From Autosport: http://www.autosport.com/news/report.php/id/121198
The Japanese manufacturer has spent its remaining four 2015 F1 engine development tokens on an internal combustion engine upgrade, plus associated exhaust parts
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Honda's chief weakness concerns the efficiency of its MGU-H.
Solving this problem requires a fundamental change in engine architecture, which means the Japanese manufacturer feels it cannot act until the winter, when more development time and tokens will become available.
Speaking to AUTOSPORT before the latest upgrade was confirmed, Honda F1 chief Yasuhisa Arai said: "The logical step is to change the compressor area, but because of the layout that's not going to happen.
"We are still looking to improve the internal combustion side of the engine, but without changing the layout."