Williams F1 technical chief Sam Michael says he feels that the new aerodynamic rules in Formula One will be the key change for the 2009 season.

The Toyota-powered team joined Renault in unveiling its new car at Portimao on the opening day of the first test session of the new campaign, with test driver Nico Hulkenberg then hitting the track to turn the first laps with the machine.

Like the remainder of the cars currently launched, the FW31 takes on a vastly different look to the car it replaces and Michael admitted that the 2009 rules had effectively led to a clean-sheet of paper when it came to design - with the aerodynamic rules being the biggest change.

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"The changes in the aerodynamic regulations are the most profound and will have the most impact on lap time," he said. "There are many immediate visual changes, but also many smaller reductions around the car through new regulation wording and exclusion zones. Starting at the front, the front wing end plate design has changed as the interaction with the front tyre is completely different, and important to control. There are no longer large barge boards - although we managed to squeeze a small one in.

"The engine cover no longer has the traditional chimneys and louvers on top for cooling, and that has forced a higher and wider exit at the rear in order to provide an effective exit for hot air. Of course the re-introduction of slick tyres is another significant change as it has an influence on the overall dynamics of the race car. Finally, of course, the introduction of KERS is another aspect to the technical picture for the year ahead."

Michael went on to give more information about the KERS system and said he felt that a fully-functioning system would be key to the chances of success in 2009.

"KERS in 2009 could be worth between two-tenths and three-tenths of a second per lap," he said. "However, once aero performance converges, KERS could start to become a greater performance differentiator and if the regulations give more scope to the technology, it could be worth anything up to a second a lap and it will be needed to win Grands Prix.

"The key decision for us with our system is to carefully balance the potential performance advantage with our ambition to improve an already strong reliability record from last season."