By Alan Baldwin
AUSTIN, Texas (Reuters) - Renault look like having no takers for their upgraded engine in Sunday's U.S. Grand Prix after Red Bull's Daniel Ricciardo said it did not look enough of a step up and Toro Rosso said they would stick with the old one.
"It's available if we want to use it. Obviously that would mean a penalty, though, so we've got to understand if it's worth it," the Australian told reporters at the Circuit of the Americas on Thursday.
"Last I heard it (the performance improvement) is not massive so from my understanding it's probably not worth taking it."
Renault have said the engine is more powerful and efficient but using it would incur grid penalties for Red Bull and Toro Rosso who have exceeded engine allocations for the season.
The two Red Bull-owned teams are Renault's only partners.
Red Bull's relationship with the French manufacturer has already reached breaking point although the two sides could continue together next season if the team cannot find an alternative.
Mercedes have ruled out a supply while Ferrari appear willing only to provide 2015 specification engines to Toro Rosso and not both teams.
Renault have struggled with the new V6 turbo hybrid power units, with their version lacking the reliability and performance of champions Mercedes.
"I think we're down such a chunk (of power) I don't think we're going to gain enough initial time to make it up," said Ricciardo of the upgrade.
"So if we start from the back I don't think we're going to make up the ground we need.
"Right now I'm probably less likely to take it (the new engine) but hopefully today they will tell me something different and we've found a bigger chunk of horsepower from it but realistically there's not a whole lot."
Toro Rosso, who have Spaniard Carlos Sainz and Dutch teenager Max Verstappen, will stick with the old specification to the end of the season.
"It's not worth it so we will commit to this year's spec that we have now, that's good enough to finish the year, and we have plenty of them," said Sainz.
Ricciardo said he was confident he would be racing next season but the concern was whether or not whatever engine was in his car would be up to scratch.
"I'm confident I'll be on the grid. It's just the concern is can we be competitive?," he said.
(Editing by Tony Jimenez)