Fernando Alonso heavily criticised McLaren's Honda engine after another dispiriting day in pre-season testing.
The two-time champion said the Honda had "no reliability and no power", with only two days of testing remaining before the first race of the season on 26 March.
But the Spaniard said he was "enjoying" driving the faster 2017 cars and added: "I feel confident that this year we will be competitive.
"I don't know at which point."
Alonso said he felt said the performance of the McLaren chassis "feels good".
But he admitted the lack of power of the engine made it difficult to properly assess the quality of the car.
"I think we are 30 km/h down on every straight," he said. "When you are 30 km/h down on every straight, it is difficult also to have a feeling on the car. Everything feels good, but you don't know what is going to happen when you arrive at normal speed."
The McLaren was actually 26.4km/h (16.5mph) slower on the straight than the fastest car at the test on Wednesday, the Mercedes of Valtteri Bottas, which set the fastest lap.
Alonso was 12th fastest, 3.7 seconds slower than the Finn.
Honda said at the launch of the McLaren car last month that they hoped a fundamentally redesigned engine would by the start of the season be producing as much power as Mercedes did last year. But the engine is currently far away from that level of performance.
"Definitely we need to improve and we must improve," Alonso said.
"The situation is far from ideal, but we are a big team. We need a reaction from everyone, stay united, working close to each other."
Honda has had a series of reliability problems with the engine in the six days of testing so far, starting with an oil-tank problem on the first day which Alonso described as "amateur".
The 35-year-old is out of contract with McLaren at the end of this season and has said he will not make a decision on whether to stay in F1 until September.
But on Wednesday he gave the strongest indication yet that he will not retire.
"I want to win races and I want to be on the podium," said Alonso, who took the last of his 32 grand prix victories nearly four years ago.
"If everything goes in the wrong direction, I will attack next year.
"It brings me more motivation to continue and win because I will not stop racing without a good feeling and a good result that I think I deserve.
"If one day I am in the car and I see other drivers in the corners do fantastic lines, they brake later than me, they accelerate earlier than me and do better starts than me, on that day I will stop and say: 'It's time.'
"What I am seeing now is really the opposite, more than ever this year and this winter.
"What I see on the track and what I see in myself is at the best level, so now it's time to attack."