US GP: Nico Rosberg to run old Mercedes exhaust in practice

The British-based outfit switched to a concept harnessing the Coanda effect from the Singapore Grand Prix, after tests at Magny-Cours showed that the new design delivered a big step forward in performance.

However, team principal Ross Brawn said that a raft of other developments that have been added to the car in recent races have left Mercedes unsure of which areas of the car are working best.

In a bid to answer this question, Michael Schumacher will continue to run the new exhaust while Rosberg uses the old one.

"One car will be with the Coanda exhaust and one will be without, which is part of an assessment programme for changes we have made," explained Brawn.

"For Friday, we will be running part of the time at least one car without the Coanda.

"There are a lot of other changes we have done on the car as well. We know what we are getting from the Coanda and we want to see how it correlates with our predictions and models.

"We want to go into the winter knowing exactly where we are, having all the information we need."

Brawn was adamant that the decision to use the old exhaust was not a sign that the team could abandon the Coanda-effect exhaust completely.

"No, it is part of an assessment of measurements we are doing," he said. "It is an incredibly complex topic the exhaust systems. It looks relatively simple but it is quite complex."