Subaru Legacy Twin Turbo

"The Ultimate Touring Car"

Technical Info

Turbo Setup for 1996 Legacy GT

    This diagram represents my interpretation of the turbocharger and intercooler setup on my 1996 model GT Legacy. The 96 model, is the sequential turbo setup. This is also the setup for a manual version of the car, but is identical to the auto version with the exception of the ECU, as I found out the hard way !

View Diagram...

How does it work ?

    For along time now, the myth of how the actual setup of the turbochargers and other myriad of things on the Twin Turbo Subaru's has eluded many people. In my endeavour to search out the truth, I found that the only real way was to figure it out myself. After about 2 hours on a 4WD Dyno I finally knew how this system fitted together. In order to modify something in any way, my philosophy is to know how it works properly first before you go playing with things, otherwise you WILL get burnt. The following information is based on a manual vehicle being run at full throttle. This makes a difference, especially in an Auto version, as in the auto, when full throttle is applied before 4000 Rpm, the second turbo tries to come in and fails abysmally. The reason I can compare Auto and Manual is that my car was both. It started out as a 4 speed Auto, and then went to a five speed manual with the auto engine and ECU. It is now a proper 5 speed, with true Manual engine, ECU, and gearbox.

    The Dyno run starts at 1300 rpm in fourth gear and car is producing 58 kw at the wheels. At 2000 rpm, the primary turbo starts boosting up. By 2800 rpm, the primary turbo is really starting to wind up and the car is producing 72 kw. This trend continues steadily to 4000 rpm where the boost then drops to nothing as the intercooler valve for the secondary turbo opens (boost valley). Within about 1 second, and at spot on 4500 rpm, the exhaust actuator for the secondary turbo opens and lets the secondary turbo start to spool up. The secondary turbo starts to spool up and produce modest boost within 200 rpm of the exhaust actuator opening, and then produces steady boost increase all the way to 116 kw at 5800 rpm. The boost remains constant at this point all the way until the Dyno run is terminated at 6600 rpm.

   To convert KW at the wheels to KW at the flywheel, the basic calculation is X1.75. This equates to my car producing around 203kw at the flywheel which would be about right. That is also running on 98 octane PULP. One important thing to note, is that when the intercooler valve for the secondary turbo opens, and the second turbo is brought in to play, the engine leans off significantly. A turbocharged engine, should ultimately be running a mixture of around 12.5:1, but at this changeover point, the mixture goes as lean as 13.5:1. Although this is significant, being that the boost is next to nothing at that stage means it should not cause any problems, but is something to be aware of. Apart from that, the mixture stays typically (Subaru) rich and finishes off at around 11.5:1 which is nice and safe. It will be interesting to see what the mixture is like with a little more boost, as my car is still running standard everything at the moment, which is about 9 psi of boost.

Solenoid Box Configuration

    After spending a considerable amount of time mapping out all of the pipes and connections for the Solenoid box, I have finally been able to put it down on paper and draw it out. The diagram is available by clicking below, and the pipe numbers are shown below with a description of where they go and/or what they do.

View Diagram...

ECU Error Codes

    This list comprises of most, if not all of the ECU error codes in the BG5 Twin Turbo Legacy.

To identify an existing fault, you will need to follow the following procedure. Please also note that the ECU light flashes to indicate the error code, the slow flashes which come first are the tens digits. The fast flashes that come second are the single digits. For example, an error code of 13 will be one slow flash and 3 quick flashes straight after the first flash, then their will be a short break and it will flash the sequence again.

Identify ECU Fault Code

• With ignition OFF, locate black ECU check connector below dashboard on left side of drivers foot well. It will be two small black connectors, one female and one male which will probably be alongside the ECU reset connectors which are green.
• Turn the ignition ON and identify how many times the check engine light flashes (if any). This will be the error code stored in the ECU memory.
• Locate the fault code in the chart below and this will identify the fault. If you feel the fault is incorrect, you can reset the ECU by using the procedure below.
• Turn ignition OFF and unplug the connectors.

Reset ECU

• With ignition OFF, locate the 2 black ECU check connectors and the 2 green ECU reset connectors  below dashboard on left side of drivers foot well. There will be one female and one male of each color (Black & Green).
• Connect the black connectors together, and connect the green connectors together.
• Start the vehicle and drive until the check engine light starts flashing. ENSURE THE VEHICLE SPEED REMAINS ABOVE 11 Km/h.
• If there is no fault, the check engine light should flash steadily. If there is a fault, the check engine light will flash the appropriate code which can be checked against the table below and the fault rectified ASAP.
• Stop engine and turn ignition OFF.
• Disconnect the two black and two green connectors.

ECU Error Codes