http://www.gt40s.com/ubbthreads/showflat.php?Cat=&Board=UBB6&Number=27165&page=0&view=collapsed&sb=5&o=&fpart=1
http://planetsoarer.com/resonator/ResonatorsAcoustic.htm
http://superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh
http://www.broaderperformance.com/muffler_flow_tests.htm

Just throwing out these links so that you guys can make some real power. No more guessing.
Enjoy!!

And for those wondering about intake manifold plenums.
The "ideal" plenum size is rougly 14 x engine capacity.
That would the Zetec's (2.0L) "ideal" plenum size about 28L.
28.3 Liters of air = 1 cubic ft of air.
Obviously the Zetec doesn't have a 1 cubic ft plenum on the intake manifold.
Its closer to 1/4 cubic feet.

http://planetsoarer.com/resonator/ResonatorsAcoustic.htm

This link deals with intake plenums (aka series resonators).
The middle of a page shows 3-axis graphs that relate volumetric efficiency (lv) (vertical axis) to rpm (n) (horizontal axis) to cylinder/plenum volume ratio (Rv) (z axis).
Two graphs are shown for various pipe length/area ratios.
As you can see, for large values of cylinder/plenum volume ratio, there is a very large bump in volumetric efficiency (which is closely related to torque)
As the cylinder/plenum volume ratio decreases (aka larger plenums), the volumetric efficiency curve is flattened (a good thing unless you like to drive in a narrow rpm band). Notably the single peak is replaced by two peaks around the original, but the amplitudes are reduced.
In simple terms, bigger plenums give more high rpm torque, but low rpm torque may suffer. It all depends on the volumes, lengths and areas. Unfortunately, I dont have the calculations for these so I cannot even suggest appropriate sizes.

As for parallel resonators... my experience with these has been the same... they remove both noise and torque.

It is a very good link, actually all of them are.
(I learned some stuff.)
Problem is, that we have four intake runners coming from the manifold, and that changes the equations.
Also the fact we have a TB butterfly opening & closing that effects the air pulses, as well as intake valve opening & closing.
Not to mention the shape of the runners, valves, intake piping, and the plenum all effect the air pulses.
Its just too hard to nail down one simple equation for intake/exhaust design.
Hell, the technical info in the links is already above most peoples understanding on here (some of it was above mine).

My equation is a "generally acceptable" starting point for design.
Ideal plenum volume for any car is going to depend on the individual engine of that car.

http://naca.larc.nasa.gov/reports/1954/naca-report-1192/

Another nice one...