Twin turbo setup Information

Discussion in 'General Motoring' started by MIKE, Jul 28, 2004.

  1. MIKE

    MIKE Guest

    Hi All

    I have recently bought a legacy with the twin turbo setup - does anyone have
    any information the how the mechanism works. A diagram would good.
    Generally the setup is good except when overtaking at highway speeds - the
    turbo lag between the primary and secondary occurs just as you are along
    side the car you are overtaking. Its quite an unnerving sensation to loose
    boost (and power) when you are on the wrong side of the road beside the car
    you are overtaking. The only way around this problem that I can see is to
    overtake in 3rd gear (as apposed to 4th like I would nominally do) - it
    certainly pulls but it feels little unkind to the engine to rev it right to
    the redline.

    thanks for the info

    MIKE, Jul 28, 2004
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  2. MIKE

    Guest Guest

    There is a valve in the exhaust system which can feed the primary turbo
    from all cylinders or share the flow between both turbos.

    There is a value between the secondary turbo output and the intercooler,
    there is a differential pressure sensor monitoring the pressure across this

    At some point around 4300 rpm but probably depends on other things the ECU
    decides to open the exhaust valve to feed both turbos. The primary turbo
    looses a lot of boost. The secondary turbo spins up and when its output
    pressure matches the pressure in the intercooler the valve between them
    opens. Both turbos are then boosting and power output continues to rise.

    You might be able to find a dyno chart on the web somewhere. If you do you
    will see that the engine peaks on the primary turbo at about 160bhp. Output
    drops to around 120bhp as the secondary turbo spins up then continues to
    around 280bph (for later MT versions).
    That is the price you pay for having a small responsive turbo most of the
    time while still having vicious top end power. There were some changes made
    on later models to try to lessen the 'boost valley'. When you get to know
    your engine it isn't really a problem.

    There is one other significant possibility. The engine is designed for 100
    octane fuel. On lower octane fuel the engine will knock. The knock sensor
    will detect this and retard the ignition and open the primary wastegate to
    cut boost. If the engine is going to knock it will knock at around peak
    boost on the primary turbo which is about the same place the twin turbo
    'boost valley' occurs.

    The drop in power is more significant than the boost valley when this
    happens. No one really knows and it may have changed between models but my
    feeling is that the ECU remembers the engine knocked and limits boost to
    prevent it in the future, my feeling is it also slowly forgets otherwise
    the engine would never boost more than it did on the hottest day with the
    worst fuel it had ever seen. If you run on low octane fuel then every now
    and then when the ECU has forgotten you will put your foot down, and
    experience a big hole in power output. You try the same thing 1 minute
    later and it will be ok. You really need to run these engines on high
    octane fuel.

    There is a site here with some
    stuff about the twin turbo Legacy. I don't think the guy's description of
    how the turbos work is quite right - maybe he had the vacuum pipes hooked
    up wrong on his car :).
    Guest, Jul 28, 2004
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  3. MIKE

    Bruce Hoult Guest

    Yevy interesting. That explains why when driving a friend's 2.0 twin
    turbo it seems really hard to make it go faster than my 2.5 non-turbo.
    The 2.5 has 165 HP and I don't like to redline other people's cars (or
    my own, for that matter).

    -- Bruce
    Bruce Hoult, Jul 28, 2004
  4. MIKE

    MIKE Guest

    Thanks for the information and the links they are very interesting.
    I had a thought while reading the information. Would connecting the output
    from the waste gate of the primary turbo to the input to the seconday turbo
    reduce the lag I'm experencing?

    Once the primary turbo reaches its set boost pressure the wastegate opens to
    prevent it from overboosting so if you send the excess exhaust to the
    secondary turbo it may at the very least spool up the secondary turbo before
    the switchover. I don't think this would make the primary turbo over boost
    as the secondary has less back presure - and if it does you could seperate
    two exhaust paths so you end up with a twin exhaust pipes.


    MIKE, Jul 29, 2004
  5. MIKE

    Guest Guest

    Sounds reasonable but mechanically a bit impossible. I think the wastegate
    is an integral part of the turbo casting and wastes into the same hole the
    turbo exhausts to.

    The exhaust valve could be used as a wastegate for the primary turbo with
    the desired result - maybe it already is.

    I'm sure there is a lot of knowledge in Japan. maybe you can find something
    on the web if you have some way to translate. I haven't looked recently.
    Guest, Jul 29, 2004
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