Resonators and mufflers

[C M Smith <cmsmith@domain.elided>]

The British refer to mufflers as silencers. A muffler is a man's woollen 
scarf or a woman's handwarmer with open ends, held across the stomach. Both 
versions of English refer to a firearm damper as a silencer.

The resonator in the exhaust is basically an  internally open box with 
baffles designed to damp the major portion of the exhaust pulses. The open 
box aspect also has a secondary reflection aspect, rarefaction waves are 
sent back up the exhaust to the ports, timed to reinforce the exhaust 
scavenging effect of the main rarefaction waves caused by the header tubes 
meeting in the downpipe assembly.

The muffler is a box with perforated tubes in it (either two tubes in 
reverse parallel or a straight through pipe with glass pack over the 
perforations) and is designed to damp out the remaining exhaust pulses 
after the resonator has done its thing. The pulses pass through holes 
punched in the tubes (best from inside to out in the "in" tube and outside 
to in for the "out" tube) which split the exhaust pulse into many tiny 
pulses all slightly out of phase. The result is a great reduction in the 
banging sound originally produced by the exhaust valves opening.

In catalyst equipped cars a great deal of the resonator function is handled 
by the cat, with the catalyst bed effectively damping out the major exhaust 
pulses leaving much less work for the center "muffler" which is actually 
usually more of a secondary resonator. Incidentally, turbo cars have even 
more aggressive muffling provided by the turbine blades breaking up the 
exhaust pulses right out of the manifold and so turbo cars tend to have 
free flowing mufflers and have kind of anemic sounding exhaust notes, more 
of a subdued roar than a proper thrum you get from a high compression engine.

Messing with factory exhaust systems is problematic these days as is 
changing suspensions or "improving" brakes. The factory boys are closer to 
cutting edge stuff in our higher end cars than in past years. Not as much 
fun for the tinkerers but great for us get in and drive it types. Post 85 
cars are pretty well sorted right out of the factory gate and improving 
them without adversely compromising positive features is a real challenge. 
Used to be an aftermarket exhaust and a set of Bilsteins was a substantial 
improvement, not any more....
Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L
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