Features 700 H1 4-Stroke Engine with EFI: The 2016 HDX has a powerful 700 H1, 695cc, liquid-cooled single-cylinder engine, except we've added a few improvements. The new closed-loop EFI system uses an O2 (oxygen) sensor to automatically adjust for ideal exhaust air-to-fuel ratio. This clean-burning engine produces excellent throttle response and superior thermal efficiency. We've also cut engine noise by 8 decibels.