Carlos Sainz believes Renault's recent weakness relative to its midfield rivals is rooted in the lack of progress of its power unit.
The French squad had a firm grip on its fourth place 'best of the rest' position in the Constructors' standings up until the latter part of the summer.
But a performance slump of its R.S.18 has since enabled Haas to outscore Renault and move within just 8 points of its midfield rival, a dwindling advantage that Sainz has pinned on a lack of engine performance.
However, the works team has so far declined to adopt the manufacturer's new C-spec engine used by Red Bull, fearing its reliability is not yet on a par with the unit's performance gain.
"I think there was a big setback with probably the engine side," Sainz told Motorsport.com.
"We didn’t expect Ferrari and Mercedes to do such a big step through the year. We thought that the B-spec was probably going to be just enough, but we’ve seen at the moment that it is not enough.
"We brought things to the chassis, probably the chassis is not that bad, but on the top speed side we are not where we want to be."
While recognizing the sheer power benefit associated with the C-spec engine, Sainz reckoned its unproven reliability still made the unit a far too risky proposition.
Red Bull's own concerns over the C-spec's durability were somewhat alleviated by the fact that the team uses a different fuel supplier than Renault, hence its acceptance to race the new engine.
“The team is fully sure that with our fuel it is just not reliable enough, so I fully back the team with whatever decision they take," added Sainz.
"I just think we need more power, which we don’t have, and we don’t get it for whatever reason.
"If we wanted to be in Q3 in Japan we just needed more power, and Honda has showed that it is possible."
Gallery: The beautiful wives and girlfriends of F1 drivers
Keep up to date with all the F1 news via Facebook and Twitter
Kimi Antonelli is sitting comfortably at the top of Formula 1’s 2026 Drivers’ Championship standings,…
Former Aston Martin team principal Otmar Szafnauer has delivered a stinging verdict on Team Silverstone's…
Isack Hadjar believes one of his biggest challenges during his first season with Red Bull…
Down the years, August 4th has seen several German Grand Prix races held at the…
Pierre Gasly believes Alpine’s upcoming Dutch Grand Prix upgrade package will be “critical” to the…
Oscar Piastri may not have the results to show for his efforts in 2026, but…