Last year's STR11 was regarded as one of the best chassis on the grid, but was held back by a year-old Ferrari engine.

Toro Rosso in currently fifth in the standings this year with 21 points, five less than last season at this point, despite running the 2017 Renault engine.

Key admitted this year's car is still lacking overall downforce, and fears the team will struggle to repeat its strong recent form in Monaco this weekend.

"I don't think we're as strong chassis wise as we have been for the past two years," Key told Motorsport.com. "We're certainly some way off our maximum downforce at the moment.

"We're at the right levels for tracks like Barcelona, and we're very similar to the group of guys we're in. It could well be the same at Monaco.

"Having said that, I think the power thing will help us at Monaco, with the sensitivity, and it's traditionally a track where we've been reasonably competitive with the mechanical grip, and the way our balance works.

"And the drivers aren't bad there either, so fingers crossed, we'll get that right."

Key, however, believes there is still plenty of room for improvement as the 2017 development race progresses.

"On the chassis side we're more immature than maybe we have been before with new regs," he added. "Because everyone started last year – no one's had 18 months at this, we've all had less than a year effectively, because the regs didn't get agreed until just over a year ago.

"So I think we're at a slightly more immature stage than we were at in the past, and I think there are some fundamentals still to find in some cases."

Toro Rosso is already 21 points behind rival Force India, who Key reckons is doing a great job.

"The guys who are getting it right are Force India at the moment," he said. "They are consistently scoring points with both cars: no dramas, good reliability, good pace and consistent strategies.

"That's allowing them to pull ahead, they are doing a great job. For the rest of us in that fight, it's all very close. It kind of hinges on every weekend at the moment; if you have a downer, you can lose a couple of places in the championship."