VW CEO Martin Winterkorn believes that there is scope for Porsche to expand its product portfolio but has ruled out developing a VW-Porsche as the companies did in the 1970s.

“A Porsche should remain a Porsche,” he told Manager Magazin in an interview.

Winterkorn, who will manage the combined VW-Porsche company, said there was room for a Porsche smaller than the Boxster, so far the smallest model in the portfolio.

Further possible new cars would be a smaller sister-model of off-roader Cayenne and another model of newly launched sports limo Panamera.

The CEO aims to boost Porsche’s sales to 150,000 units by 2012 or 2013. He did not rule out sales of 200,000 units a year by 2018. Sales this year are expected to be around 75,000, well down on recent years.

Winterkorn also said he wants to get his contract extended beyond the end of 2011, when it expires. “If I remain in good health and am still wanted, I can imagine staying longer. There is a lot to do.”