Niki Lauda has rejected suggestions Renault is trailing the pace by a figure of 80 horse power.
Red Bull's Dr Helmut Marko has mentioned the figure - dominant Mercedes' supposed advantage with its field-leading turbo V6 - on several occasions in the past weeks.
But Lauda, the Mercedes team chairman, said on Saturday: "How can anyone really know?
"The answer is that no one knows the horse power figures; not Mercedes, not Renault, not Ferrari," he told German television RTL in Shanghai.
"We all know roughly what our engines are producing, but not the differences," triple world champion Lauda continued.
"You can make an estimate based on GPS data," he said, "and how fast the cars are on the straight, but it also depends on whether the tyres are working, whether the aerodynamics are right ...
"So many factors come together that you can't clearly say that someone has 20 horse power more, or 80. We cannot judge that," added Lauda.
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