McLaren unveiled a bubble nose on the first day of testing at Barcelona, sparking speculation the device would be used at the first race in Australia on March 27.

Jenson Button's first run in the MP4-26 was unremarkable, but when he re-emerged after his first stop the bulbous nose had been fitted - it also featured rods branching out on either side of the front wing.

The consensus was it was an aerodynamic assistance, but at the lunchtime break McLaren explained it was no more than a measuring device and would not feature at any of the races.

McLaren will introduce a 'simpler' exhaust system and new floor for the Australian Grand Prix after its troubled pre-season testing period - and hopes to gain over a second per lap from the changes.

The team has been off the pace and struggling for reliability during the winter, and team boss Martin Whitmarsh said during a Vodafone McLaren Mercedes phone-in conference that he had decided the only option was to shelve the MP4-26's radical exhaust.

"I'm not satisfied with where the car was on reliability or performance in the test," he said. "We have made some fairly dramatic changes, and those changes we will see in Australia. There's some risk in that, but I think it was the right thing to do and we're hopeful that that risk comes off and the car is a lot more competitive in Australia."

He said the exhaust and floor were the headline elements in a major change to the car.

"We have a completely new floor and a new exhaust system. There are a lot of other bits and pieces, but they're the clear and obvious ones that people will see in Australia."

Whitmarsh acknowledged that the original exhaust design had been too complex and had created too many reliability issues.

"I'd say it is a simpler design than we've had before," he said. "I think the exhaust systems have become quite extreme on quite a lot of the cars. I think we in particular had very extreme solutions.

"But I think that they were not delivering, in my opinion, sufficient benefits for their complexity.

"I believe that the car isn't fundamentally a bad car. I believe that we need to unlock the exhaust-blowing potential.

"We had some very creative ideas, some of which could have worked spectacularly well. But if they were to work spectacularly well then they had to be sufficiently durable to be raceable, and frankly some of our solutions weren't, and that's why I think we had to go back on it. But I think in doing so we found some interesting performance."

Whitmarsh admitted that going straight into the opening race of the season with such major changes on the car was a risk, but felt it was worthwhile given the performance gain he expects to see.

"I think it will still be a challenging weekend, but I'm hopeful that we'll put on more than a second in performance," he said.

"That's not what you like to do after a test like that, but I think it's the right decision, and if it wasn't then I'll have to put my hand up."

While unwilling to make firm predictions about where the revised McLaren could stand in Australia, he said the team would be aiming for victory.

"I can't make any predictions, but you always have the target of going to win races," Whitmarsh said. "I'm not predicting we're going to win it, I'm saying we're aiming to win it."