After a winter of predicting a strong title challenge, Renault's decision to focus on reliability early in 2018 has left Red Bull boss Christian Horner worried.
Though insisting progress has been made after a number of failures late last year, the French manufacturer has admitted the power unit will not be running at full potential for the opening series of races to avoid problems.
Already facing a power deficit to rivals Mercedes and Ferrari, hearing the new RB14 could face a bigger disadvantage hasn't gone down well at the Milton Keynes-based team.
"It is a concern," Horner told Autosport.
"Reliability has been a big priority for Renault over the winter and they have completed more mileage than they have done in this V6 era so they are coming into this year better prepared and hopefully that will allow focus to start to centre on performance.
"We're hopeful of seeing more performance and performance converging between engine manufacturers."
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The problem Red Bull and Renault also have is the class-leading Mercedes isn't standing still, with a heavily upgraded engine of their own for 2018 with an output reportedly nudging 1000 horsepower.
"If the headline figures Mercedes have quoted [are real] then the gap will only broaden," the Briton predicted.
On the first day of testing, everything went smoothly, however, as Daniel Ricciardo topped the timings and was the only man to complete over 100 laps in Barcelona.
