The latent energy content of gasoline is fixed — 124,000 BTU — and the amount of horsepower you can make depends on how much fuel your engine can burn over time. Too much fuel and the engine runs rich and inefficiently. Too little and it runs lean. It has to be just right.
That’s where a fuel pressure regulator comes into play. Essentially, there are a few ways you can regulate fuel pressure. High-end racecars use fully electronic controls. Fuel-injected cars can use mechanical and vacuum-controlled pressure regulators. Carbureted engines also need pressure regulators, but the rules are a little different there.
To find out more about fuel pressure regulation, we caught up with Chris Mills, Senior Technical Products Specialist with DeatschWerks, a company that specializes in high-performance fuel systems solutions.
For a pressure-regulated fuel system to work, it must have feed and return lines. True returnless systems don’t have external regulators because they’re computer controlled. Those are found on the high-end racecars. Most of the racecars in NASA, use mechanical or vacuum regulators with feed and return lines.
“For a fuel pressure regulator to work, it has to be able to bleed off. It has to be able to send the unneeded or unnecessary fuel somewhere,” Mills said. “In some of the more recent late-model cars, they call them returnless, but they actually all have a return. It’s just in the tank, so the only fuel you have physically leaving the tank is what’s going to be used by the engine. It just gets returned internally.”
A mechanical fuel-pressure regulator is a simple device. Inside, you have a spring that controls a diaphragm. That spring has a set spring rate and you regulate fuel pressure by adjusting tension on the internal spring. Pressure at the fuel rail behind the injectors is determined by the setting on the spring inside the regulator.
“You’re preloading the spring,” Mills said, adding that DeatschWerks regulators use a ball bearing between the screw and the spring hat for smoother operation. “Essentially, you’re taking a linear spring and you’re preloading it so that as that spring compresses, it requires more pressure for it to compress further, and that’s really all you’re doing. As you tighten the spring, that increases the pressure at which the diaphragm inside opens. As you tighten it, fuel pressure goes up. As you loosen it, fuel pressure goes down.”
The fuel pressure on the supply side drops when you allow more pressure to bleed off to the return side. Conversely, pressure rises when you allow less fuel to bleed off.
“Fuel pressure is present because of resistance in the system,” Mills said. “A lot of people think that the fuel pump is what generates fuel pressure, but no. All a fuel pump does is spin and shoot fuel out. It’s just the supply. It’s actually the resistance in the fuel system that creates fuel pressure, so if you have zero resistance, you have zero pressure. If you have all the resistance, then you have basically as much fuel pressure as the pump is able to continue to supply fuel.”
Lots of factory pressure regulators also use a vacuum chamber. At wide-open throttle, when vacuum drops to zero, that allows fuel pressure to rise to meet the demands of the engine. Many aftermarket regulators, like those from DeatschWerks, have a diaphragm that is vacuum and boost referenced. The difference is that an aftermarket, adjustable regulator has the ability to adjust when that diaphragm opens using preload on the system.
The size of your fuel lines also is relevant to pressure, Mills explained. The smaller the fuel line, the more pressure drop you’ll have over a given length of line. Mills said that when DeatschWerks specifies a fuel line size, it targets a 1 PSI pressure drop.
With fuel injection, it’s less of a concern, but most carburetors run anywhere from 4 to 12 PSI. If you’ve got a 4 PSI pressure drop across that, you’ve just lost a large percent of the pressure the carburetor needs. Fuel pressure regulators for carbureted engines have much lighter internal springs, Mills said, topping out at about 20 PSI of fuel pressure.
DeatschWerks makes the DWR1000 regulator for engines up to 1,000 horspower and the DWR2000 for engines that make from 750 to 2,000 horsepower. The DWR2000C is for carbureted applications. The difference between the 1000 and 2000? The port sizes on the 2000 are larger.
Because, as we know, the energy content of gasoline is fixed. What determines horsepower is the amount of fuel your engine can burn over time, and a fuel pressure regulator is a critical part of the system.
“That’s basically how a fuel pressure regulator works,” Mills said. “As far as the basic function of a fuel pressure regulator, there’s really not a lot of black magic that goes into them.”