PORSCHE enthusiasts take heart. The biggest challenge the company has faced - replacing the fabled 911 - has been tackled with courage and determination. The result is a car that can stand proudly in the shoes of one of the most familiar modern icons in automobile history. Porsche has given the 911 new levels of driveability, interior comfort and practicality without diluting the unique formula - all this despite being air-cooled rather than water-cooled.