Toro Rosso tech boss James Key has assessed the performance of Renault's power unit and believes the French manufacturer has turned a "huge corner".
Renault Sport redesigned its V6 turbo hybrid engine, stating the new unit is 95% different to its 2016 predecessor.
Despite a series of minor failures which limited Toro Rosso's mileage last week in Barcelona, Key was very encouraged by the engine's performance.
"The good thing with Renault on the performance side, since we've been working with them, they've definitely turned a huge corner," said the British designer
"Our impressions are positive. Exactly where it stands in the order is impossible to say at this stage, but there's definitely a marked improvement from what we saw in the past, and last year as well.
"You can see that; it's real. It's not just a value coming out of a dyno. So from a performance point of view we're happy with what we're seeing so far."
Toro Rosso was a Ferrari customer last year before reverting back to Renault which Faenza-based outfit has worked with in 2014 and '15.
Renault has remained guarded on the issues which affected Toro Rosso's running last week, but Key ruled out they were a consequence of the installation design.
"I think the reasons for the issues clearly need to come from Renault," Key explained.
"We've had some dramas and I don't think it is installation related from what we can tell.
"Obviously, anything that has happened on the power unit side we've had discussions about that, and it doesn't appear to be do with anything specific on our car."
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