Active Exhaust

By SRT-Tom · Nov 30, 2019 · ·
  1. SRT-Tom
    It’s a fact of life- cars make noise. Inexplicably, there are those in this world who build houses next to major freeways and then complain about the speed of the vehicles going past. There are those who build houses next to airports and complain about the excessive noise. Just as there are those people, there were those who decided that cars were too loud and needed to shut up!


    This presented car companies with a problem. Their engineers were telling them that for maximum efficiency, particularly in the horsepower department, the internal combustion engine needs a free flowing path for the exhaust gasses to escape. To reduce noise, on the other hand, requires a more restrictive path.


    For stock factory cars, auto manufacturers simply used baffled mufflers to reduce sound levels and lived with horsepower efficiency losses. The solution for those that wanted the horsepower, more than the quiet, was in the aftermarket. An entire industry was built around less restrictive exhaust systems. Back in the 60s and 70s, muffler brands like Cherry Bomb and Thrush, with their straight through designs, became the most popular exhaust modification. However, when noise restrictions became extremely strict, this meant that the guy with the loud car was now under risk of being ticketed and pulled off the road.


    Back in the 1930s, American hot rodders used cable-operated valves that switched between two alternate exhaust systems. One problem was that the cables often jammed in place and had to be re-machined/replaced frequently. But they worked- perfect for sneaking quietly past that cop on traffic duty before switching over to the noise and power maker for the local traffic light drag races.


    In recent years, a lot of car manufacturers cleverly responded by programming deliberate dead-spots into the ECUs of their cars at the particular load/speed points at which cars were tested for decibel loudness, while allowing them more volume everywhere else.


    Today, car manufacturers have come up a better solution to the exhaust noise problem- active exhausts. An active exhaust consists of a “flapper” valve that opens up when the driver requires high revs, more response and big power. It bypasses the more restrictive elements of the exhaust. The restrictive elements stay in play and keep the volume down while running the kids to school or quietly cruising around. The FPV GT uses a vacuum system where the throttle body pulls the flapper valve open. Other cars, like modern Audis, use a push button on the dash that electrically opens or closes the valve.

    The active exhaust provides a simple, elegant solution to getting around the regulations on car noise, while allowing the cars to retain their distinctive exhaust note characters and power production when the need arises.

    Dodge uses active exhaust valves to reduce exhaust noise in its big 6.2 and 6.4L Hemis in order to meet and pass EPA noise regulations.


    The active exhaust valves consist of two parts. First is the mechanical butterfly valves that are welded into the exhaust pipe on each bank.


    Mounted to each of these is a smart electric actuator which has a power, ground and bus feedback wire. They are connected to the mechanical valves through a transmission spring. The spring is used to help reduce heat from transferring between the mechanical butterfly valves and the actuators.


    (Note- Due to premature breakage of the first generation of spring released, Dodge made a change to this spring beginning around 7/2015. The tightly wound flat spring (see image on right) was replaced by a more robust second generation spring (see image on left). The newer design was to put more air space between the coils to keep the spring cool and prevent breakage when attached to the hot OEM exhaust valve).


    The Active Exhaust Valve actuators communicate with the Powertrain Control Module (PCM) over a LIN Bus circuit. The actuators have three wires- positive, ground and BUS (data). Positions are communicated via computer language between the PCM and the actuator itself. The actuator has a microprocessor that communicates over the BUS wire with the PCM and vice versa. It's much more complicated than a simple switch or potentiometer. The actuator looks for a language command over the BUS to open or close.

    The Active Exhaust Valve actuator 1 (left) and Active Exhaust Valve actuator 2 (right) each has a unique part number/internal ID. The PCM can identify each actuator location by this unique ID. The PCM sends a position command to the Active Exhaust Valve actuators based on engine torque, rpm and vehicle speed. The Active Exhaust Valve actuators move the butterfly valves to the commanded position. An internal Hall Effect sensor counts the pulses and determines valve position. The valve actuators are calibrated at every Key-ON event. The PCM sends full open and full closed commands and checks actual against commanded position. If for any reason the calibration is unsuccessful, the actuators are self-diagnosing and report fault flags to the PCM over the LIN Bus.

    If the valve position doesn’t match a predetermined calibration, a performance code is set and the valves will default to the full open position. If an actuator is not attached to the mechanical valve, or the transmission spring is broken or missing, the actuator will continue to rotate indefinitely and set a fault. Also, if the mechanical valve is stuck and will not move, a fault will set.

    NOTE: There is a crossover in the exhaust, upstream of the valves that will allow the exhaust to flow through the open valve in the event one of the valves is stuck closed.

    NOTE: If the transmission spring breaks or the valve becomes disconnected from the actuator for whatever reason the valve assembly likely will cause a rattling noise with the engine running.

    There are 4 possible diagnostic trouble codes involving active exhaust valves:


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