Red Bull's Daniel Ricciardo says Renault's upgraded Formula 1 engine is not any better than the old spec, adding the French manufacturer needs to "go back to the drawing board".
Renault used 11 of its 12 engine development tokens on its new package and ran it for the first time in Brazilian Grand Prix Friday practice on Ricciardo's car only, hoping the upgrade was worth around two tenths of a second.
When asked if making the change, and starting 19th as a result of a 10-place grid penalty, was worth it, Ricciardo said: "For the grid penalties no, but for us, getting some clarity on where it is, yes.
"We have seen it hasn't really given us anything, so back to the drawing board for Renault.
"We have to try and find some more from it but I don't regret testing it because we had to try.
"We had to see if there were any positives from it and if what they saw on the dyno was replicating what we felt on track.
"In hindsight, it didn't give us any laptime, it's just for now a penalty for a little bit of knowledge."
When asked if it provided any benefits at all, Ricciardo said: "If I'm honest, no.
"I haven't see the Q3 laps, but in Q1 and Q2, I was slower than Danill [Kvyat, running the old engine] down the straight so it was a possible disadvantage."
The Australian called on Renault to up its game, especially as it now appears Red Bull is closing on a renegotiated deal with the French manufacturer for next season.
"We need something better, we need to try a different path or something if we're going to continue together next year," he said.
"I'm sure there are things we can learn from it or Renault can learn from it.
"It's now about trying to put together things they have learned and actually give us lap time now.
"I would say the last two years, we have learned a lot with Renault with these power units, but not a lot has equated to lap time.
"Hopefully there are some conclusions to be drawn from this engine and we have only run it for two days, so hopefully there is still something in there we can extract from it.
"For now, it hasn't proven to be the ingredient we wanted.
"But it's better trying it now than at winter testing. Parts of me are still optimistic."
Kvyat, who starts sixth, said he was unlikely to take the new engine for this race or the finale in Abu Dhabi.
"I think the difference was not worth it so we stick to our agreed plan," he said of the Brazilian GP.
"At the moment, no [there is no interest in taking it for Abu Dhabi].