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There is a big movement in the forced induction arena using fuel management computer like piggyback and stand alone. The reason we use piggyback computer is to trick or tell the oem ECU computer how much fuel to increase.
For example, the stock ECU is calibrated to increase so much full per rpm increased. Since we are modifying the air intake like supercharging and turbocharging as well as Nitrous, we are changing the amount of air entering the engine. If we double the amount of air coming into the engine.
Therefore, we should double the amount of fuel coming in and other factors too. In this article we will mostly focus on the piggyback computer for Lexus SC400 and LS400. Specifically, the 1UZFE motor.
Here is an over view of Apexi S-AFC: The Super AFC (Air Flow Converter) is the industry standard for sub-computer fuel control. The Super AFC is designed to fine tune and “squeeze” power out of existing engine setups by allowing the user to modify the fuel curve with precision accuracy.
The AFC, while not completely universal, comes preprogrammed with most of the commonly used air flow meter/ pressure sensor maps. By setting the AFC to one of these base programs, the user is able to adjust injected fuel amounts by +/-50%. 8 user defined RPM adjustment points allow maximum tuning in 500 rpm increments. Fuel can also be adjusted for part and full throttle situations.
Why do we need a piggyback anyway. Actually, we do not really need it. If you plan to run less than 8 psi or what your stock injector max out at. You do not really need a piggyback. With low boost a Fuel Management Unit (FMU) and the Cartech FMU is also adjustable.
Here is more info on FMU.
If a FMU work as same as piggyback, then why do we need piggyback computer like the Apexi S-AFC? That is a very good question. Piggyback become necessary when slightly larger than stock injectors are being used. For example, the stock US Spec. 1UZFE injectors are 251cc/min. If you decide to upgrade injectors like the Supra MKIII injectors, then the oem ECU does not recognized the larger injectors and the oem ECU was not calibrated for that amount of fuel being flow.
Here is another article about Injectors Upgrade. With larger injectors the car will idle very rich. Rich means lots of fuel during idle and your “Check Engine” light will come on and timing will be retard and the car will not run properly and lost of power.
If you decide to use Supra injectors like the 295 cc/min or 315cc/min, then a piggyback will help. Currently, I am using 390 cc/min from the first generation Eclipse and that is the maximum size this piggyback can be used. Larger injectors would require more complex piggyback or stand alone computer.
As you can see the below diagram, the LS400 aka Celsior is using wiring Diagram T5-a and the SC400 or Soarer is using T7-a wiring diagram. When looking at the diagram below, look for the engine reference. The U.S. Spec LS400 and SC400 both have 1UZFE
Please reference below.
Below is the updated for U.S. Spec Toyota/Lexus vehicles. The type of sensor for the Lexus is mostly Karmann. Below engine code has an error. The actually engine code is 1UZ-FE not 1UZ-GE.
The U.S. Spec Lexus LS400 and SC400 ECU is located on the driver side under the glove compartment. Once you open up the panels, you can not miss it. The LS400 using T8-a diagram and SC400 uses T7-a. The below connector diagrams are view as if you are looking at it directly. Here is a wiring diagram for the LS400 Wiring Diagram (1) and LS400 Wiring Diagram (2) Here are the SC400 ECU diagrams 92-95 SC ECU Diagram (1) 92-95 SC ECU Diagram (2).
Here is a diagram how connection is occurred. The main goal is to interfere the air/fuel signal from the Air Flow Meter or Mass Air Sensor or MAF. Interfering the signal and alter the signal is what a typical piggyback function. Below White wire from the Apexi intercept the MAF signal and Yellow send a new signal to the stock ecu. The stock MAF signal wire will be cut and the signal is altered. Please see below and the rest of the wires are getting reading like rpm, power and ground. The wiring itself is pretty simple. Here is a complete Apexi S-AFC Manual
Lexus 90-94 LS400
The picture above diagram indicates KS (number 8) is Yellow-Green is the MAF
Signal wire. You have to wire Pink wire from the Apexi closer to the ECU
Orange Wire away from the ECU and make a cut in between those
two wires. In order for your car to work properly, you must follow the
Karman wiring diagram. I messed up the wiring and my car did not run
properly. So make sure you have the right wiring.
Lexus 1992-1996 SC400:
The SC400 would have different wiring diagram according to Apexi Manual.
However, the Air Flow Signal wire is still the same. KS is the
Yellow-Green wire. Below are the wiring diagrams for the SC400. You
should follow T7-a diagram. Remember its KS-Yellow Green Wire. KS in
the SC400 ECU diagram is number 66.