McLaren have pinpointed next month's Spanish GP as the race where they will start to form a clearer judgement on their 2018 car.

In points terms, McLaren have made their best start to a season since 2014 and sit fourth in the Constructors' Championship, but their MCL33 has struggled in qualifying with the team one of only three teams yet to reach Q3 at any round so far.

A late switch to Renault engines for 2018 was followed by reliability problems with the new car in winter testing and McLaren have admitted their upgrade schedule is behind schedule.

They now expect their biggest upgrade package to arrive in time for the Spanish GP next month.

"We didn't turn up in Australia with the car we wanted because we were delayed for some reasons so a lot of parts and upgrades are on their way to hit the track," said Eric Boullier, McLaren's racing director.

"For logistical reasons it's very likely going to be in Barcelona and from that point we will see performance wise where we are."

McLaren's poor qualifying performance in Bahrain, when they lapped two seconds slower than the pole position pace, prompted what was described as an 'emergency debrief'.

Boullier admits there are no excuses and they need to close the gap to the two teams with the same engine.

"We have the same engine as Renault and Red Bull and we are behind," he said.

"There is no hiding [place], we need to understand why we are slower in qualifying, why we were better in the race and why we are behind the other Renault teams. Full stop.

"There were some questions asked in Bahrain and there is still an ongoing process because we have to address it, we will address it and we're about to address it."

Boullier added: "We all know that Formula 1 is not the easiest sport, so it's difficult, and we are definitely not happy where we are today. We have a lot coming in the pipeline so hopefully it's going to help us catch up a little bit the gap we have."

Is McLaren's car carrying too much drag?

Despite swapping an underperforming Honda engine for Renault's units, McLaren's cars have consistently remained among the slowest cars on the straights at the start of this season - raising questions about the efficiency of their car's aerodynamic package.

Boullier acknowledges the lack of straight-line speed is a problem, but says it is not all about an excessive amount of drag.

"This is obviously one of the issues, our top speed is not the best one because we are bottom of the ranking," he said.

"But it's not the only one, it's not as simple as that. If it was only a question of drag that would be easy to fix. You have to address fundamentally all the aspects of the car to make sure we are where we should be."

But were McLaren ambitious enough?

While the team's development schedule has run behind schedule, Boullier also conceded the team may have not have been ambitious enough with their design targets during the winter.

"The car has matched with the targets so that means maybe the targets were not the right ones. So we need to revise in terms of ambition what we need to achieve," he said.

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