Conversion - Electronics

Tim McCabe suggests you not undertake a TPI or LT1 conversion unless you have a thorough understanding of what each sensor does in the system. While this sounds like you have to have a degree in electrical engineering, this really isn’t as overwhelming as it sounds. Basically, the program attempts to maintain a 14.7:1 air/fuel ratio whenever the engine is in "closed loop" mode. Generally speaking, this is any time other than when the engine is first warming up or at wide-open throttle. This is determined by a closed data loop in which the MAF or MAP provides pressure data to the computer and the oxygen sensors tell the computer what the air/fuel ratio is. The computer determines the injector pulses and ignition timing based on inputs from the oxygen sensors and MAF and/or MAP using a look-up table, but then fine-tunes it based on inputs from the inlet air temperature sensor, knock sensor, and engine coolant temperature sensor. There is an excellent explanation of how a typical EFI system works contained in the beginning of the DFI Training Manual on the ACCEL website under the EMIC heading. Other aftermarket vendors also have similar (but less detailed) information, such as Fuel Injection Specialties and Ron Francis Wire Works.

Selecting a computer and engine wiring harness is a critical decision. Tony Hallo bought a complete wrecked Trans Am with a six-speed manual transmission and pulled the engine complete with computer, harness, and sensors. I was not so lucky, and had to purchase a computer and have it programmed, a wiring harness, and several sensors. Even if you get a complete setup, you may still want an aftermarket harness and custom computer program. Basically, there are three types of computers. The ’92-’93 Corvette and ’93 F-body (Camaro/Trans Am) use a computer with a removable programmable chip (PROM). The early LT1’s also use a "batch fire" system for the fuel injectors, where the bank of injectors shoots the fuel all at once rather than sequentially, as in the later system. Also, the early engines do not use a Mass Air Flow (MAF) sensor, but rely instead on Manifold Absolute Pressure (MAP) and lookup tables in the computer program to meter the fuel/air ratio. The ’94-’95 computers do not have removable chips, but are reprogramable. These contain Onboard Diagnostic Equipment I (OBD I) technology to monitor the status of the computer and sensors. The post ’93 computers are also programmed for the Vehicle Anti-Theft system, which must be disabled if you don’t use an ignition lock and key with a special anti-theft chip. Tim McCabe also builds a black box to bypass the VATS system on most GM fuel injection systems. He says "I have not tried it on an OBDII system, but it works on all the '86-'94 units. It's a simple
3-wire hook up. Let me know if you know anyone who is looking for one. I
charge $50.00 US and it has a 1-year warranty. It's a lot cheaper then getting a new PROM burned without the VATS."

The ’94 and later engines use sequential fire fuel injection; each injector shooting fuel in correct order. They also have a Mass Air Flow sensor that uses air volume across a sensor to signal metering of the fuel/air mixture. The MAF sensor is one of the highest replacement frequency parts in the system; also the most expensive at over $275.00. It also takes up a lot of space under the hood. The MAP sensor is still used, but as a secondary or backup to the MAF sensor. The third type computer uses OBD II technology and is not reprogramable; so if you use this computer, it’s best to match your engine to the stock ’96-’97 configuration. You also must have the Vehicle Anti Theft System (VATS) disabled. There are several aftermarket performance companies who will sell you a reprogrammed chip or computer and engine wiring harness. I wanted a sequential-fire system, but did not want the problems or expense of a MAF sensor. Two companies offer this configuration; Street and Performance of Mena, AR and Howell Engine Developments of Marine City, MI, who I used. Bill Howell was one of the engineers who designed fuel injection systems for GM. He purchased the computer code to delete the MAF from the ’94-’95 program. Their wiring harness includes a fuel pump relay and can be set up for full emissions, no emissions, or any combination in between. I elected to delete the Supplemental Air Induction Reaction (AIR) pump, but retain EGR, PCV, and the charcoal canister. I am also using a Vehicle Speed Sensor (VSS) geared off of my speedometer pickup to ensure smooth performance and eliminate stalling when the engine is coasting at idle with the car in motion. Howell also offer a neat 700R4 lockup feature that uses a vacuum switch and pressure switch to tell the converter to lock up in 4^th gear independent of a computer signal. Their prices are reasonable; they stand behind their product and have good customer support. Engine wiring harnesses generally range from $375.00 to $550.00 depending on vendor. Computers and chips run from $300.00- $450.00 again depending on vendor, programming, etc. If you need a complete computer, engine wiring harness, and sensor setup for your engine, plan on spending between $1200.00 and $1500.00.

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