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Fresh air valve is allowed for WRC cars.

A lot for sure. But can't say exactly, as it depends on stage characteristics, on car (turbo spool, etc), on mapping, conditions, driver, etc.

Current WRC cars I don't know, I think slightly more than R5s (as restrictor bigger so more air so more fuel needed also). R5 cars use around 55-60l per 100km in stage mode. All depends on driver...