Red Bull's engine supplier Renault has admitted it might not have closed the performance gap to standard-setting Mercedes over the winter.
Mercedes had a significant power advantage over rivals Renault and Ferrari as they dominated last season.
Renault Sport boss Cyril Abiteboul said: "We believe we have made a very big step in performance.
"We may not have erased all the gaps but we are confident we have gone a long way to making up the deficit."
Renault were believed to have a deficit to Mercedes in the region of 50bhp by the end of last season and insiders say they have made up about that amount over the winter.
But Mercedes are also said to have added at least 50bhp in performance to their engine since 2014.
Unlike last season, manufacturers can make a limited amount of developments to their engine over the course of the season.
Renault Sport, which has been restructured, said in a statement on Wednesday that on the engine "we have made some fundamental changes to gain performance and reliability".
The main changes on the turbo hybrid V6 for 2015, it said, were to the main internal combustion engine, the turbocharger and the battery.