Early drag racers learned that it made sense to mount the fuel tank on the front of their hot rod. Here, the g-forces against the fuel would aid in its delivery to the engine. As hot rods got faster, engines got larger, and fuel demands increased along with them. The need for a better fuel system helped several aftermarket companies establish themselves. Naturally, the tricky part of this development was with the carburetors. They maintained the original float design, which meant pressure had to be limited to what the needle-and-seat assemblies could handle.
You’re probably not facing this scenario, so we’ll re-focus on your street machine. You probably still plan on keeping the fuel tank in the back of the car, and you’re probably planning on making a lot more horsepower than the factory ever offered the year it was manufactured. That said, you want a fuel system that is safe, capable of feeding your engine all the fuel it needs at wide-open throttle, and is also safe enough to not keep you awake at night. Are we on target? To help you learn what should be considered when designing a fuel system, we reached out to the folks at Holley.
Ideally, you’ll have the luxury to plan your entire system at once. Sure, you can upgrade your system bit by bit, over time. But, you’ll be dealing with existing limitations, modifying less-than-ideal components, and pairing things that were not designed to work together. Sure, it’s possible, but it’s not ideal.
If you were able to design an entire fuel system, you’d probably want to do some math first. Luckily, there are some established formulas to help you determine what your engine needs. There are a couple of different ways to determine those needs, and we’ll cover all of them.
Do The Math
In order to calculate the fuel system requirements for your project, you can use the horsepower or displacement of your engine, or (especially for drag racers), we can use the maximum amount of g-force your car sees at launch.
As part of the calculation, you’ll also have to determine how much fuel the engine uses versus how much power it produces. This is called the brake-specific fuel consumption (BSFC), and the formula is calculated by measuring the engine’s fuel consumption and dividing it by the power the engine makes. It looks like this: pounds of fuel used per hour ÷ horsepower = BSFC. A lower number means you’re using less fuel to make the same power, which translates to higher efficiency. This can vary greatly, as you can imagine, depending upon the type of fuel you’re using (running alcohol uses a lot more fuel than diesel to make the same torque in similar-sized engines, for example) and whether or not the engine is naturally aspirated.
But, we’re trying to base this fuel system plan on a gasoline-powered street car that’s pushing the limits beyond what the factory equipment can reliably provide. Most naturally-aspirated street engines fall between .45 and .50, so we’ll use .50 as a generic figure.
A naturally-aspirated engine of less than 500 horsepower will require about 1/2 pound of fuel per horsepower, per hour, when running at wide-open throttle. For engines with blowers, turbos, or other forms of forced induction, .60 pound of fuel is a fair estimate. For big cubic inch engines (500-plus cubic inches) with forced induction, up to .80 pound per hour is possible.
Consider that a gallon of gasoline weighs about 6 pounds, the math looks like this: (HP x BSFC)/6=gph
If you have a 400 horsepower, naturally aspirated engine, simply multiply it by the brake-specific fuel consumption (x .5) and you have 200. Divide this by 6 (pounds per gallon), and you get an engine that wants 33.3 gallons per hour (gph).
(400 x .5)/6=33.3 gph
If you have a g-meter in your drag car, the loose rule is that 8 psi of fuel pressure is lost per 1g of acceleration. If you only have 16 psi, and you’re pulling 2g of force on acceleration, the fuel will literally stop moving within the line for a moment.
A wide number of products could help, including a baffled tank (keeping fuel available for the pickup), additional fuel pump capacity (not pressure), and a functioning fuel-pressure regulator. These will all work together to keep the proper amount of fuel at the correct pressure level to the carb inlet throughout the entire pass.
These formulas are both pretty accurate and ideally, you’ll end up at the same estimate regardless of which one you use. If you’re a math nerd, use them both and compare the results.
Fuel System Plumbing
The size of the fuel line is also a major consideration. Naturally, larger-diameter lines can move more liquid than smaller diameter lines when both are using the same fuel pressure. Traditionally, domestic cars used a 5/16-inch diameter hard line, and it worked fine for most V8 muscle cars. But, with the advent of greater power, EFI, and the popularity of forced induction, stepping up to a 3/8-inch, or larger, diameter line is a solid choice.
There are various types of lines available, too. Rubber lines, steel and stainless steel hard lines, braided-steel lines- all of these products are specifically designed to carry modern fuel, so you don’t have to worry about things like ethanol content breaking them down.
Pump Up The Jams
Most modern fuel systems rely on an electric pump located inside or near the fuel tank. Normally, unregulated fuel pressure is pushed forward toward the engine, but some of the new fuel module designs regulate the pressure before it even leaves the tank. Ideally, that fuel will be filtered before it gets to the regulator.
Naturally, there are still some who rely on a mechanical pump (operated by the turning of the engine, typically by a specific lobe on the camshaft), but adding a pump near the fuel tank to supplement feeding fuel to a reliable engine-mounted mechanical unit is a popular upgrade.
Modern cars have all adopted in-tank electric pumps, which have several advantages over in-line electric pumps. The in-tank pumps have an integral pickup, which means one less thing to concern yourself with. They are mounted to the tank itself, which is one less thing to engineer. The “clean” factor, along with the quieter nature of the design, are also advantages. Finally, since they are submerged in fuel, they stay cool when being run for extended periods. Others prefer the external, inline pumps since they can be easily inspected and serviced without the need to drop the fuel tank. It’s a personal choice.
If you’ve got sufficient pressure out of the rear-mounted tank, it will lose a bit on the way forward to the engine. A wide range of factors are stealing pressure, like friction inside the fuel line (rubber has a lot more internal friction than steel or aluminum) and the twists and turns in the routing. Gentle turns will impact flow a lot less than more dramatic turns. For example, a 90-degree turn will cause more restriction than two 45-degree fittings.
Fittings And Finishes
The type of fittings you choose makes a difference, too. The nicer AN-style connections flow a lot better than push-on barbed fittings or plumbing-style connections. Keeping the flow path as straight and smooth as possible is your goal here. There are a lot of different AN fittings out there, too. Many look pretty with their anodized coatings, but aren’t made to the high standards of established brands like Earl’s Performance Plumbing. Some cheaper AN-style fittings actually neck down on the inside of the fitting, meaning the -6 fitting you paid for, possibly necks down to a -4 inside, and will quickly become the limiting factor in an otherwise well-designed and capable system.
Fuel System Regulators…Mount Up!
Once the pressurized fuel gets to the engine, whether it’s a carb or EFI, you have to make sure the pressure level is correct for your fuel delivery setup and that it remains consistent. Some carbs (like most traditional Holleys) like 6-8 psi. Vintage little two-barrel carbs (like Holley 94s or Stromberg 97s) don’t like any more than 3 psi or so, since their primitive needle/seat assemblies can’t hold back any more than that. Modern EFI systems use anywhere from 43.5-65 psi, depending on their design. Regardless, you have to focus on the fuel pressure at the engine and make sure it’s within the designated range. This is where a fuel pressure regulator comes into play.
There are several different types of adjustable fuel pressure regulators available, but most of them work in the same basic way. They will contain a spring-loaded diaphragm, and the fuel pressure pushes against it. The spring pressure is adjustable, and once the designated fuel pressure level is reached, the spring and diaphragm will open and keep the pressure steady.
Holley even has new filter/regulators (above) that are really cool. They contain a modern fuel filter, but also incorporate a preset fuel regulator, all in the same component. This is a really slick way to team up both components in a neat, effective package.
Some regulators will return excess fuel back to the tank (called a “return-style” or “recirculating” system, for obvious reasons), while others (typically lower pressure units) are able to maintain steady pressure with just the spring-loaded diaphragm-equipped regulator (called “returnless” or “dead-head” systems). It’s best to mount the regulator (and a gauge, or gauge sending unit) as close to the engine as possible since this is the point where the pressure level is most critical.
Personally, I think designing and researching your own fuel system is a fun concept. Once you’ve done the math and determined how much fuel your engine needs (in terms of volume, gallons per hour, pressure, and psi) then you can develop the ideal system for your particular vehicle and budget. Knowing you’ve done it right with good quality components will add safety and confidence. Feeding your engine as much fuel as it needs, at the proper rate, will ensure your powerplant is giving you every bit of the power you paid for all the way to the finish line.