Conventional fuel tanks normally get the job done for weekend cruisers and mildly modified street machines, but those who are looking to get serious about performance will likely reach a point where it makes sense to make the switch to a fuel cell. Although sanctioning bodies across many different motorsport disciplines mandate the use of fuel cells in certain classes, there are benefits that go beyond simply complying with a rule set.

“For the grassroots guy, it often actually comes down to packaging,” says Holley Performance project engineer Junnior Rodriguez. “People want to add aero, or maybe they’re converting to all-wheel drive and they need room for their differential. Getting rid of the factory fuel tank will free up a significant amount of room under the car.”

In many cases, there’s also simply no need for fuel storage capacity of 20 gallons or more. “Others are looking to reduce weight wherever they can and moving down to a 10-gallon fuel cell, for example, can help. The average weight of fuel per gallon is about six pounds, so it quickly starts to add up. That’s especially true for drag racing – if the car is only being driven a quarter-mile at a time, some teams can get away with using a three-gallon fuel cell throughout an entire day of racing.”

Since most factory fuel tanks on carbureted and early EFI vehicles weren’t baffled, stepping up to a fuel cell can also help resolve potential issues with inconsistent fuel pressure and fuel starvation. And let’s not forget about the potential safety benefits, either.

“Typically, a factory fuel tank is open to the elements,” Rodriguez points out. “Whereas, with a fuel cell, you can mount it inside the trunk or in the cabin. And that means it’s much less likely to be punctured by foreign debris if there’s an incident.”

But with the range of fuel cell options and supporting hardware that Holley Performance offers, it can be tricky to pinpoint what makes the most sense for a given application. Here we’ll get some insight from the experts about what builders should take into consideration when selecting a fuel cell and the accessories that go along with it.

The Right Tools For The Job

Rodriguez suggests that before taking any specific features or design elements into account, builders should consider their packaging restraints and capacity needs.

“The first thing you need to do is ask yourself how big of a tank you want. We offer them in three, five, ten, fifteen, and twenty-gallon sizes.” Beyond determining how much room you have to work with, builders also need to consider the way the car is going to be used. For example, while a drag car might only require a three-gallon fuel cell, an endurance road racer will likely need much more capacity to minimize the need for fuel stops during competition.

Tank Design

Holley Performance’s TIG-welded fuel cells are made from 2.5mm-thick 5052 aluminum and are offered with either a flat-bottom design or a rear sump that helps to prevent fuel starvation under hard acceleration. All fuel cells come with pre-cut anti-slosh foam to clear the fuel gauge sender, though it should be noted that this foam is not compatible with E85 fuel. These fuel cells use -10AN ORB feed, return, and vent ports for high flow, and those ports can be adapted to -6AN, -8AN, and -12AN sizes without cutting and re-welding the male weld bungs.

In-Tank Versus External Pumps

Once you’ve zeroed in on the fuel cell you want to use, the other important decision to make early on is whether you want to use an in-tank fuel pump or an external one.

“One of the big benefits of an in-tank pump is the reduction of noise,” Rodriguez explains. “Some people don’t like being able to hear it operating, and that’s more noticeable with an external pump.” An in-tank pump will also run cooler because it’s inside the fuel tank, which allows the fuel it’s submerged in to function as a cooling agent. And since the pump is already submerged in its fuel source, you don’t need to worry about potentially mounting the pump improperly.

“The most common issues we see with external pumps are related to installation errors,” notes Rodriguez. “An external fuel pump is gravity-fed, so it should always be mounted below the tank.”

External pumps have merits of their own, though. Some folks prefer using them because they want to be able to hear the pump so they know it’s on and working. And in motorsports applications, having an externally-mounted pump can make service or repair quicker and easier. But it’s also worth noting that a cooler-running pump will likely have better longevity, so serviceability might not need to be such a high priority if you opt for an in-tank option.

“In most cases, an in-tank pump would be my recommendation,” says Matthew Sosa, Fuel Systems Team Lead at Holley Performance. “It’s quieter and it keeps the pumps cooler. The only real downside is accessibility – some racers want to run an external pump simply because it means it’ll be easier to get to it later.”

Feeding The Beast

The type of pump (or pumps) that is paired up with your fuel cell also plays a crucial role in your fuel system’s performance. And as with fuel cells, the process of selecting the proper pump system for a given application starts by taking a few basic questions into consideration.

“Obviously the first fork in the road is EFI or carburation,” Sosa says. “And if your application is using the latter, you should probably decide whether or not it’s going to stay carbureted. We offer carburetor-specific fuel pumps as well as EFI-specific fuel pumps, and in many cases, you can use an EFI-specific fuel pump in a carbureted application with the proper fuel pressure regulator. But the same cannot be said the other way around. You can’t use a carburetor-specific fuel pump on an EFI application.”

The next two variables to take into account are your maximum fuel pressure and your horsepower target.

“The two most common base fuel pressures are 44 psi (3 BAR), and 58 PSI (4 BAR), for example,” Sosa tells us. “For a naturally aspirated engine, the base fuel pressure will be the maximum fuel pressure, but for a boosted engine running a 1:1 referenced regulator, the total boost pressure will need to be added to the base pressure to get the maximum fuel pressure.” So let’s say start with a base pressure of 58 psi and you’re going to run 20 pounds of boost, your maximum fuel pressure will be 78 psi.

Having calculated the max fuel pressure, the next thing you need to know is what your horsepower target is and what fuel you will be running as this will allow you to calculate the flowrate that will be necessary to support that power. “For gasoline, a good rule of thumb is to take the target power and divide by 10 to get a flow rate in gallons per hour (gph)” says Sosa. So, if the target horsepower is 1000, the pump you select would have to flow at least 100 gph at the max fuel pressure. It’s also worth mentioning that E85 fuel will require higher flow rates than gasoline, so that’s something you should keep in mind if you’re planning to go that route. You can find more information about how to determine your fuel pump requirements here.

Sosa adds that while volume is the main concern when selecting a pump, there are other factors worth considering as well. “Depending on where you live and the type of racing that you’re doing, you may have concerns about heating the fuel. If you’ve got a very large fuel pump that’s pushing a lot of volume it will add a lot of heat into the fluid as it's circulating, and you live in a hot climate like Arizona, that could lead to issues like fuel boiling and vapor lock. So, aside from having the volume to support the power, you may want a system that uses multiple pumps. That way you can run on one pump when you’re just cruising around, and when you need the extra flow, you can turn on the second one.”

He recommends selecting a pump or multi-pump system with about 15 to 20% more capacity than your combination’s requirements to provide some headroom, but the need for a “fudge factor” really comes down to compensating for potential restrictions or other issues in the flow path.

“When you’re looking at a flow curve for a fuel pump, you’re looking at how much volume that pump puts out in an unrestricted system. But after the outlet of the pump you might have plumbing of various sizes, 90-degree fittings, and other things that add head pressure.”

With that in mind, he notes that it’s important to select lines and fittings that won’t cause bottlenecks in the path. And, from a performance standpoint, there’s really no drawback to using too large of a fuel line. “The only issue is cost, really. Bigger is better: There’s a downside to using a fuel line that’s too small, but there isn’t one with a fuel line that’s too large. So if you’re debating between a -8 and a -6 line for your application, and you can afford the -8, you’re better off going that route.”

The filters need to support the flow rate and should be tailored to the application, too.

“Our Billet series filters are cataloged by flow rate, so that’s pretty easy to determine,” Sosa says. “And you also want to keep the micron size in mind. For many applications, whether it’s carbureted or EFI, a pre-filter for the pump should be around 100 micron, and the post-filter should be around 10 micron for EFI, or 40 micron for carbureted applications.”

Getting Dialed In

When you’re ready to dig into the fuel cell accessories, you’ll probably want to check out the options for fuel level senders first.

“Float arm-style senders are the traditional option,” says Rodriguez. “But in a fuel cell, you have a lot of fuel sloshing around which can put the float arm through a lot of abuse, and over time, that could bend the float arm. So instead of using a float-arm design, we went with a tube-style sender design.”

The tube-style sender design improves packaging by eliminating the arm assembly entirely, which is replaced by a tube with a float ball inside of it. The design also acts like a damper, which allows it to provide consistent readings despite fuel slosh.

Holley also debuted the float-less Easy Level sender earlier this year, a programmable LIDAR-based system that eliminates the need for any sender hardware inside the tank or fuel cell. “It just needs is a clear line of sight to the bottom of the tank from the top, and since the foam in our fuel cells is pre-cut, you’re good to go right out of the box,” Rodriguez says. “All it needs to know is the depth of the tank or fuel cell.” Note that while most fuel level senders read off of resistance, the Easy Level reads off of voltage. So, if you’re planning to go this route, make sure that the fuel gauge you’re planning to use with it is compatible.

In situations where mounting location options are scarce, fuel cell mounting straps can provide additional flexibility. Available for 5, 10, 15, and 20-gallon fuel cells, these straps are made from 1/8-inch-thick mild steel and feature a black anti-corrosion finish.

If you’re planning to do a return-style system, the Fuel Cell Return Fitting Kit can also help prevent potential issues with inconsistent fuel pressure by routing the returning fuel below the surface level. “If you’re running an external pump, usually you’ll have returning fuel entering from the top,” he notes. “That can cause splashing that can aerate the fuel pump and cause the fuel pressure to fluctuate. But by having that returning fuel enter below the surface, you eliminate that potential for splashing.”

A roll-over valve is an important accessory to consider as well. In the event that the vehicle is overturned, any venting provision on a fuel tank or fuel cell can become an opportunity for fuel to leak out. If the roll-over valve becomes flipped, a stainless-steel ball moves to seal off that vent to prevent any potential spillage. This valve is included as part of the Fuel Cell Vent Kit, and it can also be purchased individually.

And if you opt for an externally-mounted fuel pump, a Holley or Earl’s billet fuel cap can provide a finishing touch to the project. Available in 6-bolt and 12-bolt configurations, these caps are a direct replacement for the standard plastic caps that are included with Holley fuel cells. “These come with new gaskets and bolts, and it also provides a larger opening,” says Rodriguez. “And whereas the standard cap opens by lifting up the handle and twisting it, the billet caps are threaded. It’s similar to a factory-style filler cap in that way, and that twist-down seal provides a bit more assurance that the cap is locked down properly.”