Easy EFI, Part 1

Piecing Together The TPI Swap Puzzle

Scott Crouse Jan 1, 2002 0 Comment(s)

Step By Step

A TPI system consists of four major induction parts: base (a), pair of runners (b), plenum (c), throttle body (d).

The ECM on the right is an EPROM unit used in TPI-equipped cars with speed density and mass air. The computer on the left uses an EEPROM for ’93- and-later Camaros.

This is a mass airflow (MAF) sensor (left) and a manifold absolute pressure sensor (right). No MAF sensor in the intake tract indicates a speed density system, but be aware that MAF systems employ a MAP sensor along with the MAF sensor.

Notice how this speed density system’s intake ducting has to snake around the alternator. A mass airflow sensor would have to fit within this ducting.

We strongly recommend using an aftermarket wiring harness to lessen the confusion of installation. The connectors are match fitted with the EFI components so you cannot plug them into the wrong places.

These are two of the 02 sensors available. The sensor on top uses a single wire and is for applications where the sensor is within a few inches of the exhaust manifold. The heated three-wire sensor (bottom) can be used farther from the exhaust port.

Be aware that as engine airflow increases, additional fuel must be supplied. TPI 305 and 350 injectors come from the factory with their flow capabilities nearly at their maximum. If you plan on modifying your engine, be prepared to swap in some larger injectors.

The fuel regulator on a TPI system is located in the middle of the fuel-injector log (arrow).

Electronic fuel injection (EFI) is one of the great innovations for the internal combustion engine. Since the ’80s, new cars have benefited from improved fuel mileage, throttle response, and all-around driveability. When these systems first appeared, hot rodders questioned their durability and complexity. Yes, fuel injection is more complex and expensive than a simple carburetor, but EFI also provides numerous benefits. EFI uses several sensors that gather information and send it to a central unit (computer) where the information is processed and sent to work controlling the fuel and spark. When everything works properly, the engine runs very efficiently.

The EFI swap territory is incredibly complex, so we’ll cover this story in two parts. Our goal is to show what it takes to adapt a fuel injection system to a carbureted vehicle. We have chosen to use a Tuned Port Injection (TPI) EFI system because of its simple design, low cost, and availability on ’85 through ’92 305ci and 350ci Camaros, Corvettes, and Firebirds. We’ll focus on a mild engine combination using a factory TPI system while other conversions will include aftermarket EFI control systems that enable hot rodders to modify engines with fuel injection for more power.

The TPI manifold system is made up of four major parts. There is a throttle body, a plenum, a pair of intake runners, and a manifold base. In this system, air enters through the throttle body and goes into the upper plenum. From there, the air is distributed by the runners to the base intake where fuel is injected, supplying each intake port with the proper air/fuel mixture. This system is known as a dry-flow–style since fuel enters the intake system at the last stage.

The electronic control modules (ECM, or computer) is the brain of the entire fuel injection system. No matter what type of EFI system your car may have, there is always an ECM. GM used two types of ECMs in TPI- and LT1-equipped cars. Early computers used an electronic programmable read-only memory (EPROM) installed in all TPI cars using either speed density or a mass airflow design.

These units include a computer chip that holds a specific computer matrix, which plugs in the back of the ECM. When an engine has been modified, this chip must be recalibrated for the new modifications. In ’93, the LT1 cars were introduced with electronically erasable programmable read-only memory (EEPROM) computers, which do not have a removable chip and require that the entire computer be recalibrated if the engine is modified.

Mass airflow (MAF) and speed density are the two different types of EFI systems available. The MAF system requires that a MAF sensor be placed in between the air filter and the throttle body. This meter measures the amount of air the engine ingests and relays that information to the ECM, ensuring that the correct amount of fuel and spark timing is present in the cylinder. A MAF system is more likely to accept engine modifications without recalibrating the computer because its computer matrix design is more adaptable and can measure the amount of air entering the engine. These systems work well but the MAF sensor makes things more cumbersome and expensive.

A speed density system uses a manifold absolute pressure (MAP) sensor to measure engine load based on the amount of vacuum present. Once the engine load has been measured, the computer determines the correct fuel and spark delivery based on a pre-determined amount of airflow. A speed density system is more compact than the MAF system but its ECM is sensitive to engine modification and requires recalibration when the engine is modified for more power.

When performing an EFI swap, there are several things to consider. If you use a MAF system, the engine compartment can become cluttered and engine brackets and accessories may interfere with the intake ducting. Switching to TPI Corvette or Camaro brackets isn’t always feasible, as additional bosses in older cylinder heads may not be present. In this case your best option is to customize the intake tract tubing. Another problem facing these engines is differing manifold bolt patterns. The ’87-and-newer Camaro TPI intake manifolds used on iron-head 305s and 350s use a different intake bolt pattern. The four inner bolt holes will not properly line up with the typical small-block head. You will have to elongate the intake holes to match the cylinder head or use heads with the ’87-and-later intake bolt pattern. The Corvette TPI intake for aluminum heads did not make this change, and these intakes will work on the common small-block intake pattern.

Connecting the ECM to all of these sensors leaves you with two options. You can use a factory wiring harness or purchase an aftermarket one from companies such as Painless Wiring, Holley, Street & Performance, Howell Engineering, and EFI Wiring. When selecting the correct harness for your combination, you must be aware of the components in your system. You must determine whether you are using an early or late MAF system, a speed density system, or an LT1 application. It is also important to know the type of transmission you will be using. If you’re going to use the factory harness we recommend gathering all the pieces from the same car to lessen confusion. An aftermarket harness typically comes with four wires to be wired into the electrical system and the rest are connectors that will only mate to the proper EFI fittings. We would recommend this route as it will make it easier.

TPI systems require the use of a computer-controlled HEI distributor that appeared in two different forms. Small-cap distributors with external coils offer firewall clearance while large-cap distributors with internal coils are the most common HEI units available. Another area to consider is camshaft and distributor gears.

Here is a quick rundown of the needed sensors within a TPI system. Both speed density and MAF systems employ a MAP sensor. All EFI systems also use an inlet air-temperature sensor, coolant-temperature sensor, throttle-position sensor, an 02 sensor located in the exhaust system, and possibly a knock sensor in the block. Later EFI systems also monitor vehicle speed from the transmission.

Because fuel is injected in an EFI system, the fuel must remain under constant pressure. An EFI fuel system is a high-pressure circular system that flows fuel from the tank, through the pump and filter, to a regulator, then to the injectors. Excess fuel is bypassed and returned to the fuel tank. Injector size varies depending on the demands of the engine. For example, a 305ci engine comes with a lower-rated pound-per-hour injector than the 350ci engine. When you’re ready to plumb these lines be sure the fuel system you use is equipped with high-pressure lines such as steel or AN-braided. Also pay attention to the fitting sizes as the fuel rails often use different size Saginaw fittings (5/16 and 3/8).

An LT1 and LT4 engine/EFI swap from a ’93 to ’96 Camaro or Corvette can be the way to go if you are looking for more factory horsepower. These EFI systems are entirely different from the earlier TPI systems but function according to the same principals. The EFI system for the LT1-powered Camaro uses a one piece intake manifold design with a front-mounted distributor and an EEPROM computer. This EFI system is responsible for the fourth generation’s sequential injector firing pattern compared to the TPI system’s batch-firing system. Within the LT1 and LT4 swap there are many other small but important basics to include with this system, so do your homework first.

The best piece of advice we can provide is to buy a book on EFI conversions. There are so many different injection systems and variables involved with each one, that the information is extensive. Tuned Port Induction Specialties (TPIS) offers two books for under $20 each on TPI and LT1/LT4 engine/EFI swaps. Spending a few dollars here can save you days of hassle and confusion in the long run. There is no reason to fear the world of fuel injection as thousands of people before you have ventured into this realm and returned with stories of broadened torque curves, better throttle response, improved fuel mileage, and lower emissions.

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