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Between 1997 and 2010, the regulations mandated that World Rally Cars must have been built upon a production car with a minimum production run of 2500 units. A number of modifications could be made including increasing the engine displacement up to 2.0L, forced induction (including an anti-lag system), addition of four wheel drive, fitment of a sequential gearbox, aerodynamic body modifications, weight reduction to a minimum of 1230 kg and chassis strengthening for greater rigidity.
Unlike the requirements for the preceding Group A cars, manufacturers were no longer required to build "homologation specials" in order to meet approval. The base model did not need to have all the characteristics of the WRC car, as evidenced from cars such the Peugeot 206, 307, Citroën Xsara and Škoda Fabia, which during this period had no road car variant with a turbocharged petrol engine or four wheel drive.
Citroën C4 WRC
To limit power, all forced induction cars were fitted with a 34 mm diameter air restrictor before the turbocharger inlet, limiting the air flow to about 10 cubic meters per minute. The restriction was intended to limited power output to 300 hp although the some WRC engines were believed to produce around 330–340 hp.[citation needed] Engine development did not focus on peak power output but towards producing a very wide powerband (or power curve). Typically, power output in excess of 300 hp was available from 3000 rpm to the 7500 rpm maximum, with a peak of 330–340 hp at around 5500 rpm. At 2000 rpm (the engine idle speed in "stage" mode) power output was slightly above 200 hp.[2]
