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Thread: Modification Path Guide

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    Full ISDC Club Member bang's Avatar
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    Modification Path Guide

    I was on Bob Rawle's site and came across a good piece which I'm sure will be useful to some. I never knew that one of the first mods you should do is to replace the fuel pump.

    Standard car upgrade

    A car is capable of yielding improvement even as standard, very noticeable improvements are available. Our own MY03 JDM STi in standard form gained 40 bhp and 45 ft lbs torque from just a remap. The major gain comes from enhanced throttle response and improved mid range punch. It is recommended that the fuel pump be upgraded since this is recognised as being prone to failure and/or weak performance even in a standard car. A STi of model year 2001 onwards cannot be remapped unless the fuel pump is changed.

    Stage 1 upgrade

    This is the first step on the tuning ladder and is the upgrade most people go for when looking to improve on the factory car�s performance.

    Induction � high flow panel filter improves breathing by allowing greater airflow through the air box, capable of supporting up to 350 bhp.

    Recommended filters :-
    Subaru STi filter
    K & N panel filter
    Green panel filter
    Fuel pump � essential when any increase in performance is contemplated since the factory standard pump is weak and prone to failure, the failure mode is subtle since in the majority of cases the pump simply starts to deliver less flow into any given pressure demand causing lean running. The capacity selected should reflect the performance objective.
    Recommended fuel pump :-
    HRC 255 in tank pump
    HRC 290 in tank pump
    HRC 340 in tank pump
    Roger Clark Stage 2 in tank fuel pump
    Exhaust system � there are two options at this level, A full decat from turbo back or a sports cat down pipe and decat mid/back section in addition. The sports cat should be a 100 cell item to ensure maximum gains whilst still remaining emissions friendly. Suitable systems are recommended to be three inch to allow for future upgrades however a good two and a half inch system would give similar results if this level is determined to be the limit of tuning aspirations.
    There are several systems that could be recommended, for down pipes the Miltek sports cat is known to give good results. For the full decat down pipe then Janspeed or Hayward and Scott manufacture similar specification parts that give known good results. For the mid section and back box the HKS "Silent" is excellent with both Janspeed and H & S producing product that works well.

    The remap, once again the remap of the factory ecu ensures that the best is gained from the above modifications and can bring from 60 to 80 bhp and similar torque improvements dependant on model and model year of car.

    Fuel choice is very important and strongly influences potential gains, for the best opportunity then Shell V Power or Tesco Momentum (99) must be the norm. 97 ron Super is then next best with 95 ron allowing some gains but mainly forcing a remap for safe engine performance when using the low fuel grade. One fuel to avoid is BP Ultimate which is not suited to the Impreza engine.

    Stage 2 upgrade, this assumes the engine remains standard

    If a stage one tune has already been applied then the next steps are as follows.

    Tubular headers � replacing the factory headers improves the mid and high rpm range performance, if fitted without map adjustment then the boost response is slower and boost creep can take place so the use of these is only recommended when the ecu is also adjusted to take full advantage. Note that it is important than the pipe diameters be sized according to the turbo in use, failure to take this into account can lead to turbo lag.

    Induction � an induction kit replacing the factory air box allows even greater air flow and opens up the breathing, ideally complimented by an uprated air intake pipe to suit the chosen turbo option.

    Recommended induction kit :-
    RCM80 strongly recommended for the new age car it is larger than the factory air pipe, it can only be installed at time of remapping and the car will not run properly unless remapped. A version is available to fit the MY99-00 standard mass air flow meter tube. Both are MAF friendly.
    K&N 57i for the MY99-00 cars this is MAF friendly and is simple to install.

    Stage 3 upgrade

    If stages one and two have already been applied then the next steps are as follows.

    Turbo � beyond stage two, the only way to further improve the performance of a car is to increase the turbo capacity, depending on the model year and model, there are a variety of options that could be considered taking account original turbo specification. All standard Impreza variants are capable of supporting up to 400 bhp providing that the right supporting modifications are employed. To achieve this level would require the use of a hybrid turbo and this could be installed to the normal factory position.

    Recommended turbo for 400 bhp
    MD321H supplied by Lateral Performance this turbo has good response and can achieve 400 bhp at 1.4 bar boost which is within the limit for the standard engine.
    RCM400 virtually identical to the MD321H this turbo also has good response and can achieve 400 bhp at 1.4 bar boost which is within the limit for the standard engine.
    Alternatives can be used however it is prudent to check what experience of use is available to validate any manufacturer�s or suppliers claims.

    Fuel Injectors � if the turbo is increased in capacity then the injectors have to be increased in capacity also to match.

    Recommended Injectors for 400 bhp
    Up to model year 2000 550cc side feed
    Model year 2001 upwards 650cc top feed
    The above are standard sized injectors readily available from sources such as Lateral Performance or Roger Clark Motorsport, larger sizes may be used but their suitability should be checked with our experience before purchase.

    Front mounted intercooler � the factory fitted top mount intercooler is not recommended for use at 400 bhp, the MY02 onward STi top mounted intercooler and bonnet scoop/under bonnet tray is the best combination and can be used however some necessary compromises in the remapping process will be required to ensure safe use which may preclude the car achieving the desired performance level.

    An efficient Front mounted intercooler overcomes the above limitation and allows optimum improvement using more advanced ignition timing and leaner fueling.

    Recommended Front Mounted Inrtercoolers
    Hybrid, cost effective and efficient this product should be the minimum choice, cheaper options are available (Autobahn 88 for example) however fail to deliver the same gains. Harvey Smith is the best known UK supplier of these products.
    APS, expensive but with a proven track record, available from a variety of APS dealers including Roger Clark Motorsport.
    HKS, again expensive but utilising a very efficient core enabling some of the lowest charge temperature rises to be maintained.
    The remap, bearing in mind the engine is still standard, great care must be exercised since over optimistic tuning can lead to engine damage. Our experience of this upgrade level is extensive.


    upgrade path.PNG

  2. The Following 4 Users Say Thank You to bang For This Useful Post:

    dontommy (24-08-2017), Gary sti (24-08-2017), hassen27 (24-08-2017), Sheriff (24-08-2017)


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