How to Build an Off-Road Fuel Cell using Holley HydraMat

10 min read

How to Build an Off-Road Fuel Cell using Holley HydraMat

10 min read

Fuel delivery has long been the bane of everyone from stock car racers to rockcrawlers, and the pressure demands of fuel injection have only made this situation worse. Hard cornering, fuel sloshing, and degraded foam are just some of the contributing factors to fuel starvation. And no one who is racing wants to have ten pounds of extra fuel in their tank that is unusable due to a poor pickup location. In 2014 Holley came up with an innovative solution to all of these problems: it’s called HydraMat.

M715 Hydramat 1

HydraMat has a 15-micron pore size, so it essentially acts like a pre-filter for your fuel. The mat is compatible with gasoline, E-85, alcohol, ethanol, methanol, and two cycle oil blends as high as 20:1, but you can’t run HydraMat in your diesel truck.

How does HydraMat work?

Holley’s HydraMat completely revolutionized fuel delivery with a product that is still unrivaled in the market. Surface tension and fluid wicking are the secrets of the HydraMat’s amazing ability to draw fuel from nearly any area that has contact with fuel and store it in its internal reservoir. As an area of the HydraMat is uncovered, the tiny pores of the media seal off thru surface tension, forcing fuel to be contained in the reservoir and drawn from other areas of the mat where fuel continues to be available. All the while the pump is able to pull from inside the HydraMat allowing it to avoid fuel starvation.

The sheer size and coverage area of the HydraMat allows it to pull fuel from virtually any area in the tank or cell eliminating the need for specialty reservoirs and pick up pumps, saving cost and greatly reducing complexity. This patent pending design also does an excellent job preventing aeration, which helps your fuel pump live a long, happy life.

M715 HydraMat fuel cell build

There are a lot of different fuel cells on the market, but sometimes you need something more custom. That was the motivation for Dr. Smash creating this fully fabricated aluminum fuel cell for a customer that wanted zero compromises. With the big bottom of the tank, Holley HydraMat will ensure that none of the fuel goes to waste.

Custom Fuel Cell Fabrication

Eric Bevans (aka Dr. Smash) recently used two HydraMats in a custom thirty-gallon fuel cell he built for the back of a customer’s M715. “This is a big, heavy truck with 43-inch tall tires,” Bevans explained. “The customer plans to take this on extended trips through the desert and wanted plenty of fuel capacity, right down to the last drop.”

Dr. Smash started by measuring twice, then had aluminum sheets cut up to create the top, bottom, and sides for the fuel cell. He built his own filler neck with a flap to keep fuel from spilling out, and added bungs for the fuel hose, return line, and rollover valve. Like many of us, degrading foam has caused Dr. Smash to pull his hair out in the past, clogging fuel filters and killing pumps. No foam was used in this custom fuel cell. While baffles are not a necessity with HydraMat, some people like wearing suspenders with a belt, so baffles were added to the sides of the cell running crossways to prevent sloshing when accelerating and decelerating.

M715 HydraMat sizing

HydraMats are most commonly used flat on the bottom of the tank, but they can be folded, such as up the back wall of a fuel cell in a rock buggy to optimize fuel scavenging on vertical climbs. In this case, bigger is better when sizing your HydraMat...just ensure you can get it into the tank opening.

Hydramat Sizing

HydraMat is available in eighteen different shapes sizes to accommodate different tank sizes and openings. Square, rectangular and “X” shaped mats are offered to provide easy installation through various sending unit access holes or fuel cell cover plates. Retrofit kits are also available for stock tank vehicles to ease installation of aftermarket EFI retrofit kits like the Holley Sniper EFI system. Dr. Smash chose to use two “X” shaped HydraMats (PN 16-102) in the bottom of the baffled fuel cell he constructed for the M715 for redundancy. These will be plenty to feed the mild Small Block Chevy engine under the hood.

The general rule of thumb is to install the largest HydraMat you can reasonably fit inside your tank. Larger sizes offer more fuel capacity in the internal reservoir, increased filtration, and less restriction for the fuel pump. They also cover more square inches in the tank for maximum fuel scavenging, ensuring the mat is touching fuel even in extreme driving conditions. HydraMat is not cleanable, however they will provide approximately ten years of usable filtration performance, depending on variables including the fuel type and quality, how clean your fuel system is, and how much you drive your car. Severe duty applications such as racing should inspect the filter at regular intervals to ensure the HydraMat is not restricted by debris.

Holley VK040048

Holley offers combo kits, such as this 1978-86 Jeep CJ combination (VK040048) under one part number that includes a new galvanized steel fuel tank, internally regulated high pressure in-take fuel pump capable of supporting fuel injection, HydraMat for reliable fuel pickup at any angle, and a pre-calibrated sending unit.

Drop-In Tank Options

Holley also offers HydraMat as part of its drop-in returnless and return-style fuel pumps. Universal pumps are available in a variety of volumes to feed even four-digit horsepower engines, as well as direct fit pumps for popular muscle cars. If your tank is too far gone to salvage, Holley offers new fuel tanks complete with a high pressure, in-tank fuel pump featuring HydraMat for Camaros, Barracudas, Mustangs, Jeep CJs, and more.

M715 HydraMat Securing The HydraMat

Dr. Smash cut slots in the baffles of the fuel cell to allow the HydraMat to slide underneath and lock in place. Tabs were also added to each side to ensure that the HydraMat didn’t float when the tank is full. If you don’t want to get this custom, Holley has off-the-shelf mounting options.

How To Hold HydraMat In Place

Dr. Smash TIG welded tabs inside the tank he was fabricating in order to hold the HydraMat in place. The aluminum tank was laser cut and uses slots to fit together prior to being welded. He took care to make the tabs tight enough to keep the HydraMat secure, while keeping in mind that he would have to reach through the opening on top of the tank to install or replace the HydraMat. If the mat is allowed to float or move, its effectiveness could be reduced. If you aren’t building a custom fuel cell, Holley has a couple of clever options for how to secure your HydraMat.

If you have a steel tank, Holley offers a kit with magnets that can be used to hold the HydraMat securely to the floor of the tank. The magnets are very strong, and have studs that correspond to the holes on the corners of the HydraMat. The magnets are designed to be bolted to the various HydraMats to prevent unwanted movement of the mat inside of steel fuel tanks or fuel cells. If you are using an aluminum or polyurethane fuel cell, the magnets obviously won’t work. Holley offers click bond fasteners that are specially designed stud mounts that can be installed in custom tanks or fuel cells with large openings. These studs are epoxied into the tank to secure the HydraMat and prevent unwanted movement.

M715 HydraMat plumbing

Dr. Smash built the fuel cell with baffles to prevent sloshing fore and aft, and then positioned two HydraMats inside the cell on each side of the tank to ensure adequate fuel delivery at any angle, no matter how little gasoline is in the cell.

Fuel System Plumbing

The Holley HydraMats used here came with a bonded aluminum fitting that you can add a 3/8-inch NPT fitting to, such as a hose barb or threaded fuel line. Other HydraMat models come with 1/2-inch NPT fittings or 5.8-inch Quick Connect fittings. Alternatively, if you are using a TI 450LPH or similar fuel pump, HydraMat is available with an 11mm outlet to hook directly to the bottom of the pump. Multiple HydraMats can be used in a single tank by installing a “T” or “Y” fitting in the feed line to the fuel pump, which is exactly what Dr. Smash did in this application. The self-sealing design will pull fuel from the tank area that still has available fuel.

Holley recommends the use of a convoluted PTFE hose or a rubber hose designed for in-tank fuel use, such as the 3/8-inch in-tank compatible hose that Dr. Smash chose (PN 26-161). This hose can be attached to the HydraMat before or after insertion into the fuel tank, depending upon the tank opening size. In this case, Dr. Smash attached the hose first and torqued the NPT fitting to 24 ft-lbs. If your tank has sharp edges, such as in this instance, wrap tape around the edges of the tank opening to prevent damaging the HydraMat. Duct tape is thick and strong, but leaves behind a sticky residue. Dr. Smash used blue painter’s tape to protect the HydraMat during installation.

The return fuel line should be plumbed in the tank so the discharge point is directed towards the pickup connection point and approximately one inch above the HydraMat surface, if possible. This will keep the HydraMat wet under extremely low fuel conditions and provide maximum scavenging at steep angles or when low on fuel.

M715 Hydramat truck

The fuel cell Dr. Smash is building will find its way into the back of this Kaiser M715 military truck. The truck has been re-powered with a Small Block Chevy and uses custom leaf springs and one-ton axles capped with 43-inch tall Super Swamper SX tires.

Hydramat M715 tank creation

Eric Bevans had the 10-gauge aluminum cut out to his specifications and then welded the entire cell together. The shape is intended to fit behind the rear axle of the truck under the frame where it will assist with weight balance and provide a low center of gravity. Slots in the top panel allowed Bevans to weld the baffles into place and seal up the tank.

HydraMat Magnets

If you have a factory steel tank and want to add HydraMat to combat sloshing and other pickup-related issues, Holley’s magnetic mounts are a great option. They connect with studs to the corners of the mat and then hold the HydraMat securely in place in the bottom of the tank.

HydraMat barb fitting

A simple 3/8-inch nylon hose barb was used in this application to connect the HydraMat to the fuel line. Threading the nylon into the aluminum nut bonded to the HydraMat only requires about 20 ft-lbs to be tight

HydraMat safety wire

Even with barbed fittings, we recommend using hose clamps on all junctions to prevent the hose from coming loose in the tank, particularly in off-road applications. Dr. Smash used safety wire on all connections inside the tank.

HydraMat Fuel Cell Opening

The opening of this fuel cell is large enough to get your hand inside, however the baffles add another layer of complexity to the job. It is helpful to be double jointed! It is also helpful to put tape around the opening in the tank to protect both the Hydramat and any exposed skin from getting cut.

Parts List

Part NumberDescriptionLeg WidthAcceptable 
Tank Opening
16-100 15 x 15 cross-center outlet – 3/8” NPT 2.5”3.15” & larger
16-101 15 x 8 cross-center outlet – 3/8” NPT 2.5”3.15” & larger
16-102 15 x 8 cross-offset outlet – 3/8” NPT 2.5”3.15” & larger
16-103 8 x 8 cross-center outlet – 3/8” NPT 2”3.15” & larger
16-104 11 x 11 cross-center outlet – 3/8” NPT 2.5”3.15” & larger
16-105 15 x 3 rectangle – center outlet – 3/8” NPT N/A2.07” & larger
16-106 11 x 11 square – center outlet – 1/2” NPT N/A4.00” & larger
16-107 15 x 15 square – center outlet – 1/2” NPT N/A4.00” & larger
16-108 8 x 3 rectangle – center outlet – 3/8” NPT N/A2.07” & larger
16-109 24 x 15 rectangle – center-side-side outlet – 1/2” NPT N/A4” x 6” & larger
16-110 30 x 14 rectangle – center-side-side outlet – 10 AN N/A6” x 10” & larger
12-924 30 x 14 rectangle – side outlet – 10AN N/A6” x 10” & larger
12-925 30 x 14 rectangle – center-side outlet – 10 AN N/A6” x 10” & larger


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