By Patrick Johnston
SEPANG, Malaysia (Reuters) - A new promising nose box failed to lift the concerns of McLaren's Jenson Button that his team could struggle to match the pace of the Mercedes at the notoriously quick Malaysian Grand Prix on Sunday.
The Briton was fourth fastest in Friday's opening session but was down in eighth in the second 90-minute run, with team mate Kevin Magnussen fifth and then 12th as first Lewis Hamilton and then Nico Rosberg set the pace for Mercedes.
"We're finding it tough out there today especially in the heat," Button told reporters behind the McLaren garage where track temperatures peaked at more than 50 Celsius.
"We're not as strong as we'd like to be in high-speed corners and, when you have downforce issues, they're further amplified by the hot weather.
"So our long runs were tricky in terms of both tire degradation and outright pace," said Button.
"The upgrades we brought here seem to be giving us something so that's encouraging but there's a lot of work still to be done."
Button, who finished third in the season-opener in Melbourne two weeks ago behind race winner Rosberg, said the team would need to keep working on their downforce issues throughout the season in order to start challenging for victories.
The first of those was the new nose box and McLaren racing director Eric Boullier was pleased with its introduction, saying he was confident engineers would work to fix the issues they encountered in the second run.
"This morning's session was an important one for us because it gave us the first opportunity to carry out on-track correlation of our new nose box," he said.
"Initial impressions were positive. Both Jenson and Kevin were immediately able to put in competitive lap times this morning and both guys also reported that our car felt good.
"This afternoon was a little trickier. The changes we'd introduced between the two sessions didn't make the improvements we'd been hoping for."
Magnussen's car came to a halt in the morning session before being pushed back to the garage and the 21-year-old then complained on the radio during the afternoon run that the car was oversteering.
The Danish rookie, who finished second on his debut in Australia a fortnight ago, said he had learnt a lot from the experience of driving in the testing Malaysian conditions ahead of Saturday's qualifying session.
"I've never driven a car in these sorts of temperatures," he explained.
"The track temperatures are so hot here that the tires are really suffering but it's the same for everyone so we need to try and get on top of things before tomorrow's sessions.
"I'm going to try and set the car up in order to protect the rears," said Magnussen.
"There's always something you can improve and we've currently got a lot to think about and a lot of data to sift through tonight.
"For me, overheating the rears is my biggest problem, the tires are behaving very differently from how they did in Australia so that'll be my focus."
(Editing by Tony Jimenez)