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The team, who have to lead the drivers' and constructors' championships this season, started the season with a performance advantage of nearly a second over the rest of the field, but this has been clawed back in recent weeks.

After Jenson Button's win in Bahrain - his third in four races - the .

Brawn's chief rivals on pace so far this season, and , will also have upgrades to their cars in Barcelona.

Of these three, Red Bull are the only outfit without the that dominated paddock discussion at the first three races of the year.

The device makes a big difference to the performance of a car fitted with it - as much as 0.5 seconds per lap, according to most F1 designers - so the fact that Red Bull are so quick without it speaks volumes for the quality of their car.

But it is also likely to mean that they still have another big step forward in pace to come compared to their rivals - which, if they remain competitive in Spain, could be an alarming thought for Button, among others.

For the three teams that dominated F1 last year, though, this race could define their season.

McLaren, Ferrari and BMW Sauber have all struggled to one degree or another this year - and all have upgrades for their cars in Barcelona.

McLaren and Ferrari will both have their first version of a double-decker diffuser - so it will be fascinating to see where that puts them over the weekend.

BMW, meanwhile, have admitted that while they do have an aerodynamic upgrade for this track which makes such a heavy call on a car's efficiency through the air, they one of the trick floors.

They have also taken the decision to . Which, given that Kers only escaped being canned this year because of BMW's support for the device, has raised a few eyebrows.

BMW drivers Robert Kubica and Nick Heidfeld finished at the back in a dispiriting outing in Bahrain.

Only a fool would write BMW off but, right now, the Mole finds it hard to see much of a chance of them returning to their winning form of 2008.