Induction Kits and ECU Calibration

The Mass Air Flow or MAF sensor found on all turbo charged Imprezas is one of the most vital engine components for safe, smooth power production. Furthermore, one of the most common 'upgrades' fitted to these vehicles is a replacement induction system. This 'upgrade' often upsets the air flow metering and causes poor driveability and/or engine damage. This article details a method for successfully integrating an aftermarket induction kit onto the Impreza, retaining driveability, safety and increasing power.

Credit must be given to Lachute Subaru (one of our EcuTek partners) for the measurement data published in this article.

Induction Kit Aims

The goal of replacing the standard induction kit on an Impreza is to derestrict the passage of air from the ourside air to the turbo. If done correctly, the result is improved turbo spool up, faster throttle response and higher top end power. If done incorrectly, the result is often higher intake temperatures, lean running, poor driveability and in some circumstances engine damage.

Standard Induction System

Standard Intake Scoop, Resonator & Air Box

The standard induction system consists of an air scoop located behind the headlamp assembly, attached to a resonator assembly located in the inner wing of the car. The pipework from this resonator (aka snorkel) emerges from the inner wing, immediately attaching to the air box containing the air filter. From the air box, the intake air passes through the Mass Air Flow sensor, then passes under the intake manifold and into the compressor of the turbo.

Advantages of the standard system

  • The standard air scoop is mounted high in the engine bay. This makes it resistant to ingress of water from puddles in wet weather. The design of the inner wing snorkel helps to remove any water that is drawn in by this scoop.
  • Air drawn in is cool, since the air comes from the edge of the engine bay, and at speed, all air will be drawn from outside.
  • The standard system is very quiet, since the resonator in the wing helps to filter out the noise from the air flow.
  • The standard ECU is calibrated to work perfectly with the standard induction system.
  • Unless it is necessary to produce more than 350 BHP, the standard induction system is quite adequate - all high power Japanese Impreza models use the standard intake system. The exception is the STI S202 which uses different pipework.

    Disadvantages of the standard system

  • At very high powers, the long path that the air takes causes a pressure drop which limits power and throttle response.

    Choosing an Induction Kit

  • Select an induction kit that draws in cold air, either by boxing the filter in the engine bay or by mounting or piping the intake from the inner wing. Ensure that the intake does not scoop up puddles if your area of the world has lots of rain.
  • Avoid kits with square edges and rough castings. Smooth pipework with bell mouthing is best. The kit below from Graham Goode Racing is ideal, though needs boxing or mounting in the wing to draw in cool air.

  • If possible, choose a kit that retains the standard mounting pipe for the Mass Air Flow sensor. If this is not possible, see the remainder of this article, which describes how to recalibrate the ECU for the new induction system.

    The process detailed below requires reprogramming of the standard ECU and is provided for informational purposes.
    This process may be performed by one of EcuTek's skilled partners.

    Mass Air Flow Sensor Operation

    A mass airflow sensor assembly consists of a sensor element mounted inside a tube of a known cross sectional surface area (CSA). The sensor works by measuring the heat lost from the sensor's heated wire or film to the air that passes it inside the tube. The greater the mass of air moving past the sensor in a given period, the greater the cooling effect. The sensor produces an output voltage that increases with increased air mass passing across the sensor. This voltage can be converted to a mass air flow rate.

    In order to make sense of this voltage, the ECU is calibrated so that it can convert this voltage into an air flow rate. Below is the standard air flow sensor scaling map present in every Impreza.

    Incorrect Operation after Induction Kit Installation

    The cross sectional area (CSA) of the tube is extremely important to correct metering of air flow. For a given mass air flow rate, the proportion of the air that passes over the sensor element decreases as the size of tube increases. In real terms, if the sensor element is placed in a larger diameter housing, the sensor will produce a lower output voltage for the same mass of air passing down the tube. This voltage will then be converted by the ECU into an incorrectly low air flow rate, resulting in incorrect fuelling. In order to cure this problem, the ECU must be recalibrated to understand the new relationship between the air flow sensor output voltage and actual mass air flow rate.

    ECU Calibration for a New Induction Kit

    In order to correctly calibrate the ECU for the new induction kit, the relationship between air flow and output voltage of the air flow sensor must be measured. Initial thoughts would suggest using a flow bench to do this, though very few tuners will have access to a calibrated flow bench capable of flowing 300 grams per second of air.

    In actual fact, all Subaru tuners do in fact have this flow bench in their workshops - the vehicle itself ! By plumbing the the two induction kits in series (the standard system and the new system), it is possible to draw the same mass of air through each induction kit. With a mass air flow sensor in each induction kit, it is possible to measure the sensor output from each kit. By taking the vehicle through a wide air flow range on the dyno, it is possible to measure the relative outputs of the two sensors when mounted in the two induction kits. A tool such as DelatDash External Sensor Logging may then be used to record the data for later analysis.

    The result of recording this data is a graph showing the relative outputs of the two induction kits for a range of air flows.

    But what does this data tell us ?
    Well, taking the highest output voltage that was recorded in this test, the standard induction system outputs 4.5 Volts - looking at the ECU's air flow sensor scaling, the ECU will interpret this voltage as 255 grams per second of air flow. Now, for the same air flow (remember, the two kits are measuring the same air flows rates because they are plumbed in series), the aftermarket kit outputs 3.8 Volts - looking at the ECU's scaling map again, the ECU will interpret this as 150 grams per second of air flow. That is a 41% error !

    Ok, so this induction kit would destroy the engine if we plugged it on now. How can we fix it ?
    Quite simple really. We change the scaling map in the ECU so that when it sees an air flow sensor output of 3.8 Volts that it is in fact 255 grams per second of air flow. After a number of measurements and smoothing the graph, the following new air flow sensor scaling map was produced.

    This new calibration was then programmed into the ECU, providing dramatic improvements to this car.

    For more information on this specific induction kit conversion, please contact Benoit at Lachute Subaru.