On my std S1 with std exhaust (with individual tailpipe exits) the LH pipe worked a lot more than the other at idle. I put this down to the fact that the exhaust gases will have a natural tendency to want to continue in their direction of flow - i.e. into the backbox and straight through, and not turn back on itself. Similarly, the outer radius (RH) pipe on the Eliseparts replacement (with single tailpipe exit) seems to do more work at idle and discoloured due to heat first, again presumably because the gases have a tendency to flow the straightest (outer) path.

S2 Elise

Perfectly normal.

111s exhaust has a flap that stays closed at low rpm/light throttle to increase torque.

When you use more throttle/higher rpm this flap opens for more power/noise.

You can disable it I believe.

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Sample exhaust sound clips

RS Extreme with motobuild silenced cat

Motobuild with no cat

Janspeed

Eliseparts

EliseParts CRP and Sports Exhaust

Oval Janspeed exhaust and CRP

Larini

Larini2