Re: <E36> '98 Muffler Mechanism?!?!?!?

["Robert T. Chafin" <rtchafin@xxxxxxxxxxxxxx>]

Dean Schindell <schindel@domain.elided> wrote:
<Subject: re: 
<Michael asks about the flap in the inside exhaust pipe near the end of the
<muffler.

<This flap has 2 purposes:
<1.  to increase low end torque by increasing backpressure below 3000rpm
<2.  to reduce noise below 3000rpm.

<As you might have guessed, it's closed below 3000rpm and opens above that.

Several cars have had these "flaps" or valves in their exhaust systems:
contrary to popular belief, they are not intended to "increase low end
torque" - backpressure doesn't do that, at least not with modern injected
computer-controlled engines.  What the valve does is permit your car to
meet legal (pass-by) noise requirements and give satisfactory subjective
(interior) noise characteristics at idle and low speeds (as Dean noted),
while opening and reducing exhaust backpressure for improved performance at
higher speeds.  Opening the valve effectively short-circuits the muffler -
if you use an electric circuit analogy, the muffler is an impedance device.
 Typically, there is a slight delay in the opening of the valve, i.e., it
opens slowly enough that the pass-by noise test is over before the valve is
fully open.  
Nissan used this system (and my former employer holds the patents) on a
couple of Japan-only models whose names escape me at the moment.  And
Mitsubishi has used them on the 3000GT VR-4 and 929 for several years.
Weight, cost, and complexity don't generally justify the miniscule
performance increase, especially on 4-valve engines.  When we developed one
for the ZR-1 Corvette, though, that car *really* sounded great at full
song.  Tennessee 130 down by Lynchburg may not have recorded yet.
Robert

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