Understanding The LT Direct Injection Fuel System for Swapping (2023)

| How-To - Engine and Drivetrain

Jefferson BryantWriter, Photographer

The Gen-V LT-series engines are a little bit different from the LS series, particularly in the fuel system department. All LT-series engines are direct-injection, meaning the fuel is pressurized between 2,000 and 2,900psi (2,175 for LT1, 2,900 for LT4), and injected directly into the combustion chamber, much like a diesel engine. DI engines have much greater fuel economy because the ECM has far better control of how much fuel is being burned.

(Video) Gen V LT Direct Injection Horsepower Secret: Tech Tip Tuesday

To supply fuel to the high-pressure direct injection system, there are two fuel pumps, an electric supply pump in the tank and a mechanical pressure pump underneath the intake. The mechanical pressure pump runs off a tri-lobe wing on the camshaft. Aftermarket upgrades to bolster fuel flow can be made through the camshaft. Comp Cams offers cams with extra and different shaped fuel pump lobes which can increase fuel flow as much as 74 percent. The electric supply pump in the gas tank is different from a standard electric pump as well.

Instead of a basic fuel pump and regulator, the factory supply pump is controlled by the ECM through a fuel pump module to control the base fuel pressure as it reaches the mechanical DI fuel pump. A special pressure sensor in the fuel line monitors the pressure of the fuel, which is maintained at 72 PSI at 45 GPH. Rather than use a regulator, the pressure is managed through Pulse Width Modulation or "PWM" control of the pump. Essentially, the ECM switches the voltage and current sent to the pump on and off at a very fast rate to control the speed of the pump, ensuring full pressure at all times with no delays. With a typical electric fuel pump, when you romp on the throttle, there is quick surge in fuel flow followed by a lull as the pump recovers from the sudden loss in pressure. If plumbed properly, a port-injected engine won't see too much performance drop from this scenario. In a DI engine, this would cause a serious issue as the mechanical pump needs full pressure at all times to maintain the 2,000-plus PSI needed to correctly operate. This complicates the fuel system for the LT-series engine swap. You can't use just any old fuel pump. DI fuel pumps have to be PWM capable, and not all electric fuel pumps are.

Another factor is that the LT fuel system is returnless; this was done to keep the fuel temperature down. Because hot fuel does not cycle through the pump, to the engine, and back to the tank, the fuel temp remains even. Returnless fuel pumps are rarely suitable for EFI use without PWM control, and the ones that are available cannot support the type of pressure and flow needed for an LT. The pump requirements are 72 PSI at 45 GPH (Gallons Per Hour) and an 84 PSI pressure relief to be compatible with the Chevrolet Performance control system, which is a pretty high burden for a street-driven electric fuel pump. So where does that leave us for LT swaps?

(Video) Gen V LT Swap Fuel System

There are two options for retrofitting LT DI-compatible fuel systems: a PWM control system or pump/regulator/return line system. Many swappers already have an existing electronic fuel system in their vehicle. Swapping an LT into that scenario could mean buying new parts, but all is not lost. It is possible to bypass the PWM-style pump system by using a standard electric pump with a return line and regulator set to 72 PSI at 45 GPH. That is fairly high for a continuous-use street pump, so make sure your pump is capable of providing the required flow and pressure. The duty cycle of that pressure and flow rate will certainly be high and the lifespan of the pump will be quite low. 72 PSI is high for a street fuel system, and so is the flow rate of 45 GPH. A typical retro-fit EFI inline fuel pump can flow 42-45 GPH, but only at 15 PSI. This means you need a much larger fuel pump to supply the engine with enough fuel to operate. You will suffer from hard starting along with drivability and hot fuel issues. Switching to the PWM system gives you greater control.

Installing the PWM controller is simple plug-and-play, but the fuel pressure sensor and fuel pump wiring is a bit tricky. First, you need an inline adapter with a pressure sensor port positioned between 5- and 85-degrees to the horizontal flow of fuel, according to the GM manual for the fuel controller. This is fairly easy, as there are plenty of these fuel sensor adapters out there. The problem is that most of the adapters are for 1/8" npt fittings, and not the 10mm threads required for the GM sensor. Finding a 1/8" npt male to 10mm male adapter is difficult. At this point, there is no commercially available sensor adapter for the LT-series pressure sensor. What you can find easily is a -6 AN male to 10mm adapter. To use this, you need an aluminum fuel log or Y-block fuel splitter and a -6 to 10mm male-male adapter. This allows you to connect the sensor into the fuel system. We made one with a leftover piece of fuel rail from another project.

(Video) LT Swap fuel system

Wiring the pump itself takes a little patience. There are three wires coming off the pump module, yellow with a black stripe, gray, and a smaller gauge black wire. Unlike a typical installation, the power wires are only 14 gauge. PWM control also eliminates the need for power relays. The yellow/black wire is the ground, the gray wire is the power side, and the small black wire is the shield. If you are using a GM pump with a shield pin (such as a 2014 Corvette or truck), connect the small black wire to that pin, if you are using a pump without a shield pin, leave the wire un-terminated and tape it to the other wires. Because of the nature of PWM control, there is a very real potential for EMI (Electro Magnetic Interference) from other electronics in the car. To eliminate this from interrupting the control signal, the two main power control wires are twisted with a 3rd shielding wire. The Chevrolet Performance wiring harness only comes with a short length of wire; most cars require longer wires. To maintain the shielding, you have to twist the wires at a minimum of 27 twists per 8 foot of wire. The best way to ensure that the wires are correctly twisted and won't unravel is a braid. Braid the three wires together in a loose, consistent manner, it does not need to be a tight braid, wrapping the wires around each other every 2 inches or so. Do not use crimp connectors for these wires, make sure you solder them well and use shrink tubing.

For our install, we used an Aeromotive Phantom 340 in-tank pump kit, wired to the Chevrolet Performance ECM and fuel control module in a 1971 Buick GS with a new 6.2-liter LT1 crate engine. The fuel pump was installed into the factory fuel tank, and wired directly to the GM fuel control module. The wires were extended roughly 10 feet and wrapped with Painless Performance Classic Braid.

The hardest part of the installation was the pressure sensor; the rest is fairly simple, once you understand what it takes to make the system work. The sensor needs to mount as far from the engine as possible, in our case, this was at the transmission crossmember. The harness has a short lead, so this is as far as you can get it without extending the harness. As this is similar to the C7 Corvette harness, we left it.

(Video) How to Locate and Identify for Your Gen V Fuel Rail Pressure Sensor for an LT Swap, by PSI LT1 L83

Installing the proper fuel system to feed an LT-series engine gives you the best fuel control possible. With PWM, the fuel pressure is nearly perfect at all times, reducing spikes and lag in your engine's performance. It may take a little bit of time to sort out the details for your install, but in the end, the results are worth the effort.

Sources:

Aeromotive Inc.
www.aeromotiveinc.com

Chevrolet Performance Parts
www.chevroletperformance.com

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FAQs

How does a direct injection fuel system work? ›

Engines with gasoline direct injection produce the air-fuel mixture directly in the combustion chamber. Only fresh air flows into the intake port through the open intake valve. The fuel is injected directly into the combustion chamber by high-pressure injectors.

Is the LT1 direct injection? ›

All LT-series engines are direct-injection, meaning the fuel is pressurized between 2,000 and 2,900psi (2,175 for LT1, 2,900 for LT4), and injected directly into the combustion chamber, much like a diesel engine.

How much fuel pressure does a direct injection need? ›

With the fuel being directly sprayed in the combustion chamber area, it allows for more power and better fuel economy. Normally fuel injectors need about 46 to 65 psi of fuel pressure to make them operate. The GDI fuel injectors will require upwards of 2000 psi.

What are the three modes of operation in GDI system? ›

The engine management system continually chooses among three combustion modes: ultra lean burn, stoichiometric, and full power output. Each mode is characterized by the air-fuel ratio.

What are the disadvantages of direct fuel injection? ›

The biggest drawback of direct fuel injection is DI can lead to clogged fuel systems and engine carbon fill up.

Is direct injection more powerful? ›

Due to its fine atomization of the air-fuel mixture, direct injection produces more power than other systems because it allows for greater control over ignition timing and air/fuel ratio. It is more efficient and requires less maintenance to use direct injection engines rather than port fuel injection engines.

What size are stock LT1 injectors? ›

Re: LT1 fuel injector size? There are fast LT1 Stockers running 24 lbs and 30 lb injectors. Personally I would use the 24lbs at 60 psi. Higher pressure atomizes the fuel better.

What is good oil pressure for a LT1? ›

factory oil pressure specs are 22psi at 1000rpm, 30psi at 2,000 rpm, and 33psi at 3,000 rpm. So yes, that will be normal. The relief valve (should) stop oil pressure from exceeding between 55 and 75 psi.

Can high fuel pressure damage injectors? ›

No permanent damage as long as the pressure doesn't remain high for long. The valves must be fully closing. Even a few thousandths of separation between valve and seat would result in very little to no compression at all, the engine would spin freely when cranking and wouldn't start.

How long should a fuel injection system hold pressure? ›

The basic leak-down test

For the specification we must consult service-data, but the system should hold some pressure for about five-minutes.

What is the correct fuel pressure? ›

It is between 35 and 65 pounds per square inch (psi) on most vehicles.

What is the meaning of direct injection? ›

Direct injection is a diesel engine injection system in which the fuel is injected directly into the engine cylinder. Direct injection diesel engines will often start from cold with relative ease. With direct injection, the fuel enters the cylinder late in the compression stroke.

What is the best type of fuel injection? ›

The sequential fuel injection system eliminates the only disadvantage of MPFI and is the most widely used fuel injection system today. In a sequential fuel injection system, the fuel injectors function with respect to the cylinders they are connected to.

What is direct injection vs fuel injection? ›

The difference is how they atomize the fuel: direct injection uses very high pressure and is sprayed directly into the spark plug area to ignite. Port fuel injection uses heat from the valves to atomize the fuel before being ingested into the cylinder when the valve opens,” Kociba said.

What is the advantage of direct fuel injection? ›

Direct fuel injection gives greater fuel efficiency because of a higher level of precision over the amount of fuel injected into the cylinder, the timing of the injection and the spray pattern. This precision also gives the engine greater power, which allows for a smaller engine.

Is direct injection better than carburetor? ›

Fuel injection is fast overtaking carburettor systems. Almost every new vehicle today comes with fuel injection because of the advantages of higher power, better fuel economy and easier maintenance.

Does direct injection use more fuel? ›

Direct injection improves combustion efficiency, increases fuel economy and lowers emissions. Both systems use electronic fuel injectors to spray fuel into the engine, but the difference is where they spray the fuel. With port injection systems, fuel is sprayed into the intake ports.

Does direct injection increase horsepower? ›

The technical answer is yes; fuel injectors can add 10 extra horsepower at peak.

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