Hot Rod Handbooks: Tuned Port Fuel Injection

Adding Chevrolets Tuned Port (Electronic) Fuel Injection system to the Small Block Chevrolet used in your Street Rod, Street Machine, Kit Car or daily driver will improve its driveability, performance, and fuel efficiency, in addition to giving the engine compartment a clean, "High-Tech" look. A substantial number of these systems have been used in Jaguar V-8 conversions since the small block chev gained prominence, and the TPI from 1986 is gauranteed to produce excellent results. This book is a compilation of a vast store of information, gathered over a period of time in an effort to give you a basic introduction to the Tuned Port Electronic Fuel Injection system in general and, specifically, how to incorporate this system into your Hot Rod, or for that matter, any other vehicle powered by a small block Chevrolet Engine.

Chevrolets Tuned Port Fuel Injection System, or TPI as it is more commonly referred to, first appeared on production vehicles in 1985 and, since then, has been used on all Corvettes and on many specially equipped Camaros and Firebirds. GM advertising claims of up to a 20% improvement in torque, horsepower and mileage, over equivalent carbureted engines have proven conservative, if road and dyno tests caried out by the various automotive enthusiast magazines are to be relied upon. As the following table illustrates, the horsepower and torque ratings of the basic 350 engine used in GM cars since 1980 have increased substantially, with only minor changes other than the fuel injection system. While a portion of the power improvements can be attributed to increases in compression ratio and decreases in engine friction, especially in the later engines with roller tappet camshafts, the majority is attributable to the overall efficiency of the Computer Controlled Tuned Port Electronic Fuel Injection System.

Table 1 - 350 Chev HP Readings
1980- 1981
190 hp @4400 rpm
280 Ib/ft @2400 rpm
8.2 to 1
4-barrel carburetted.
205 hp @4200 rpm
290 Ib/ft @2800 rpm
9.0 to 1
Cross-fire twin throttle body injection.
1986- 1987 (centre bolt valve covers)
230 hp @4000 rpm
320 Ib/ft @3200 rpm
9.5 to 1
TPI w/aluminium heads
245 hp @4300 rpm
340 Ib/ft @3200 rpm
9.5 to 1
TPI w/aluminium heads and roller lifter camshaft. Serpentine belt system.

Fuel consumption figures are harder to compare because of the effects of different rear end ratios, transmission types, tire sizes, vehicle weights, etc. However, after examining the evidence from owners of V-8 powered Hot Rods utilising 350 TPI engines and TH-700-R4 overdrive transmissions, we have found these cars can consistently produce mileage figures 3 to 5 mpg better than similarly equipped (ie, same transmission and rear axle ratio), carburetted, 350 cu. in. V-8 cars.

About Fuel Injection

The GM TPI system incorporates twin forward-mounted air cleaners passing air from the grille opening back through a Y-shaped duct to the Mass Airflow sensor (MAF) positioned upstream from the cast-aluminium throttle body assembly (FIGURE 2.  Air Flow). The throttle body has dual internal throttle blades with a cast-aluminium plenum and eight individual tuned aluminium intake runners connected to a criss-crossed intake manifold baseplate which holds the fuel injectors (FIGURE 1.  Fuel Flow). Fuel is distributed to the injectors from a dual fuel rail assembly.

FIGURE 1. Fuel Flow

Runner length is tailored to achieve inlet system resonance for optimum throttle response in the normal range of engine speeds. It is designed to take advantage of the air pulses set up by the opening and closing of the intake valves. The tuned runner length results in a higher density air charge timed to arrive as the valve opens and optimise cylinder charging. All eight injectors fire simultaneously during each crankshaft revolution. During the first of two revolutions that complete a firing sequence, fuel is injected at the base of the intake valve while it is still closed. The second injection is sprayed into the airstream entering the chamber while the valve is open. Fuel from the first injection is already vaporised; this contributes greater fuel atomisation to optimise efficiency. The fuel pressure regulator reads changes in manifold vacuum and makes appropriate adjustments to maintain a constant fuel injection pressure of 44 psi.

FIGURE 2. Air Flow

Based on information taken from the MAF sensor and engine Coolant Temperature Sensor (CTS), the computer determines the pulse width of the fuel injectors. The computer reads the sensors and calculates fuel requirements every 12.5 milliseconds. The precise nature of fuel injection and the torque enhancing intake runner system are chiefly responsible for the increase in power and torque over conventional (carburetted) intake systems.

FIGURE 3. Basic Electronic Fuel Injection Signal Paths

Fuel delivery is via a high pressure fuel pump and vacuum modulated pressure regulator. A fuel return line is essential, and must be included in your fuel tank/delivery setup. Also essentila is a good fuel filter. Pressure to the injectors must be a constant 35 to 45 psi per injector. All fuel delivery and return lines must be of sufficient quality to handle high pressure TPI systems. If you are converting an earlier carburetted setup, these items must not be overlooked, or your new TPI setup will not work.

FIGURE 4. Electronic Fuel Injection - Fuel Path

Obviously, the simplest and most expedient way of using a TPI system with your project is to obtain a complete Corvette, Camaro, or Firebird engine with a TPI unit already installed and use the complete assembly in your vehicle. It is extremely important to be sure that you are getting ALL of the attendant sensors, relays, connectors and, especially, the COMPLETE wiring harness.

Later, we will endeavour to itemise and explain the functions of these various components in an attempt to familiarise you with their function, appearance, and location.

Mass air flow sensor variations

This manual concerns itself ONLY with 1986 through 1989 TPI systems which use a Mass Air Flow sensor. The advantage of using a system that incorporates a Mass Air Flow sensor (see  Mass Air Flow Sensor), such as the 86 through 89 units, is that the system is self-adjusting (within certain design limits) to different displacement engines, or modified engines with a higher air flow capability, such as would be required when improved cylinder heads or performance camshafts are utilised during the engine build-up. Our research has shown that installations of these units in engines as large as 406 cu. ins. have been successful and trouble free. The larger displacement does not seem to have an effect on the MAF sensor operation, and its integration with the ECM.

1990 and later Corvette, Camaro, and Firebird systems are now being installed without this Mass Air Flow sensor. The following is included for information on the post 89 TPI:

The LT1 with Opti-Spark was introduced in 1992 as the basic engine in the Corvette. It appeared in the Firebird, Camaro Z28, Caprice, Buick and Cadillac in 1993, which wass the last year for EPROM's in these cars. In 1994, OBDII EEPROM computers were used in the Corvette and F Body Cars, and batch injection disappeared to be replaced with sequential injection. In 1996 the LT4 appeared for the Corvette, rated at 330 HP with 340 lb ft torque for the manual version. It looks the same as the LT1, except for the heads and valve train. The valves are larger (2.00 Int / 1.55 Exh from 1.94/1.50), air passages are larger to enhance volumetric efficiency, hollow valve stems, aluminum roller rocker arms and stronger valve springs. By giving the camshaft more lift and a slight overlap at the end of the combustion cycle, the EGR was eliminated. The compression ratio is 10.8:1 (premium fuel only) compared to the 10.4:1 LT1.

The VERY important difference since 1989 is that, by designing the TPI to be used without a Mass Air Flow sensor, they are calibrated with what is referred to as a Speed Density system and it is necessary to use these systems on the specific displacement engines for which they were originally designed and calibrated, that is, a 305 cu. in. system on a 305 cu. in. engine and the 350 cu. in. system on a 350 cu. in. engine. Speed Density systems use a Manifold Absolute Pressure (MAP) sensor instead of a Mass Air Flow (MAF) sensor to measure engine load.

You may have noted that we did not include the 1985 TPI system. While the 1985 unit also uses a MAF sensor, it is basically a one-year only system as far as the electronics are concerned in that it utilises an Electronic Control Module and burn-off circuitry that are not compatible with the later 86 through 89 systems. In addition, the self-diagnostic capabilities of this first year unit are not as sophisticated as the later 86 through 89 units. For these reasons we prefer not to use them. In terms of driveability and performance, there is no perceptible difference between the 85 systems and the later 86 through 89 systems. The differences are more noticeable in terms of serviceability and factory, or after-market, parts availability. The desirable 86 through 89 units can be identified by the ECM I.D. number which is 1227165 for all applications Corvette, Camaro, or Firebird 305 cu. in. or 350 cu. in.. The 1985 system can be identified by the ECM I.D. number which is 1226870, for all 305 cu. in. and 350 cu. in. systems.

While the #1226870 and #1227165 are not electronically interchangeable, they are physically interchangeable in that they both have the same type of harness connectors with the same number of terminals (one 24 pin connector, and one 32 pin connector). Since the ID on these ECMs consist of a stick-on tag, which can easily be removed, it is very important to be sure you are getting the correct unit. Both ECMs have a 13/4" x 51/2" access plate on one side. On the 1226870 ECM, the access plate is at the extreme opposite end of the unit from the harness connectors. On the 1227165 ECM, the access plate is offset slightly from the centre of the unit. When the access plate is removed from the 1227165 ECM, a very clearly defined, removable memory calibrator (MEM-CAL) is visible, whereas on the 1226870 ECM unit, removing the access plate reveals a number of separate microchips and substantial printed circuitry. Page refers.

Although the ECM is the same for all '86 through '89 units, the system is fine-tuned to the vehicle engine/transmission combination in which it is installed by the use of a specific Programmable Read Only Memory (PROM) unit which is a part of a removable Memory Calibrator (MEM-CAL) insert which is plugged into the ECM. In most cases the MEM-CAL, which is in the ECM that comes with the engine/harness assembly you start with, will work adequately for normal driving. If the MEM-CAL is missing from your ECM, or if you're interested in optimising the performance of your unit, we can recommend GM Part #16082183 for 305 cu. in., or #16075399 for 350 cu. in. and larger displacement engines. In addition, there are numerous after-market companies supplying upgraded MEM-CAL units for higher output or specialised performance applications.

Vehicle Anti-theft System

All 1986 and later Corvettes, some 1988 and all 1989 and later Camaro/Firebirds, are equipped with a theft deterrent system called the Vehicle Anti-Theft System or VATS (later known as the Pass-Key system) . When equipped with this system, the vehicles ignition key contains a small resistor pellet, the resistance of which must match the special VATS decoder installed in the vehicle. If the decoder does not see the proper resistance, it will not send the correct signal to the ECM and, accordingly, the ECM will not enable the injectors to pulse ON to inject fuel into the engine, thereby preventing an engine start. If you are not sure what year or model vehicle the TPI system and harness you are using is from, check for a wire (various colour codes are used depending on year and model) in terminal B-6 of the 24 terminal ECM connector. Page refers. If there is no wire in this location, the vehicle from which the harness was removed was not equipped with a VATS system and can be used as is.

If the harness has a wire installed in terminal B-6, the vehicle was equipped with a VATS system. The MEM-CAL in the ECM must be replaced with a MEM-CAL from a vehicle which was not equipped with a VATS system, such as the previously mentioned GM Part #16075399, OR an after-market VATS defeater module MUST be added to the system. In the U.S., the cost of a new NON-VATS MEM-CAL from a GM dealer is approximately half the cost of a VATS defeater module. Taking this into consideration, plus the fact that the Camaro/Firebird wiring harness is longer in the area between the ECM and the engine sensors than the Corvette harness, we prefer to use 1986 and up Camaro/Firebird harnesses from vehicles that were not equipped with a VATS system. As previously mentioned, there are ways to neutralise the VATS system. However, we have found it much simpler to merely select the correct TPI/ECM harness assembly initially. Actually, any factory TPI harness can be modified to work correctly, but since, as just mentioned, the Corvette harness is substantially shorter than the Camaro/Firebird harness, all of the wires to the ECM, in some installations may have to be lengthened, to allow mounting of the ECM in a convenient location inside the passenger compartment or trunk (boot) of the vehicle. We have found the area underneath the passenger seat to be the preferred location for many applications, especially T-Buckets and HiBoy Roadsters.

Harness modifications MUST BE SOLDERED PROPERLY. Use heatshrink tubing on the soldered joint to minimise the possibility of corrosion by the resin.

Because of the low voltages used, poor connections, or corrosion of any kind, will wreak havoc with this system as is detailed later in the Oxygen Sensor information section (see  Oxygen Sensor).

While we prefer to use factory-type wiring harnesses, and modify them as necessary to work in the Jaguar and other non-Original Equipment Manufacturer (OEM) applications, we should mention that there are also a number of after-market wiring harness assemblies that are available to simplify the installation of TPI units into cars other than those in which the TPI systems were factory installed. One of the major reasons the stock harness is the preferred wiring system in non OEM installations is that it can usually be obtained complete with the fuel injection system/engine assembly at the time of purchase (a free-bee), whereas the after-market harness assemblies run in the $US300.00 - $US600.00 price range.

Our research has shown that, of all the aftermarket wiring harnesses available, by far the most adaptable and value-for-money unit is the Painless Wiring harness for the early TPI systems. A detailed description of this wiring harness, and the necessary information needed to incorporate this harness into a particular configuration can be found under  Painless Wiring.

If you are interested in using an after-market type of harness, we would suggest you do some research into the different types available. One definite recommendation we can make, however, is to make absolutely sure the harness you use has, what is referred to as, closed loop capability and does not operate the injection system in either the open loop or limp mode.

Hot Rod Handbooks
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