In Part 1 of the Holley Performance Fueling Guide, we talked about how to plan and build out your fuel system when making the switch from carburation to electronic fuel injection. Here in Part 2, we’ll take a closer look at the factors which indicate that a vehicle’s factory EFI system is being pushed beyond its capability, and find out how to determine what upgrades you’ll need in order to get the system up to snuff.

OEMs typically engineer some headroom into their fuel systems to ensure that a few bolt-ons won’t push the factory components beyond their limits. But if you’re making a big jump in horsepower, the stock hardware might not be up to the task. And this shortfall can cause a number of issues that ultimately equate to reduced performance and increased component wear.

“When you start making significantly more horsepower than stock and you’re really leaning on those factory fuel system components, you’ll start to notice diminished performance near the top end of the powerband, where the engine is typically consuming the most amount of fuel,” Holley Performance EFI Product Manager Harrison Alford notes.

“If you have a fuel pressure sensor that you’re data logging, when that occurs you’ll often see the fuel pressure start to drop off in those areas. In some cases the performance drop off might not be dramatic, but if it feels like the car is nosing over in those situations and the RPMs start to come down, it’s a good indicator that your system isn’t providing enough fuel to meet the demand. In some cases it could feel like a surge or a hesitation. And in either situation the air/fuel ratio might start to go lean, and depending on the application, the EFI system’s ‘closed loop’ control will likely detect that lean condition and try to address it by adding more fuel. But this not a viable long-term remedy – it’s more of a reaction by the EFI system because of that drop in fuel pressure. It’s not actually correcting the problem.”

Finding The Bottleneck

In order to address the issue, we first need to figure out what component – or components – are creating the situation. To avoid making a costly miscalculation, Alford recommends starting with the basics first before moving on to other variables in the system.

Shop Holley's EFI-rated fuel filters here.

“If I see a drop-off in fuel pressure, the first thing I’m going to do is check to make sure my fuel filters aren’t clogged up. If it’s a street application running on pump gas, maybe it’s something as simple as a bad tank of gas, or maybe the station you filled up at doesn’t clean their system as frequently as they should and the filter just did its job, trapping those contaminants. The filter could also just be undersized for the application, which can cause it to not flow enough once it gets contaminated a little bit quicker than it would if were a larger size.”

“With most EFI systems – both factory and aftermarket – there will be some means of data logging capability available, and that will allow you to determine what the injectors’ current duty cycle is,” Alford explains. “I should note that factory EFI systems may require purchase of aftermarket components to have logging capabilities. Regardless, most data loggers will calculate the duty cycle based on the injector pulse width, or in some cases, it maybe be based off of the commanded fuel flow versus the amount of available fuel flow from the injector size. Once you get into the range of an 80 to 85% duty cycle and beyond, you’re reaching the tipping point and you’ll want to start looking at moving to a larger set of injectors if you plan on more power. At that point the injector is being pulsed very rapidly, especially at high RPM. Most manufacturers, including Holley Performance, will recommend a duty cycle of no more than 85% duty cycle when operating at peak flow. Most injectors are basically going to be ‘static’ at about 90% duty cycle, which means you’re trying to pulse them on and off in such short intervals that they’re essentially wide open and always ‘on’ in that situation. What that means is you’re losing dynamic control over the fuel delivery. For most applications, the duty cycle ‘sweet spot’ is somewhere in the 60 to 75% range.”

If the logs indicate that the injectors are operating in the appropriate duty cycle range, the next place to look is the fuel pump.

“OEMs normally engineer some headroom into their fuel systems so that small upgrades and other variables don’t push the factory fuel system beyond its capability,” he tells us. “Bolt-ons like an intake manifold, a set of headers, a cat-back exhaust, and a tune usually won’t create a extreme fueling issue. For common OEM performance applications 20 to 50 horsepower above stock isn’t cause for concern. It’s the bigger modifications – like turbos, superchargers, nitrous, and significant changes to the engine internals – that tend to lead to this.”

Appropriately Sizing Your Upgrades

If you’ve determined that your injectors are undersized, or that your fuel pump can’t keep up with the consumption demand (or both), you’ll likely need to make some upgrades. But Alford cautions against simply buying the biggest replacements that your budget allows for.

Shop high-performance fuel injectors here.

“It can be tempting to go with a much larger injector. You might be trying to think ahead so you only have to buy those injectors once, but you typically want to size injectors based on the power that the engine is going to be making right now with maybe a little headroom. A 500 horsepower naturally aspirated pump gas engine and a 1,000 horsepower boosted engine running on E85 are going to require different sized injectors. If you size the injector for the latter and run it on the former, although it will net you a lower maximum duty cycle, in most cases it will commonly cause poor idle quality along with drivability issues at throttle tip-in and part-throttle operation. In that situation, the injector is too large to be controlled precisely enough at the lower pulsewidths required to deliver the appropriate volume of fuel for the circumstances.”

An oversized fuel pump can create its own unique set of issues, too.

“Because the engine is consuming significantly less fuel than the pump is capable of supplying, you’re going to be returning an excessive amount of fuel back to the tank if you’re running a return-style fuel system,” he points out. “And that can introduce additional heat into the fuel, which can lead to drivability issues of its own as well as premature component failure with heat being the primary cause.”

To correctly size both the injectors and the fuel pump, you first need to know the engine’s approximate peak horsepower output at the crank. Once you have that figure, the first part of the formula for sizing both the injectors and the pump is more or less the same.

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“Once we have crank horsepower, we then need an estimate of our brake-specific fuel consumption,” says Alford. “Without the use of a dyno to determine fuel flow and an exact horsepower output number, we need a ballpark figure with a built-in margin of error for safety.

The basic rule of thumb for brake-specific fuel consumption for a gas-powered, naturally aspirated engine is half a pound of fuel per horsepower per hour. In this formula, that would be expressed as 0.5. So if we have a naturally aspirated gas engine that makes 500 horsepower at the crank, we’d multiply 500 by 0.5, which gives us 250. What that means is that, at 500 horsepower and B.S.F.C. of 0.5, on average the engine would be consuming roughly 250 pounds of fuel per hour [250 lb/hr].”

Fuel Flow Formulas

From here the formulas for determining injector size and fuel pump size start to deviate.

“For the fuel injectors, since most are rated in pounds-per-hour and we’ve just calculated for pounds-per-hour, we just take that 250 and divide it by the number of injectors that we have in the system. So if we have an eight-cylinder engine with one injector per cylinder, we’d divide 250 by 8. That’s going to give us the pounds-per-hour, per injector – in this case, 31.25.

250 ÷ 8 = 31.25

But it’s important to note that this would be the maximum flow at 100% duty cycle. So to factor in the duty cycle, we’d take 31.25 and divide by the decimal equivalent of the desired maximum duty cycle. If we wanted that 80% duty cycle, we’d take 31.25 and divide by 0.8, which gives us 39.06.

31.25 ÷ 0.8 = 39.06

So I’d look for an injector that’s a little bit above that – for example, a 42 lb/hr injector. Generally speaking, you’d round up to the nearest available size.”

Fuel pumps, meanwhile, are typically rated in gallons-per-hour (gph), so we’ll need to convert pounds-per-hour into gallons-per-hour in order to size the pump appropriately for the application. To do so, we’ll divide by the calculated lbs/hr figure by the weight of the fuel. “The average approximate weight of a gallon of fuel is six pounds, so we’d take that 250 and divide it by 6. That gives us about 42 gallons per hour. That’s what the engine is going to be consuming at peak horsepower in this particular example.”

250 ÷ 6 = 42

Once you have this gallons-per-hour figure, you’ll want to look for a fuel pump with a slightly higher rating to build some headroom into your system. “I wouldn’t go out and look for a fuel pump rated for exactly 42 GPH for this application,” says Alford. “A lot of fuel pumps are rated at ‘free flow,’ which is zero PSI of fuel pressure.”

As we discussed in Part 1 of this fueling guide, as pressure increases, flow naturally decreases.

“So while that fuel pump is rated for 42 gallons at free flow, at operating pressure it might only flow 28 gallons per hour, which is well below what we actually need here. What I would look for is a pump that flows a little bit more than I need at the desired operating pressure I’m going to be at. Since we’re talking about upgrading a factory EFI system, the two common operating pressures you’ll see in these types of applications are 43 psi and 58 psi. So usually what I’d do is factor in at least a 20-to-50-percent margin in there by taking that calculated number of gallons per hour and multiplying it by, say, 1.5. In this case that would net me 62.5 gallons per hour, so I’d look for a fuel pump that flows around 60 gallons per hour at 58 psi of fuel pressure. The pump I actually purchase may be labelled as ’80 GPH’, but again that’s typically at free flow.

Another potential problem is if you buy a pump that delivers exactly the required volume, such as 42 gph in our example, at the rated pressure. At peak horsepower the fuel pressure would basically drop to zero because the supply from the pump is equal to the demand from the engine. So, in order to maintain the correct fuel pressure, we need to have a supply that’s greater than the maximum demand of the engine. In this case, the engine is asking for 42 gph at peak horsepower, and pump is able to supply 60 gph, which means it’ll be able to maintain the correct fuel pressure no matter what.”

Read more on how to integrate the right fuel pump into your build, including return or returnless systems, the advantages of brushless moto pumps and more here.

He also adds that when you’re upgrading your fuel pump, it’s important to ensure that the wiring you’re using is appropriate for the application.

“Often times the factory wiring is just good enough for the factory fuel pump in terms of the wire gauge that’s used. When you move to a higher-flowing pump, you’re often going to have an increase in amp draw, and that may require a step up in wire gauge. In many cases we’ll see factory wires that are 14- or 16-gauge, whereas an aftermarket pump might require a 12- or 10-gauge. If you get into really, really big pumps – or mutli-pump systems – you might even want an 8-gauge wire. Often times a simple way to upgrade the wiring is a relay kit with larger gauge wire, you can use the factory fuel pump wire to trigger the relay and have the 10- or 12 gauge wire go straight to the pump”

Lines And Fittings

If you’ve made a big jump in horsepower, you may discover that the diameter of your vehicle’s fuel lines aren’t large enough to properly feed the engine. While a pressure loss between the pump and the fuel rail is a good indication that the fuel lines are too restrictive for the application (provided the filter isn’t restrictive), Alford says that the following rule of thumb can make it easy to figure out whether or not your lines are still up to the task.

“Generally speaking, a 3/8-inch line – or an equivalent -6AN line – will safely support up to 600 horsepower. A 1/2-inch line, or an equivalent -8AN line, is capable of supporting 800 horsepower, and a 5/8-inch or -10AN will be capable of supporting 1,000 horsepower or more. That’s a reliable baseline to work from.”

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He adds that the lines mentioned above can often support a little more horsepower if the fuel system has minimal restrictions in it. And in some cases, those restrictions can actually be caused by the route that the fuel lines take between the tank and the engine.

“You might find that the factory fuel line routing has some tight bends in it and things like that. But fluid doesn’t like to turn, so having a route that’s as smooth as possible between the tank and the engine is ideal, and it gives us the best chance to extract all of the capability that fuel system has to offer. So if OEM system has a lot bends and kinks in it, that might be an instance where we’d ditch the factory lines and either go with an upgraded OE-style line, or we may look to do something that’s AN hose. Beyond the benefits of a larger diameter line, this might also give us the opportunity to re-route the line in a way that’s more free-flowing.”

Alford recommends using the factory routing as a starting point and using a frame channel when you can in order to keep the line protected and away from heat sources like downpipes and the exhaust system. “The idea is to protect that line from the environment – road debris, rocks, and whatever else might get kicked up while driving, especially if it’s a street application. Also keep in mind that you’re going to need to consider where components like the fuel filter and regulator are going to be mounted, and how you’ll route the lines to those components. Those factors will help determine what route makes the most sense.”