Over-enthusiastic(?) CIS desmog | FerrariChat
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Over-enthusiastic(?) CIS desmog

Discussion in '308/328' started by Ehamilton, Dec 27, 2010.

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  1. Ehamilton

    Ehamilton F1 Rookie
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    Jun 13, 2010
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    Durham, NC; USA
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    Eric Hamilton
    #1 Ehamilton, Dec 27, 2010
    Some previous owner (or previous owner's mechanic) removed the fast idle system, removed the vacuum limiting valve, and disconnected the AAV. Now, I'm not going to mourn the fast idle system, smog pump, cats, or the EGR.... but the AAV?!

    So two questions:

    1) To get the car running smoothly in cold weather I have to set the warm idle speed a bit higher than I like. Of course I could work around this by shoving the carpet forward when I start the car, and pulling it back when the engine is warm :D, but I'd kinda rather try hooking up the AAV instead. I have power at the electrical connectors, the AAV opens wide when I put it in the freezer and closes up when I warm it, so should work fine. Question: In the factory setup, which two ports on the intake go the AAV? One upstream and one downstream of the throttle, of course, but I'd as soon use the same ones as the factory.

    2) What is the purpose of the vacuum limiting valve, and what bad things will happen if it's not there? The workshop manual (Mondial 8 2v is the one I'm using because there isn't one for the GTSi 2v and as AFAICT it's the same motor) says that the vacuum limiting valve "operates only on deceleration so to avoid excessive mixture leanout"; but that's no answer at all - any engine will go way lean when the throttle is closed at high revs, but cylinder pressures and temps are so low then that there's no risk to the engine. Is the vacuum limiter just there to control emissions on deceleration?
     
  2. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    #2 finnerty, Dec 27, 2010
    Last edited: Dec 27, 2010
    I am assuming this car is a 2-valve, injected, US-spec model, correct ?

    If so, that engine needs both the AAV (auxiliary air valve) and the vacuum limiting device (valve) for important functions other than emissions control. So, I would put them back on / re-connect them if I were you.

    The other components you mention have only to do with emissions and "the fast idle warm up" function. Fortunately, the early CIS engine has fairly "dumb" emissions control systems which act independently and without ECU feedback loops --- they are not truly integrated into the car's engine management --- so they can be removed with no complications or detriment to engine performance.
     
  3. Ehamilton

    Ehamilton F1 Rookie
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    Eric Hamilton
    #3 Ehamilton, Dec 27, 2010
    yep - 81 GTSi.

    That I'll buy, but I'm not asking IF the vacuum limiter is necessary, I'm asking WHY it is necessary. What is it about this motor that makes it necessary to avoid high vacuum on deceleration?
     
  4. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    #4 finnerty, Dec 27, 2010
    Mainly it has to do with the ignition advance / retard which is controlled by intake plenum vacuum. It's not so much to do with high vacuum on deceleration in general, rather for situations of high "over-run" where the RPM's are high, and the throttle is closed quickly. Because of the 180 degree crank on that engine, it takes longer to wind down than a 90 degree crank engine. So, with rapid throttle closings, the 308 injected engine goes lean (as the CIS fuel delivery wants to cut off), and the ignition is retarded heavily due to the vacuum control ---- this can lead to misfiring and excessively lean mixtures which could be damaging. Under these conditions, the vacuum limiting device allows metered air to bypass the closed throttle, which raises the air flow sensor plate in the fuel distributor and allows the injectors to get some fuel they would not otherwise get.

    So, it's kind of a combination of a CIS system / 180 degree crank / Marelli vacuum ignition application scenario. There are other ways to mitigate the lean over-run problem, but for whatever reason, Ferrari chose to do it with the vacuum limiting device.
     
  5. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    #5 finnerty, Dec 27, 2010
    Last edited: Dec 27, 2010
    EDIT ---

    Before someone jumps down my throat, I probably should have said "moves" rather than "raises" because to be technically accurate the AFS plate actually moves "downward" in the Ferrari 308 application. In most other applications (all the German cars of the same era that also use CIS systems , for instance), the air flow sensor mechanism is inverted, and the plate is pushed "up" by the intake air flow.
     

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