TBI Fuel Injection Conversion

Four million GM pickup trucks can't be wrong

General Motors introduced TBI—throttle body fuel injection—in the mid-1980s. This story shows how to adapt TBI fuel injection to any small-block Chevy. While fuel injection scares many shade-tree mechanics, it shouldn't. It's not as complex as you think, and the benefits are surely worth the trouble to figure it all out. To prove our point, we recently converted a '48 Ford sedan running a 4-barrel-fed 327 small-block to TBI using stock GM parts and a wiring harness from Painless Performance. A brave new world beckons.

What's TBI?

TBI uses a standard-appearing intake manifold and a "throttle body" with two injectors that squirt fuel into the manifold. Think of it as a carburetor with brains. While tuned-port fuel injection (TPI) gets all the press, TBI delivers the same improved driveability and fuel-economy benefits. TBI systems have been standard equipment on millions of GM pickups and sport-utility vehicles since 1987; the components are proven and bulletproof. TBI also emphasizes low-end torque—good news for any old-truck owner or hot rodder. And if you convert from a carb to TBI, all existing bracketry for the alternator and A/C compressor can be retained.

How EFI Works

Electronic fuel injection (EFI) is simple in concept: Sensors send information on various engine functions—vacuum, throttle position, exhaust, vehicle speed, coolant temperature—to a computer. The computer kicks all this data around its circuit board for a millisecond or so, then tells the fuel injectors and distributor how much fuel and timing to deliver. The result is an engine that receives exactly what it needs when it needs it, thus performing as efficiently as possible.

What's TPI?

These days, quite a few small-block Chevy-powered conversions run GM tuned-port (TPI) fuel injection, yanked off a Camaro or Corvette. TPI uses an individual fuel injector for each cylinder and a swoopy air plenum and runner. TPI is a logical, costly option for rodders seeking high-rpm power and an engine bay that looks like the business end of the space shuttle.

Conversion List

A TBI fuel injection system requires the following components: A TBI throttle body and intake manifold, a computer (a.k.a. ECM; electronic control module), a GM electronically controlled distributor, an electric fuel pump, a fuel filter, one pressure-safe post-pump filter, and a fuel return line to tank. Also required is a MAP (manifold absolute pressure) sensor, an electric spark control module, fuel pump relay coolant, a temperature sensor, a knock sensor, one oxygen sensor, an oil pressure switch, a vehicle speed sensor, and wiring harness.

While TBI components can be found at wrecking yards and swap meets, we opted for a ready-to-go package from Arizona TPI Specialists, which specializes in GM fuel injection systems. Arizona TPI and Painless Wiring recommend TBI components from a 1987-90 GMC/Chevrolet pickup or Suburban. In fact, the Painless Wiring harness will only work with computers from these vehicles.

Converting

The conversion involves three basic steps: swap the intake manifolds and distributors, mount the fuel pump and fuel lines, and route and connect the wiring harness and computer.

It's not as complex as you think, and the benefits are surely worth the trouble to figure it out. Switching manifolds and distributors is straightforward. One hiccup was finding a place for the water-temp sending unit—which is NOT to be confused with the coolant-temp sensor for the ECM. TBI manifolds use a threaded opening at the right rear of the manifold for the heater-hose outlet, and place the coolant sensor up front near the thermostat housing. Where does the water-temp sender go? On late-model heads, it can go in the head itself. However, since our 327 runs early "fuelie" heads, we moved the gauge sender to the heater hose opening and used a nifty thermostat-housing riser from Vintage Air to connect the heater hose.

Next up: fuel pump, filters, and fuel lines. On our '48, we decided to make life easy by mounting the fuel pump and filters outside the stock tank. (On GM cars, the pump is inside the tank.) Because TBI systems run at a low pressure (between 9-12 psi, compared to 45 psi for TPI systems), a soft return line is sufficient. We ran ours down the right-side frame rail, lashing it to the hard fuel-feed line. The pump itself is a high-speed vane-type pump that spins at 3,500 rpm.

While the TBI uses low fuel pressure, we wanted the fuel delivery system to be completely safe and leak-proof. The hose pros at Oil Filter Service in Portland, Oregon recommended Aeroquip's super-slick Versil-Flare flareless tube fittings and Aeroquip AQP Socketless hose and fittings. This combination looks trick and will never leak. Still, standard worm-gear hose clamps are adequate for TBI systems—but make sure to use high-pressure fuel-injection line. (Note: when you get your throttle body, make sure you get the fuel line fittings and a couple of inches of tubing. The fittings are a metric, double-bump flare—tough to find later.)

The next step was running the return line into the tank. The return tube should be within one inch of the tank bottom, ensuring that the tube opening is always submersed under the fuel level. Here, we contacted Classic Instruments, who supplied a fuel gauge sending-unit that uses a return tube as the float mechanism's mounting stanchion. Very slick. It simply slips into the stock tank-mounting hole.

Wiring Harness

With the fuel pump, filters, and lines in place, next is the wiring. We spread out the Painless Performance harness to make sense of the wires and oddball connectors that sprouted everywhere. Like all Painless Wiring harnesses, all connectors and sections are identified by small tags. Do not remove any tags until you are completely finished. Study the harness. Read the directions. Read them again; and once again. Get a sense of how it all works before you connect or snip any wires.

We mounted the ECM to the firewall with Velcro, which actually makes a very secure attachment and helps isolate the computer from vibrations. The harness routing is contingent upon where the computer is placed. We mounted ours under the dash between the A/C evaporator and the fuse panel. Behind a kick panel is another logical spot. If you're totally out of space, Painless Wiring makes an extra-long harness for placing the computer under the seat. We mounted the ECM to the firewall with Velcro, which actually makes a very secure attachment and helps isolate the computer from vibrations.

Next, we drilled a 1 5/8-inch hole in the firewall to slip the harness into the engine compartment. This is easily the most bewildering moment. There are seemingly a zillion connectors. We carefully fished them through the hole and arranged them in their general positions. All connectors are unique to the sensor they feed. You can't connect them wrong. Making it all look neat, though, is a time-consuming process. So, take your time.

Getting Sensory

Three sensors require special attention. The MAP sensor connects to the manifold vacuum, informing the ECM about engine load. It must be mounted higher than the vacuum source with the hose connector pointing down. Painless Performance places the MAP sensor inside the car, for a clean look. Arizona TPI recommends mounting the MAP sensor within 12 inches of the vacuum source. Try it inside first. If the ECM shows a MAP sensor error, you may have to move it closer.

The vehicle speed sensor tells the computer the relative speed your car is traveling. It screws into the speedometer cable outlet on the transmission. On our '48, the X-member opening was too tight for the sensor. We used a 4-inch mini-speedo cable to move the sensor out of the X-member.

The oxygen sensor monitors exhaust gas content. It goes into the exhaust system within 12 inches of an exhaust port. We drove our Ford (carburetor and all) to a local muffler shop, which welded the threaded boss into the downpipe just below the exhaust manifold mounting flange. By doing this first, we avoided unbolting any part of the exhaust system.

The ALDL

With all engine compartment sensors and wires hooked up, we turned our attention under the dash. We found homes for the fuel-pump relay, fuses, ESC (electronic spark control), and Assembly Line Diagnostic Link (ALDL) connector. This 12-pin connector is used to retrieve diagnostic error codes that are stored in the computer. Painless mounts the ALDL on a small bracket with a small Check Engine light. This light comes on when the ignition is turned on and goes out once the engine fires—unless there's a problem. If there is, it stays illuminated, announcing the problem. If the problem is momentary, the light will go out, but the ECM will remember the error. To find out what happened, you jump two terminals on the ALDL, which causes the check-engine light to flash error codes. Then you simply fix the problem. Try that with a carburetor.

Ready To Fire

Finally, everything was connected, plumbed and mounted. We then primed the pump by loosening the fuel line until gas flowed out. We re-tightened the clamp. Next, we turned the key one click. The Check Engine light came on and the fuel pump cycled for two seconds. We engaged the starter, and the engine spun around about six seconds or so, fired right up and idled.

On the road, the car exhibited crisp, flawless throttle response—no hesitations off the line, no cold-start stumble. In fact, to start the engine on a cold morning requires simply turning the key. The engine fires immediately and idles smoothly. On the road, fuel economy is up and driveability is improved. And re-jetting for changes in altitude is a thing of the past; the computer automatically adjusts fuel mixture.

Research

To describe fully all the nuances and technical background of fuel injection would take a book. And, in fact, there are several tomes that do just that. All have comprehensive overviews of GM fuel injection basics plus extensive troubleshooting techniques. They're available from Motorbooks International (800-826-6600):

"Chevrolet TPI Swappers Guide" by John Baechtel, "How to Tune & Modify Chevrolet Fuel Injection" by Ben Watson, "Tuned Port Fuel Injection" by Choco Munday, "Chevrolet TBI & TPI Engine Swapping" by Mike Knell, "Hot Rod Wiring" by Tim Remus and Dennis Olverholser.

Electronic fuel injection is not every rodder's preferred induction system. But if improved gas mileage, driveability, easy troubleshooting, and start-every-time convenience appeals to you, consider a Chevy TBI system.

Resources

Arizona TPI, 717 E. Hacienda Drive, #S105, Tempe, AZ 85281, 602-921-2500

Classic Instruments, P.O. Box 1216, Crooked River Ranch, OR 97760, 541-548-1940

Lokar Performance Products, 10924 Murdock Drive, Knoxville, TN 37922, 423-966-2269, www.lokar.com

Oil Filter Service, 615 SE Market St., Portland, OR 97214, 800-628-9332

Painless Performance, 9505 Santa Paula Drive, Fort Worth, TX 76116, 800-423-9696, www.painlessperformance.com