McLaren say their new car will not suffer the reliability problems that blighted the team last year.
Raikkonen has expressed concern about the McLaren's engine
The team's engine partner, Mercedes, admits it has had teething troubles but says the car is now performing well ahead of the opening race on Sunday.
"The lap times improved continuously throughout the tests and the long runs were OK compared to the fastest," said Mercedes sports chief Norbert Haug.
Haug said the car's reliability had improved steadily in recent weeks.
In January, McLaren driver Kimi Raikkonen said he was worried about Mercedes' new V8 engine - introduced this year after a rule change.
Haug admitted the team had had problems at a test in Barcelona in late January but added: "In the following five and a half weeks or about 40 days until the final test at Valencia on 1 March, our team showed what it is capable of, be it chassis or engine-wise.
"Several times, our new engine stood the strain of two race weekends and up to 50%, on the circuits of Barcelona and Valencia as well as on the dynos [engine test beds]."
McLaren's 2005 campaign was dogged by reliability issues as the team was beaten to the driver and constructor world titles by Renault.
Regular wins were interspersed with problems that either forced Raikkonen into retirement or relegated him to grid positions from where it was impossible to beat rival Fernando Alonso.