Firebird and Camaro LS Swap: Fuel System Guide • LS Engine DIY

As has been mentioned before, the LS engines were not expressly designed to go between the frame rails of the first- or second-generation F-Body. Every modern LS engine is fuel-injected while every early F-Body car carried a carbureted engine. Fuel injection has many different requirements than carbureted engines. To run the Efisystem on an F-Body requires installation of more fuel system plumbing and either an internal or external electric fuel pump for the injection system. You also need to integrate the wiring into the main harness and install the drive-by-wire pedal. Even with these challenges, LS engines have been effectively installed in the 1967–1981 F-Body chassis and have delivered phenomenal performance.

The early Gen III engines from 1997 to 1998 and some 1999 engines used a return fuel line system while the 1999 and newer LS engines used a returnless fuel system. If you are installing a later-style Gen III or Gen IV engine, you don’t have to route a fuel line back to the tank. If you’re installing an LS with a return fuel line system, a regulator is installed between the fuel rail and tank to provide consistent fuel pressure. Then hoses must be run to a 3/8-inch supply fitting and 5/16-inch return fitting on the fuel rails.

The FAST fuel injection system feeds the Chevy Fast Burn 385-ci engine in Mary Pozzi’s 1973 Camaro. (Photo Courtesy Mary Pozzi)

This tech tip is from the full book, SWAP LS ENGINES INTO CAMAROS & FIREBIRDS: 1967-1981.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

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Throughout this chapter, I show you various ways to build a fuel system for your LS-equipped F-Body car. I discuss the use of stock gas tank modifications all the way up to a custom-built stainless gas tank with a trick off-the-shelf pump. Of course there are many ways to complete the task and I can only show a few of them. This should help you decide the direction of your build.

By today’s modern fuel injection standards, the first two iterations of the F-Body and its fuel system were crude and rudimentary. The stock F-Body used a mechanical fuel pump, which was driven off the camshaft, and it drew fuel from the gas tank to the engine. In addition, the LS engine has far greater fuel pressure requirements than the original carburetor-equipped F-Body. Carburetors were fairly reliable and easy to fix, but they featured a static setup and didn’t change to accommodate varying conditions. They were prone to carbon build-up and tended to blow out power valves if the car wasn’t driven properly. Today, the fuel delivery system no longer requires engine power to drive the fuel pump and requires an electric pump to delivery fuel needs. The old system only needed a paltry 8 to 10 psi at the carburetor while an LS engine with an Efineeds a constant 58 psi at the fuel rail. This means that it’s a necessity to upgrade your fuel system with an electric pump and a fuel pressure regulator. This means more forethought and some blueprinting of the new fuel system in your F-Body.

Proper Planning

A fuel system needs to be planned out prior to purchases being made and brackets formed. A solid fuel delivery system also requires a properly wired system. This means that you have to ask yourself what your plans are for your car so you can buy the correct combination of parts. One question that many folks face is, “Am I going to upgrade in the near future?” If the answer is yes, you might want to step up to a slightly better system to plan for the long haul.

A carbureted system needs roughly 8 psi to operate well under idle and under heavy loads. Unfortunately, the newer engines with EFI (and this includes the LS engines) require much greater fuel pressure and fuel system sophistication. The key number to remember here is 58 psi. If you are running a stock or mildly upgraded LS engine, this is all you need in terms of fuel delivery. If you are running an LSA or LS9, you might need to step up to a much better fuel delivery system that requires more than 58 psi. However, 58 psi is sufficient for most LS engines turning out 400 to 700 hp.

Fuel injectors require high pressure to deliver fuel into the combustion chamber in a microsecond. In order for the injectors to do their job, they need a constant supply of high-pressure fuel in sufficient quantities (flow rate) or the engine ceases running. In addition, the injectors need this much pressure to keep an even and consistent spray pattern. The system requires a constant high-pressure fuel to be available at all times. By comparison, a carburetor is vacuum-operated and doesn’t require high pressure to mix air and fuel together to feed the engine.

Fuel Pump

I went to Carl Casanova, the guru of fuel delivery, to gain insight and advice for planning and installing a fuel system. Carl has been known around the pro-touring and hot-rodding fuel delivery arena for quite some time. You probably have seen his shiny red Gen I in multiple articles in various print and online publications. He teamed up with Hector Guerrero at Rick’s Tanks to make a system that functions like factory equipment, with easy installation and the reliability of a rental car.

I’ve laid out a pre-filter (left) and the pump (right) to show you what it may take to make a system work. I left the post-filter out because you may not install one after-filter. The Corvette filter with the return line is a popular and practical choice.

The regulator (if you chose to use one) also needs some planning as to how to route the fuel lines and a vacuum line. Most regulators on the market require it.

A relay is a fairly simple thing to wire. You need a constant 12-volt source such as a battery terminal or node, a ground, and an activator wire to the device you want to run. It clicks on when power is needed without overloading the system. It protects your vital electrical components as well as provides much-needed power to things with heavy loads. (Illustration Courtesy Holley)

Holley’s fuel pump relay kit is essential when installing a new pump. The relay provides power to the pump only when necessary. This prevents the pump from burning out prematurely and it shuts off when the key is in the off position.

When it comes to fuel pumps, there are two basic kinds: in-tank and external. Both systems have advantages and disadvantages. The in-tank pumps tend to run quieter and cooler because they are immersed in fuel inside the tank. They also tend to perform at higher pressures. The trade-off is that they are often more complicated to install and relatively costly. To modify a stock system to work with an in-tank pump requires a greater investment of time and potentially more money.

To make your system an in-tank setup, you need the pump submerged in the tank as deep as possible but maintain a strong fuel feed. You don’t want it choked off because of improper mounting. You need to mount the system carefully and properly for your particular setup and needs. This means you have to find a creative mounting system that is not only fuel-resistant but also sturdy enough to handle a bit of abuse. Adequate tank pressure is necessary to keep the pressure up between the pump and the fuel line.

External pumps make the job a lot easier to install or replace in case of a failure because you can simply access the external pump where it’s mounted to the chassis. With an in-tank system, the gas tank needs to be drained and dropped and then the pump has to be extracted from the tank. In-tank pumps are less expensive to purchase, but if you need to change a pump it costs a lot more in the long run. External pumps are typically noisier because they lack fuel to dampen the sound and they are exposed to the elements. In addition, the pump can transmit vibrations through the chassis to create noise.

The best and most logical place to put an external pump is as close to the gas tank as possible and at or below the fuel level. These pumps work best as “pushers” of fuel, rather than “pullers” of fuel. Pump life is extended because gravity is used to draw fuel to the pump and therefore a reduced load is placed on the pump to enhance longevity.

The other major problem with an external pump is that the fuel line is only pressurized after the pump. This means that the system relies on gravity to feed the pump. These pumps can provide some suction but should be gravity-fed for optimal results. This means that your system can lose its siphon under hard cornering. If you plan to do any hardcore racing or severe lateral g’s, you should really look into an in-tank system.

Matching the fuel pump to the engine’s fuel demands is vital. I recommend determining the amount of fuel needed for the engine, adding 10 percent for safety, then choosing the fuel pump that best matches the fueling need. A 255 lph fuel pump is usually a good starting point for comparison.

Determining which fuel pump is right for you requires a number of steps. First, you need to determine your engine’s horsepower and torque target. It’s typically safe to aim high here because it’s better to have more fuel pump capacity rather than not enough. Second is finding the engine fuel efficiency and third is finding the maximum fuel pressure and flow volume you’ll be running. Finally, you need to know the available voltage under load from the engine and flow volume at that particular voltage.

I spoke with an Aeromotive tech specialist about engine fuel efficiency, and he informed me that typically naturally-aspirated engines make roughly between .4 and .5 lb/ hp/hr (pound per horsepower per hour). The tech strongly recommended engine dyno testing to determine your actual efficiency number, but these are good starting points. Engines with a nitrous addition often develop .5 to .6 lb/hp/hr and those with boosted applications are usually least efficient at ranges from .6 to .75 lb/hp/hr.

So, for example, if I used a 500- hp naturally-aspirated engine I would multiply 500 by .4 and get 200 pounds, which would be the expected fuel pump requirement. However, if I used a 500-hp engine that was boosted, I could potentially multiply that by .75 and get 375 pounds of fuel pump requirement. Aeromotive also informed me that this also plays a big factor in injector choice. Normally most engine builders like to use a duty cycle of .8, which gives them a window of 10 percent for unexpected occurrences.

Therefore, using our example of 500 hp and a naturally aspirated engine, I would get something like this:

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Next is finding the base fuel pressure for the engine, and of course, fuel system requirements change for boosted or nitrous applications. As pressure goes up, often pump volume goes down. It is important to check the fuel pump’s specifications and read the flow charts for accurate pressure ranges. (If this is something that you find challenging, the Aeromotive tech line is a great place for asking questions about fuel pumps.) In the case of my build, I knew going in that my LS3 as well as most LS applications require a constant 58 psi for the engine to run optimally throughout the power band.

Finally, you need to consider voltage. If the pump does not have the required power it underperforms and does not run at peak efficiency. Higher voltage means that it increases power output so it is crucial to match your alternator’s output with the fuel pump you are using to complement the entire system.

The following focuses primarily on fuel-injected setups, as those seem to have the most confusion. If you plan to run a carbureted LS engine, a lot of it doesn’t apply.

External Fuel Pump Assembly

Several approaches can be used to install a fuel system. Often the most practical way to do this is with an external pump assembly. A few key features are critical when constructing a new delivery system. The best way to do this is to plan well ahead of your build so you don’t end up having to redo it or buying the wrong parts. Remembering that an external pump works best as a pusher of fuel (not a puller) commands your next moves. You certainly want gravity to feed your pump, so it’s best to place the pump as close to the tank sump as possible.

A typical setup consists of a pre-filter, pump, after-filter, fuel pressure regulator, return line, and feed line that is typically regulated by a vacuum source coming from the engine. The location of these components is dependent on available space. A lot of folks run the regulator on the firewall for easy access and that’s typically a good idea. Keep in mind that you need to run a vacuum line and a return line back to the fuel tank. Some racing circuits do not allow this so you might have to get creative. For most street drives the firewall is the most useful, accessible, and logical place to put the regulator.

Because external pumps are “pushers,” you want to mount the pump as close to the gas tank as possible. The most efficient way is to build a bracket that mounts between the front of the fuel tank and the rear axle assembly and rear suspension. Keep in mind that exhaust and suspension become major factors when you stray from stock components. For example, in this build adding an external pump assembly is impractical as the rear suspension is far too cumbersome to work around.

Fuel Level Sending Units

A lot of sending units are on the market right now. The key is to determine what gauge you’ll be using. Almost all GM gauges and fuel level units use a 0- to 90-ohm reading. Virtually any company can supply you with a gauge that fits this criteria. The important factor here is that you correctly match the gauge to the fuel level unit in terms of ohms. Wiring is fairly straightforward; simple ground and positive wires usually do the trick.

Filter

I cannot emphasize this part enough: It is crucial that if you choose to run an external pump it is essential to use a pre-filter to catch debris before it gets to the pump. It’s common knowledge to run a filter after the pump. It is also important to purchase the correct high-pressure filters as well; the old carburetor filters give out too easily for these kinds of pressures.

The Corvette fuel filter eliminates the need for two lines to be run to the engine. In this installation, the owner installed AN fittings over the hose ends to allow for braided hoses. (Photo Courtesy Street & Performance)

A neat trick for systems running a return line (which is most of them) is to run a C5 Corvette external fuel filter and regulator. If you have an LS engine that is 500 hp or less (which is probably stretching the limits of the C5 fuel pressure regulator), you might want to seriously consider GM PN 10299146. Not only does it act as a regulator designed specifically for LS engines, it does triple duty as a filter and provides a return line to keep the number of pressured lines to a minimum. Again, this has the standard 3/8-inch quick connection for the inlet, and a 5/16-inch quick connection on the return side. So make sure to plan on purchasing the correct number of adapters for your project.

Hose and Fittings

You need to use the correct hose for the correct component; otherwise you risk hose failure and a fire. Not all fuel lines are created equal. Fuel lines rated for carburetors are inadequate for the pressure of fuel-injection systems. Typically, an EFI system applies triple the pressure to the hoses as fuel hoses for carburetors. In fact, a carburetor fuel pressure is 8 to 11 psi while the fuel pressure for an LS3 is 60 psi and most LS engines have 40 to 60 psi. If you rupture a fuel line, it can easily create a fire and your car can potentially burn to the ground. You must use hoses that are rated to handle the pressure from your fuel system. You must use the correct fuel-injection rubber hose or, even better, braided fuel line.

I used a series of these clamps to keep the fuel line away from heat and other hazards. You can use any method you want to hold them down. I prefer self-tapping sheet-metal screws because they can be reused. Pop rivets also can work equally well; it’s your choice.

Using the stock hole in the floor for the fuel-level sending unit, I slipped a grommet over the wires and positioned it in the floor. I then replaced the access cover and it looks nearly factory.

I recommend using braided (PTFE) lines over rubber lines for a few simple reasons. They prevent premature wear by being much more durable and resist wear better than the rubber lines. In addition, they are less likely to kink compared to rubber lines. It might be advantageous to use hard lines for long runs under the car and add a few adapters. I prefer to keep the system as basic as possible (in this build I used one long piece of -8AN braided fuel line from the pump straight to the fuel rail). You can use either -6AN or -8AN fittings for mild to medium applications.

When I saw the quick-connect ends of the fuel pump and the fuel rails on the stock LS engine, I thought I had run into a big compatibility issue. Thankfully companies have come up with reliable adapters to help fit stock fuel lines to AN-sized fittings and hoses. Here, for example, I used Russell’s (PN 644000) to connect the fuel pump to a -8AN fuel line (mine are black). A -6AN line is fine up to about 700-hp engines; bigger than that and you might want to step up to -8AN.

Stock Fuel Tank Replacement

Removing a stock tank is not difficult and I’ve included a couple of quick tips to help you make the job a lot less messy. Depending on your vehicle, you might have a two-piece downspout, such as on my 1968 test car, or it could be like the 1969 version where the spout is built in. With either downspout, disconnecting the spout from the car is the first priority. The fuel line connections to the tank come next (these are located through the rear passenger-side wheel well on my car).

Make sure you remove all the old gas or run the risk of getting a lap full of nasty old gasoline. Siphoning the tank is fairly easy but can be a pain. Doing this lightens the load when you pull the tank out.

Two bolts hold the tank between the rear bumper and the trunk. A deep-well 9/16-inch socket does the trick. Make sure to remove the electrical connectors as you slowly lower the tank. I used a board and a jack to guide the tank out of its cocoon.

This tech tip is from the full book, SWAP LS ENGINES INTO CAMAROS & FIREBIRDS: 1967-1981.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

Fuel Tank Removal and Installation

Step 1:

To remove the old gas tank in a 1968 Camaro I first have to remove the fuel filler neck cover to reveal the rubber housing protecting the filler neck. On the 1969, the filler neck is attached to the tank. On the 1968, I used a socket to unbolt the necessary pieces and slid them out of the way.

Step 2:

Use a 5/16-inch socket and ratchet to remove the four outer bolts that anchor the fuel filler neck.

Step 3:

Once the bolts have been removed, the old filler neck is easily lifted from the car.

Step 4:

Remove the screws or fasteners that secure the license plate. Once removed, you have access to the two clamps that hold the gas tank and filler neck together. This is also a good place to access the tan fuel gauge wire.

Step 5:

Before lowering the tank, remove any attachments, such as the ground wire, gauge lead wire, and fuel feed line, which is located right behind the passenger-side rear tire. Typically, a small worm gear clamp is all that holds the fuel tube in place. Be prepared to cut it off if it’s cracked or the rubber has gone bad.

Step 6:

Use a deep-well 9/16-inch socket to remove the two nuts that hold the tank straps in place. Even though the tank had been replaced in the past five years, it was still a difficult job to remove the large and unwieldy gas tank. The nuts and bolts are exposed to the elements so these fasteners typically rust and therefore it’s much more difficult to remove them. Using a thread lubricant to spray the nuts makes the job much easier.

Step 7:

I recommend using a jack and a flat board to support, lower, and remove the tank from the chassis. Raise the jack high enough so the board makes even contact with the bottom of the tank. Remove the nuts from the tank straps and balance the tank on the jack. Slowly lower the jack until the gas tank is close to the floor and can be properly moved. Turns out I had about 3 to 4 gallons of fuel I couldn’t siphon out. It got recycled to lawn mower duty.

Step 8:

Using a flathead screwdriver and a hammer, gently drive the ring off in a counterclockwise direction. The ring holds the sending unit in place and usually takes a few taps for removal.

Step 9:

Carefully place the fuel tank on the floor jack for installation. The tank is a fragile box fabricated of sheet metal so handle it and install it with care. Move the tank into the proper position for installation. Gradually raise the jack and make sure the tank doesn’t bottom out against the trunk. Don’t dent the tank. Essentially everything goes back in the order in which it came out. Don’t worry about the space between the gas tank and the trunk floor; it has a good 1/2 inch of clearance.

Step 10:

Everything is ready for hook-up. The downpipe for the fuel filler neck comes next and the wiring harness quick-disconnect. I used a rubber grommet to protect the wires and slipped the gas tank filler hose over both pieces. Afterward, two large worm-gear clamps were used to prevent leaks and secure the bond between the filler and the gas tank.

Step 11:

Cinching the two 9/16-inch nuts slowly secures the tank to the car and forms the correct bends for the straps (they begin as straight pieces).

Step 12:

Route the rubber shroud over the filler neck and secure it in place with the four machine screws.

Tank Replacement Sources

The following are a few companies that sell stock replacement fuel tanks.

Robb McPerformance

Contrary to some popular hot rod lore, a stock F-Body gas tank can be converted to fuel the LS engine to supply its fuel injection system. You need to install an aftermarket fuel sending unit in the tank, and this process requires a significant amount of work. If you are up for the task, it is a good way to save a few bucks along the way.

You need to install a fuel-sending unit that is entirely different from the stock unit. Many in-tank fuel pumps are compatible with the stock F-Body tank and the LS engine series. Aeromotive offers the in-tank 340 Stealth Fuel Pump and Racetronix offers a kit for LS engines that has a drop-in wiring harness and a Walbro GSS340 fuel pump. For my particular build, I selected a kit from Robb McPerformance (RobbMc) that does the trick.

This is a comparison between the stock sending unit and the RobbMc sending unit that is designed for fuel-injected applications. Beefier is a good adjective.

The RobbMc sending unit has been labeled with R for return and F for feed lines. The -8AN male ends allow for a quick assembly

The RobbMc sending unit is a direct fit for a stock tank. You need to install a new rubber O-ring gasket so the sending unit is sealed well. The red and black wires connect to your fuel level gauge with the red going to the positive and black to the negative side of the gauge.

I spent a few extra bucks and had them add an -8AN option that allowed me to run the -8 line to and from the fuel tank. This is a direct drop-in replacement for the factory unit and gives me an option to run a return line via this sending unit or just cap it and not use it. RobbMc told me that it would be best to run an external surge tank with their system to aid in fuel delivery and stop unwanted fuel starvation under lateral forces. They recommend a 1-gallon upright tank between the fuel tank and the fuel filters.

As far as I know, no one makes a tank for this specific purpose, so you might have to make one to fit your custom application. I have found a few surge tanks on eBay going for roughly $250; the route you wish to take is up to you.

Tanks Inc.

You can also install an internal pump and sending unit in the stock gas tank to complete the task of plumbing a fuel system in your F-Body. The system comes from Tanks Inc. and requires a fair amount of fabrication and fitment, but if you are looking for a low-buck way to go, this might be the ticket.

The system uses an internal pump assembly that needs a large hole cut in the fuel tank to allow the pump to rest inside, as the stock hole isn’t big enough. As of this writing, Tanks Inc. has changed their design from this version; so current units may look slightly different.

At the same time, you reuse the stock sending unit opening. You have to remove the collar that’s welded on from the factory to make this work, but it allows you to run a fuel level sending unit without interfering with the new pump. The important factor to remember is the ohm reading from your gauge, which typically is 0 to 90 ohms for a standard GM gauge.