Hot on the heels of the new F-duct controversy, Mercedes has turned the heat back onto Red Bull.

Amid complaints from rival teams including Red Bull about its innovative aerodynamic concept aboard the W03, Mercedes' Ross Brawn admitted he sensed a diversionary tactic.

"The discussion about our system has diverted the focus from the exhaust issue," he said in Australia.

The FIA has clamped down hard on the exhaust-blown diffusers seen throughout the grid last year, but much pre-season chatter focused on some teams' post-ban 2012 solutions that reportedly fly close to the boundaries of the new rules.

But now Germany's Bild newspaper reports that Mercedes suspects Red Bull is also bending the rules in another area.

The report said Mercedes' sound analysis indicated Renault-powered Red Bull is deploying an "illegal engine trick", apparently involving the turning on and off of individual cylinders.

Mercedes' competition vice-president Norbert Haug commented: "There is no official protest by us. But there are some questions that we are asking the FIA."

Red Bull's Dr Helmut Marko hit back: "No one will find anything. We comply with the rules."

German specialist magazine Auto Motor und Sport, meanwhile, reports that Melbourne winner McLaren currently has the most sophisticated exhaust solution on the 2012 grid.

"I wouldn't say that it's illegal," said Peter Sauber, "but it's borderline."

Teams including Mercedes, Lotus, Toro Rosso and Williams reportedly used much more conservative solutions in Australia.

"First we wanted to see what is allowed and what is not," said Williams engineer Mark Gillan.