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Exhaust bypass valve

Discussion in '348/355' started by Mitch Alsup, Sep 18, 2009.

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  1. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,742
    #1 Mitch Alsup, Sep 18, 2009
    Last edited by a moderator: Sep 7, 2017
    It has amused me to read the exhaust bypass vavle threads. But it got me (a EE by education) why not just leave the factory bypass control (mor or less) as it is and add some new logic to the problem to give the user some control over the situation.

    See figure below: On the left is a highly simplified diagram of the electrical circuit. The exhaust computer has a signal line that when energized, drives the solenoil that couples the vacuum accumulator to the bypass actuation mechanism.

    Now given that this diagram is not misleading: consider that DC current only flows inone direction. This means diodes can be used to isolate other inputs so the exhaust computer does not "see" the other inputs driving the solenoid. When these other inputs source current into the solenoid it will open as if the exhaust computer was doing it.

    I have conceived ot two additional input sources. The first is that the bypass valve is always on (open), the second is where the bypass is set ot open when the throttle gets to a certain opening position. This first is a simple switch to source current into the solenoil, the second is two switches in series. On sources current to a microswitch with is placed either in the throttle inside the cabin, or under the plastic work and runs off of the throttle actuatoin assemble. In either case, a microswitch is used to complete the circuit.

    In case the Exhaust computer is looking at the signal line to see if the (malicious) user has disconnected the signal line, a 1K to 10K ohm resistor is used to give the illusion that the solenoid is still under sole control of the exhaust computer.

    Thus with 2 handdfulls of less than $1.00 parts, one can have 3 kinds of actuations: {Factory, Always Open, Open at throttle}.

    This leads to the question as to why the alerations to the bypass valve actuation mechanism are so expensive?

    With a little more logic I can even get a NeverOn situation for those who might have done something to the y-pipe cats and still need sniffed each year.
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  2. eulk328

    eulk328 F1 Rookie

    Feb 18, 2005
    2,800
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    #2 eulk328, Sep 18, 2009
    Some people prefer the bypass valve to fail in the open position if there is some failure of the valve, vacuum lines etc. The Capristo fails in the open position (spring loaded to open) the OEM does not (spring loaded to close).

    On the 5.2 version you could, of course, simulate the vacuum solenoid coil resistance however the computer looks at the change in exhaust temperature, via a thermocouple, just after the bypass valve. It can "see" if the temperature went up or not when the valve supposedly opened. If not, you get a lamp/error code.

    I believe the 2.7 does not have this exhaust gas temp. probe on it in this location.


     
  3. MarkCollins

    MarkCollins F1 Rookie
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    Jul 2, 2002
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    South England
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    Mark Collins
    #3 MarkCollins, Sep 19, 2009
    Hi Mitch, I was considering a similar setup, I used the 355 valves and solenoids on my P4 replica to create a straight through option :eek:

    I had initially considered having a second solenoid to feed vacuum to the valve with a (cabin) switched control and a t-piece in the vacuum line however you can't really use one way valves with the vacuum otherwise in theory once open the valve would remain open

    So I was just Yesterday giving thought to the electrical solution wondering what was stopping me putting a switched 12v to the existing solenoid however i note your mention of diodes and resistors to keep the current electrics happy, will the Solenoid receiving a second 12v signal cause a problem?
     
  4. jm3

    jm3 F1 Rookie

    Oct 3, 2002
    4,364
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    JM3
    #4 jm3, Sep 19, 2009
    Last edited: Sep 19, 2009
    I have already done this, and with a little careful checking with a multi meter and my fingers crossed, I did not use diodes.

    A simple switch, in the console.

    My kids call it "The Tunnel Switch"

    It works awesome. The 1% that is not awesome is that if driving for a long time with it open, and then gong down a long hill with very little throttle, the computers will go into some kind of limp mode. The limp mode can be reset by turning the key off, and restarting, even while still rolling. Obviously, because of the occasional (and repeatable) limp mode, I think it is completely silly to have the bypass deleted. And the limp mode is proof to me that a switch is right way to go. (other people have reported no limp mode with a bypass, but I can reproduce it on a certain hill)

    Benefits:

    warms up cats faster, so passes smog test easier.
    probably better for 02 sensors.
    can be quiet (believe it or not sometimes I listen to the radio)
    wife doesn't complain on long (4-5 hour) trips.
    can go on a drive with my brother and talk about stuff when we are stuck behind another car.
    It idles better
    I know for sure the cats (all four) are happier, because I have accidentally warmed up the car in the garage with the "Tunnel Switch" open, and it smelled like a 1982 Chevy with a pellet-bed cat going up a two mile hill. (rotten egg)

    Disadvantages:

    When driving by two blonde milfs in a Solara convertible, the switch must be operated manually.

    Mitch:In my opinion factory operation is actually pretty good. The bypass opens at nearly full throttle and 3500 (i think) in every gear but third, when it opens a little higher. (search my past posts for exact specs, I cant find it right now) However there is no "Milf mode" .
     

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