Honda drivers Jenson Button and Anthony Davidson topped the timesheets on the fourth day of testing at Barcelona, however, both drivers were running with V10s when they posted their best times.

Button, in a 2005 chassis, completed an aero programme alongside a suspension evaluation in the morning session, while Davidson continued his work in the 2006 'concept car', completing further shakedown running, along with a short aero evaluation.

The two drivers switched cars for the afternoon giving Jenson his first run in the 'concept car' and his first experience of the V8 engine in this configuration. He ran through a short tyre evaluation. In total, the new eight-cylinder car completed 84 laps during the day.

Driving the 2005 car, Anthony completed a further 30 laps after lunch, setting his best time of the day.

Third quickest was Ricardo Zonta, who was joined by teammate, Ralf Schumacher, getting his first experience of the new TF106.

Ralf's debut in the TF106 was interrupted by a massive engine failure but the RVX-06 engine had done a strong mileage before the incident occurred, in a car fitted with a new oil system. Ricardo Zonta, meanwhile, had no disruptions to his third day of testing and made good progress, closing to within 2.5s of the leading Honda.

After an unspectacular day on Wednesday, the Williams-Cosworths were on better form today, with Webber and Rosberg finishing fourth and eighth, on the Grove outfit's third and final day of testing at the Spanish GP track.

"This week was a fantastic first test for the new Williams, Cosworth and Bridgestone partnership," said a delighted Sam Michael, "and it is a great credit to everybody in Grove and Northampton to have been able to conduct such a thorough first test together.

"Although we experienced the normal teething problems, the mileage we've achieved and the progress we've made on set-up and understanding the tyres and how the car responds to the V8 has been really good.

"Mark worked on set-up changes today and did some tyre testing for Bridgestone, while Nico spent this morning on set-ups but we lost some time on his car repairing a gearbox problem."

Pedro de la Rosa was the leading McLaren driver, the Spaniard joined by Juan Pablo Montoya. Having opted to 'rest' on Wednesday, the Woking team lost further track-time today when the Colombian suffered an unspecified technical problem. Later in the day, he went off when (former teammate) Ralf Schumacher's engine detonated right in front of him.

Juan Pablo will be joined in by Gary Paffett tomorrow, for the final day of the test.

Renault had three drivers in action, with Franck Montagny joined by World Series champion Robert Kubica and former Formula Renault Eurocup V6 champion Giorgio Mondini - both drivers making their F1 debut.

Montagny continued to focus on chassis set up around the simulated V8 power output and also started working on tyre development. Kubica and Mondini both familiarised themselves with the car.