I wanted to find out once and for all what effect on power across the rev range, different muffler types have on my car, a 306 gti6. So with the help of the local friendly exhaust shop, I had a flange joint fitted on the pipe just behind the muffler for a small price and they lent me several mufflers to test out on a dyno! I assume the results are pretty applicable to other 2 lt NA motors.
The first graph is power and torque with a reasonably quiet (but significantly louder than standard) reverse flow walker muffler. The shop was busy so they couldn't spare time to get the car back on the hoist to switch the other mufflers and do more runs.
Instead of going back to the first shop later to test the other mufflers, I decided to book into another shop so a)I could test all 3 mufflers one after another, and b)could compare the results for the same reverse flow muffler between the two shops.
The second graph shows power and air-fuel ratio. The lowest Blue line is the reverse flow muffler. It's not on the graph but power came out to 96kW (vs 98kW from the first shop, but same curve shape). The Green line is with the STANDARD OEM muffler, power 99kW. The Red line is with the Walker straight through muffler, also 99kW. Had a look inside the muffler and it was a well made perforated diagonal pipe with nothing sticking in the way of gas flow.
Anyway, I was surprised to say the least that there was NO power difference between the standard muffler and the straight through. Before the run, my assumption was that at least at high revs the straight through would give better power. It doesn't. The straight through muffler is also much louder than the reverse flow muffler (which in turn is louder than the standard).
I highly recommend the second dyno shop, Turbo tune. They did 4 power runs (two with the straight through muffler..first run, and fourth run..and both times identifical power curve) and changed mufflers 3 times for $100.
So it seems that peugeot have fitted a quiet but excellent flowing muffler, sufficient for the engine's requirements. Also, if you do feel the need to spend big $$$ in any of the 'performance mods' such as exhaust, intake etc, it's worth spending the extra small $ to see if it actually does make a difference or whether power has actually gone backwards.