McLaren-Honda did not reveal the level of performance they expect in 2016 as they unveiled their new car.
The former champions were ninth of 10 teams last year as Honda struggled with its engine on its return to Formula 1.
McLaren chief Ron Dennis repeated his confidence the partnership would win but said: "We'll make no predictions about when those wins will come."
Jenson Button said the car "really looked the part" and team-mate Fernando Alonso said he was "raring to go".
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Button, the 2009 world champion, will be first behind the wheel of the MP4-31 at the start of pre-season testing at the Circuit de Barcelona-Catalunya on Monday.
He said: "I'm not about to make any over-optimistic predictions - F1 is far too unpredictable to do that - but from what I've seen so far the aero detailing on the car looks fantastic and I'm really looking forward to beginning the process of testing our innovations."
Two-time champion Alonso added: "Last year was a tricky season for all of us but we learned a hell of a lot.
"The whole car is beautiful - it's particularly nicely packaged from an aerodynamic point of view and I'm 100% ready for the challenge ahead."
He said the new car was "impressive" and added: "It is a privilege to be part of this project."
However, the low-key nature of the comments from key figures provided on a team statement suggests a degree of realism about the step forward McLaren can expect.
The car is an evolution of last year's chassis, which was one of the more competitive on the grid but was hamstrung by the Honda power-unit's lack of performance.
The Japanese company has made changes to the areas which caused the engine's weaknesses - which were focused on its energy-recovery system.
Honda F1 boss Yasuhisa Arai said: "We won't know exactly where we stand until the end of the two tests."
It is not expected that Honda will have been able to close all the gap to the standard-setting Mercedes engine over the winter.