There's still a lot of work to do - Button
By Andrew Benson and Sarah Holt
BBC Sport at Silverstone
Lewis Hamilton and Jenson Button's McLaren team have been forced to abandon a major upgrade they had planned for the British Grand Prix.
The 'blown diffuser' - which increases downforce and therefore cornering speed - caused problems in Friday practice and has been taken off the car.
But engineer director Paddy Lowe told BBC Sport they would continue with a new front wing, which was also fitted.
He said they were still optimistic of challenging Red Bull for victory.
"We were competitive in Turkey without the blown diffuser," he said, "and that's a similar sort of track to here.
"The blown diffuser is an influence in terms of performance around here but we've got other new parts on the car that we're very happy with so we're reasonably optimistic."
Lowe said McLaren dropped the blown diffuser for this race because it was causing the bodywork to overheat and the car proved difficult and unstable to drive on Friday.
However he does expect for the diffuser to return for the next race at Hockenheim in Germany on 25 July.
Highlights - British GP second practice
During practice at Silverstone, Hamilton ran off course a number of times at the new 160mph Abbey kink and Button also complained of a poor balance to the car's handling.
Championship leader Hamilton's best time in practice was more than a second behind Mark Webber's pacesetting Red Bull.
The blown diffuser is the fashionable F1 upgrade at the moment as other teams seek to claw back Red Bull's raw performance advantage.
The device relocates the exhaust exits from the top of the bodywork to lower down, where the gases blow into the 'coke-bottle' shape at the rear of the car and through into the diffuser - the upturned part at the back of the car's floor.
This is a crucial area in the car's overall aerodynamics and teams believe that the blown diffuser can provide a lap-time gain of about 0.5 seconds.
But because the exhaust gases are so hot, it can be difficult to manage the raised temperatures.
And if the gases are not going exactly where they were intended, they can disrupt the car's aerodynamics rather than improve them - this could lead to the instability Hamilton and Button were experiencing on Friday.
Lowe said McLaren needed to do more work on the device to fully understand what was happening.
Test driver Gary Paffett returned to the McLaren factory on Friday evening to drive the team's simulator, which was set up to replicate the car without the new diffuser to give the team's engineers some data from which to establish a baseline set-up for Saturday practice.
Ferrari, the third team in the championship battle, introduced the device at the last race in Valencia without problems and are continuing with it here at Silverstone.
Hamilton, who leads the drivers' championship by six points from Button, said on Friday: "We haven't got the optimum out of the car yet but more is to come. We have to nail it on Saturday.
"I don't know if [the update] makes as big a difference as I thought.
"It's something you have to tweak, to adjust but at least it's running well and I do feel a difference.
"We have the potential in the car to go faster but it wasn't just bolt it on and go faster."
Button added: "We haven't had the perfect day. It has been trickier than I expected in the car."
Hamilton feels McLaren have made progress
Button's best time, 13th fastest, was 1.966 seconds behind Webber's best time around the modified Silverstone circuit.
The defending world champion said he had struggled with the balance of his car and to make matters worse had a broken heat shield - which protects the rear suspension and bodywork - during the second session.
Sebastian Vettel took pole and a dominant win for Red Bull at Silverstone last year and after victory last time out in Valencia, the team laid down their marker in Britain.
Vettel and Webber, who between them have captured eight pole positions this season out of nine races, topped a practice session apiece with Webber clocking the overall fastest time of the day.
"The Red Bulls are still very, very fast for sure," warned Hamilton, who won the British Grand Prix in the wet in 2008.
"I can't stand here and say we are faster than them no problem.
"I definitely think we've made a step forward from Valencia. This is track more like Barcelona where you'd see normally a second gap between us and Red Bulls but I don't think you'll be seeing that gap tomorrow.
"It'll be a lot closer if we get the set-up right."
Button added: "The Red Bulls look very fast and it will be tough to challenge them.
"It's very difficult to say but on high fuel the car seems to work pretty well but on lower fuel it's a bit more tricky.
"[Qualifying] is an area we have to be stronger in - at the moment we're not but there is so much information it's just a very different car from what we've driven before.
"I think we can still do a good job this weekend but it's not going to walk in the park."
Webber said he was expecting the fight for pole to be much tighter than Friday's form suggests.
"I think some other teams are sandbagging a bit but we're happy with our programme and looking forward to qualifying," Webber added after practice on Friday.
"McLaren aren't showing everything. I think they're waiting to unleash the car tomorrow. Definitely expect them to come right back up to us tomorrow."