cavitation occurs, which destroys the pump in a matter of minutes. A common misconception is that georotor pumps do not produce vacuum on the inlet side, meaning they must be gravity fed by the fuel tank. In reality, georotor pumps can generate significant vacuum. As the vacuum increases, the boiling rate of the fuel decreases, and the fuel to turns to vapor. This causes cavitation, which is an air bubble imploding. Cavitation is like setting off a bunch of tiny explosions inside the pump, it doesn’t take long, even a few minutes, to destroy the pump. By mounting the pump as close to the tank as possible, with a good gravity feed, you reduce the amount of vacuum generated by the pump, eliminating the vapor issues.
Choosing a pump that suits your needs
Regardless of the style of pump you choose, you need to know what size to get. While the old penchant of bigger is better is often true, there are situations where too much fuel pump can actually cause problems. A fuel pump is designed to move fluid and build pressure. A typical carburetor requires 7-14 psi of fuel pressure to ensure the bowls stay full. EFI systems require wildly different pressure, but typically range from about 26 to 60 psi, more with boost.
Recently we had a 400 hp small-block with EFI that required 26 psi of fuel pressure. Because of some fuel delivery issues, we ended up strapping a monster fuel pump that was capable of 100 psi and could feed a 1500 hp-carbureted engine. We noticed that the fuel pressure gauge would slowly creep up to almost 30 psi, even while the car was driving because the fuel pump was too big, causing the engine to run rich. It was simply pushing fuel past the built-in regulator on the TBI set up. A separate high-performance regulator could solve that problem, however the point is, too big can be a problem in certain situations.
There are several key variables that are used to determine the fuel requirements of any given application. Flywheel horsepower, engine fuel efficiency, also known as Brake Specific Fuel Consumption (BSFC), Maximum fuel system pressure and pump flow at that pressure, along with available voltage at the pump under load and the pump flow volume at said voltage all factor into the equation. Yikes. While that sounds a bit complicated, it really isn’t. There are some simple formulas that we can use to extrapolate the information needed.
The Simple Math
More pressure requires more fuel pump volume.
Take a 1500 hp NA engine, with a BSFC of .45, multiply the two figures together (1500 x .45) and we get 675 lbs of gasoline. Take the same horsepower in a turbo application with a BSFC of .65 (1500 x .65) and the result is considerably higher at 975 lbs of fuel. This just goes to show that a fuel pump that says it is capable of feeding 1500 horsepower may not be enough for your 1500 hp application.
For a naturally aspirated engine with an alternator, this might be all you need, but if you are running forced induction or nitrous, or not running an alternator, then there are a few more considerations. Forced induction alters the fuel pressure needs with engine load. As the fuel pressure demand increases, the pump volume decreases.
For example, the Aeromotive A1000 pump on a carbureted NA engine, set to 9 psi running on 13.5 volts flows 791 lbs/hr, feeding 1582 hp @ .5 BSFC. On the same engine with EFI, regulated to 43.5 psi on 13.5 volts, the volume drops to 614 lbs/hr, feeding 1228 hp @ .5 BSFC. Add in some boost–6 psi, an 8:1 boost reference regulator, intercooled, pressure regulated to 91 psi on 13.5 volts and the flow craters to 370 lbs/hr, feeding 616 hp @ .6 BSFC. This demonstrates a flow reduction of 53%. More pressure requires more fuel pump volume.
The other consideration for race fuel pumps is voltage. This variable is far too often overlooked as a given, but drag cars that run off a battery alone will have less available voltage to the fuel pump. Just like an engine needs fuel, an electric motor needs voltage to function. As the voltage drops, the motor’s speed drops, reducing both pressure and volume of the output. Considering the Aeromotive A1000 pump—at 80 psi, the volume will increase 40% when the voltage is increased from 12 to 13.5 volts. For non-alternator cars, a belt-driven pump would alleviate the voltage variable.
Belt-drive vs. Electric Pumps
The mechanical fuel pump has been around since the very beginning of the automobile, but major advances in the last 10 years have made the mechanical pump once again viable options for serious high-horsepower engines.
“Mechanical pumps have flexibility that electric pumps don’t have,” states Crow. “The Aeromotive belt-drive pumps start at the 500-700 hp level. The primary advantage of a mechanical pump is they offer positive displacement and are being driven by unlimited horsepower. It takes power to operate a pump; a belt or hex drive pump may take 5 hp, whereas an electric motor takes considerably more power to operate.”
Electric pumps consist of 2 components, the electric motor and the pump. Belt drive pumps are just pump section, simplifying the overall design.
An electric fuel pump’s motor speed is directly tied to the volume it produces. The faster you turn the pump, the more it will produce. The limiting factor to the electric pump is that as the flow requirements increase, the power it takes to move that fluid increases. It takes a massive motor to spin a fuel pump with lots of pressure and high volume. As the pressure goes up, the work load increases. Most fuel pumps at 150 psi are essentially stalled because the motor slows down to the point that it can’t produce any flow. This is why build tolerances are so crucial in a fuel pump. “Aeromotive runs tolerances of .005”-.01”, which is what makes them so efficient, Crow stated.
The beauty of the mechanical belt drive is that as the engine spins faster, the pump runs faster, providing the flow you need without relying on battery voltage. Aeromotive changed the landscape for EFI on high-horsepower engines with their belt-drive pump in 2004. Before that, the only way to feed a 1500+ hp EFI engine was to run multiple electric pumps or a second battery just for the fuel pump. All Aeromotive belt drive pumps run a georotor design, where other belt drive pumps use the external spur gear pump, with two gears that mesh externally.
“External spur gears pump less volume at low rpm than a true georotor internal gear design,” explained Clow. “At low RPM, specifically during start up, external mechanical pumps can’t produce enough volume and pressure for the fuel injectors to properly fire. With a georotor mechanical pump, you can get enough volume, but you need to alter the crank timing in the ECM, increasing the open time for the injectors so that the engine will fire with less fuel pressure. Once the engine starts, the belt drive pump will supply the full amount of fuel.” For the convenience factor, a small electric priming pump for starting the engine with a belt drive system can be used. This is not an issue for carbureted engines with mechanical pumps. Once the bowls have fuel, the engine will start.
One of the drawbacks for a belt-drive pump is the location of the pump in relation to the fuel cell. Belt drive pumps have the ability to draw as much as 20-inches of vacuum because they are so efficient. As we discussed earlier with georotor electric pumps, the problem that can arise from this is vapor lock. As the amount of vacuum increases, the boiling point of the fuel decreases. To combat this, you need to consider the fuel line feeding the pump and the design of the fuel cell. As the length of the fuel line increases, the diameter of the line must increase to keep up with the flow.
Internal or external bypass?
Another benefit of a belt-drive pump is that they don’t need a return line to effectively regulate the fuel pressure. Because they don’t produce the same volume and pressure all the time, there is not a continuous overflow of fuel. That does not mean you can’t use a return line with a belt-drive, but it is not required. For most high-powered electric pumps, an external bypass regulator is required.
High pressure and electric pumps do not like being deadheaded (pressure back up to stop the flow), it leads to cavitation and overheating. Brett from Aeromotive offered a suggestion for the location of the external bypass on EFI engines-“Install the regulator after the EFI fuel rails so that the fuel is in the rails at pressure. This ensures that there are no losses in flow or pressure when the injector needs the fuel.” Sounds like good advice to us.
Whether you want the convenience of fast start up, simple installation and constant fuel volume of an electric pump or the high-volume flow without the need for additional battery voltage, the choice is yours. In any case, make sure you research the fuel needs of your application and make sure it matches the pump you choose. It just could mean the difference between a win and a loss.