Amid swirling speculation about the future, Honda is still pushing to improve its 2017 engine.
There are claims that McLaren and Honda will divorce, but the struggling Japanese marque's recently-launched 'spec 3' unit has been reliable so far and powered Fernando Alonso to the fastest race laptime in Hungary.
"I don't know if this is the end of the big reliability problems," said a McLaren source. "You never can be sure."
It is reported that McLaren intend to make a decision about their next engine partner in September, but a few days before that, Honda's 'spec 4' will apparently make its bow at Spa-Francorchamps.
Marca said that it will feature an extra 15 horse power, with another 40hp to come after that, getting the Honda to within 45hp of the leading Mercedes and Ferrari.
"We will see if Honda gives us another evolution after the summer. We need it," said team boss Eric Boullier.
McLaren are now second from bottom in the constructors' championship on 11 points.
