Winterbottom qualified fifth and fourth for the respective heats despite suffering from an undiagnosed issue.
Team principal Tim Edwards told Speedcafe.com that the team elected to change the engine rather than work on diagnosing the problem.
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“It just wasn't quite right, so rather than playing around changing coils and injectors and trying to chase what the problem was we just elected to change it,” he said.
“Frosty noticed something in the first qualifying session, but it was difficult to quantify what it was in the data.
“We can see in the microsectors that he was very quick in the corners but just struggling a bit in a straight line.
“It's hard to say what lap time he would have done without the problem, but for a driver to notice (it's significant).”
With two hours between qualifying and the opening race, the Prodrive crew made short work of the job.
“To get it done and having it cranking over in 36 minutes is a great job by the team,” said Edwards.
“It's moments like these that the teams thrive on because they earn their money.
“We saw it with (Craig) Lowndesy's car in New Zealand. It's times like this you appreciate how good the teams are.”
Heading into today with a 239 point leader over nearest challenger David Reynolds, Winterbottom could wrap up his first title as early as tomorrow.