Adrian Newey expects Toro Rosso and McLaren - thanks to progress from Honda - to be a threat to Red Bull next season.
Red Bull will use a Renault power unit after failing to secure a supply from another engine manufacturer for 2016, though it will rebrand the engine as a Tag-Heuer. However, Newey is not expecting much progress from Renault compared to the other manufacturers and he believes both McLaren and Red Bull's sister team Toro Rosso are likely to move ahead.
"Toro Rosso, our sister team by having a 2015 Ferrari, will be considerably ahead of this year's Renault power unit," Newey told Motorsport.com. "I believe Honda, what we have seen, will make a good step. I think their internal combustion engine is a reasonable engine.
"Their problem this year has been that they made the recovery units on the turbo, the MGU-H, far too small. But that is an easy problem to address over the winter.
"So, next year's going to be very difficult for us. It's really down to, I think, what happens next with the sport, for 2017 and beyond. Is there desire from the governing body to allow private teams such as ourselves to properly have competitive engines or not?"
Honda motorsport boss Yasuhisa Arai recently told F1i the engine manufacturers target is to produce a "more sophisticated" version of its 'size zero' power unit in 2016.
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