| KEY TAKEAWAYS |
|---|
| • Fuel injection runs at roughly ten times the pressure of a carburetor. Most self-learning throttle body EFI systems need around 58 to 60 psi, which rules out a carb-era mechanical pump and low-pressure hose. |
| • In-tank pumps run quieter and cooler than external pumps and are the preferred setup for EFI. External pumps must be mounted low and gravity-fed. |
| • Use PTFE-lined hose for EFI fuel lines. It resists ethanol and stops the fuel smell that rubber-lined braided hose can leave in a garage. |
| • Filter twice: a coarse pre-pump filter protects the pump and a fine post-pump filter protects the injectors. |
| • Hot Rod Hardware stocks FiTech fuel injection, fuel pumps, EFI-ready tanks, filters, and AN hoses and fittings. |
Bolting on a throttle body EFI system is the easy part of the conversion. The fuel system behind it is where most EFI problems start. An EFI fuel line kit, pump, regulator, and filters have to deliver steady high pressure without starving, heating, or leaking, and the low-pressure parts that fed your carburetor will not do that job. This guide goes deeper than general fuel system advice and focuses on what fuel injection specifically requires: pressure, pump placement, return versus returnless plumbing, hose and fitting selection, filtration, and tank pickup.
Why EFI Changes the Fuel System
A carburetor typically wants around 5 to 7 psi. Most self-learning throttle body EFI systems, including FiTech units, run around 58 to 60 psi, and port-injected swaps such as LS engines run in a similar range. Everything between the tank and the throttle body has to be rated for that pressure: pump, hose, fittings, filters, and regulator.
EFI also wants steady pressure under every condition: hard acceleration, long corners, and a quarter tank of fuel on a hot day. A carburetor's float bowl hides brief interruptions in supply. An injector does not, and the engine leans out the moment pressure drops.
If you are still on a carburetor or want the fundamentals of tanks, lines, and venting, start with the Classic Car Fuel System Build Guide. This article picks up where that one stops.
Choosing an EFI Fuel Pump
In-tank vs. external
In-tank pumps sit submerged in fuel, which cools them and keeps them quiet. They are the preferred EFI setup. Many builders use an EFI-ready fuel tank with a built-in pump mount and baffled pickup; Tanks Inc. builds tanks for many classic applications.
External inline pumps work when an in-tank setup is impractical. Pumps push fuel far better than they pull it, so mount an external pump low, close to the tank, and below the tank outlet so fuel gravity-feeds into it. Keep it away from exhaust heat.
Sizing the pump
Size the pump for your engine's horsepower at EFI pressure, not at free flow. Pump flow ratings drop as pressure rises, so check the flow chart at your system's operating pressure. A 255 lph class pump is a common choice for naturally aspirated street engines up to roughly 500 horsepower. Boosted engines need more. Browse fuel pumps by rated flow and pressure.
Keeping your existing tank
If a new tank is not in the budget, a fuel surge tank or command-center style module solves the starvation problem. A low-pressure pump fills a small reservoir near the tank, and a high-pressure EFI pump draws from that reservoir. FiTech's Fuel Command Center works this way. When you delete a mechanical pump, cap the boss with a fuel pump block-off plate.
Return-Style vs. Returnless Plumbing
A return-style system runs a feed line to the engine, a regulator near the throttle body, and a return line back to the tank. Excess fuel circulates continuously, which keeps fuel cool and pressure stable. It takes two lines and is the most common choice for throttle body EFI.
A returnless system places the regulator at or near the tank and runs a single feed line to the engine. It is simpler to plumb and common on LS swaps that use a filter-regulator unit. The tradeoff is fuel temperature control and fewer tuning options.
| Factor | Return-Style | Returnless |
|---|---|---|
| Lines | Feed plus return | Feed only |
| Regulator location | Near the engine | At or near the tank |
| Fuel temperature | Cooler, fuel circulates | Warmer in hot weather |
| Common use | Throttle body EFI | LS swaps, factory-style setups |
Follow your EFI manufacturer's recommendation. Some systems are designed around one approach.
Fuel Line Kits: Hose, Hard Line, and Sizing
Hose types
PTFE-lined braided hose is the standard for EFI. It handles high pressure, resists ethanol blends including E85, and does not let fuel vapor permeate through the hose wall. Rubber-lined braided hose works but can leave a fuel smell in a closed garage over time. Basic rubber fuel hose and worm-gear clamps are not designed for EFI pressure.
An AN fuel line kit bundles hose with matching fittings, which saves sorting part numbers one at a time. Earl's Performance Plumbing is a common source for PTFE hose and AN ends.
Line size
For most street EFI builds, a -6 AN (3/8-inch) feed and -6 AN return cover the job. Higher-horsepower or boosted builds may step the feed up to -8 AN. Undersizing the feed causes pressure to drop at high load, exactly when the engine needs fuel most.
Hard line plus flex
Most builders run hard line along the frame and use short sections of braided hose where the line meets the tank and the engine, the two places that move. Stainless or nickel-copper hard line resists corrosion. Secure lines with cushioned clamps, keep them away from exhaust, and plan routing around the exhaust system; the Custom Exhaust Kit Builder's Guide covers clearance from the other side.
Fittings: AN, NPT, and O-Ring Boss
AN fittings seal with a 37-degree flare, metal to metal. Do not put thread sealant or tape on an AN flare. NPT pipe threads seal on the threads and do need sealant rated for fuel. O-ring boss (ORB) ports seal with an o-ring and are common on pumps, regulators, and filters.
Most EFI systems mix all three, which means adapters at the pump, filter, regulator, and throttle body. List every port and its thread type before you order, then buy the adapters with the hose. The hoses and fittings collection covers hose ends, adapters, and unions.
Filtration and Tank Pickup
EFI uses two filters. A coarse pre-pump filter, often around 100 microns, protects the pump from debris without restricting its inlet. A fine post-pump filter, often around 10 microns, protects the injectors. Never put a fine filter on the pump inlet, since the restriction can cavitate and overheat the pump. See fuel filters for both types.
Many classic tanks have a simple pickup tube with no baffling. Under hard cornering or braking with low fuel, fuel sloshes away from the pickup, and the EFI loses pressure. Baffled tanks, sumps, in-tank pump modules, and surge tanks all solve this. Pick one before you finalize the rest of the plumbing.
Wiring the Pump
An EFI pump draws enough current that it should run through a relay with properly sized power and ground wires, triggered by the EFI controller so the pump shuts off if the engine stalls. Voltage drop to the pump reduces its output, so short, heavy wiring pays off. For the chassis side of that job, see universal wiring kits. LS swap builders should also read The LS Swap Revolution for how fuel and wiring fit into the larger swap plan.
FAQ
What fuel pressure does throttle body EFI need?
Most self-learning throttle body EFI systems run around 58 to 60 psi. Check your specific system's documentation, since requirements vary by manufacturer.
What size fuel line do I need for EFI?
A -6 AN (3/8-inch) feed line with a -6 AN return covers most naturally aspirated street EFI builds. Higher-horsepower and boosted builds often step the feed up to -8 AN.
Can I use my mechanical fuel pump with EFI?
Not as the main supply. Mechanical pumps produce carburetor-level pressure, far below what EFI needs. A mechanical or low-pressure pump can feed a surge tank, with a high-pressure EFI pump drawing from the surge tank.
Do I need a return line for EFI?
Not always. Return-style systems are the most common choice for throttle body EFI and keep fuel cooler. Returnless systems use a regulator at the tank and a single feed line. Follow your EFI manufacturer's recommendation.
What is the best fuel line kit for EFI?
Look for a PTFE-lined braided hose kit with AN fittings, rated for EFI pressure and compatible with ethanol fuels. PTFE resists E85 and prevents the fuel odor that rubber-lined hose can cause.
Does Hot Rod Hardware sell EFI fuel system components?
Yes. Hot Rod Hardware stocks FiTech fuel injection, fuel pumps, EFI-ready tanks, filters, and AN hoses and fittings. Call 507-527-1020 for help planning your system or browse the fuel and air catalog online.
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