A boost bottle is completely different kettle of fish altogether. They are/were mainly (but not exclusively) used on 2-strokes and are fitted between the carb (or throttle body) and the engine's inlet port (in the inlet manifold or intake tube). The Yamaha boost bottle was known as YEIS (Yamaha Energy Induction System). The theory is that, even with a reed valve, some air/fuel mixture will end up flowing back in pressure waves towards the carb when the piston is in certain positions and ports are not completely open or closed, so the extra volume provided by the boost bottle (YEIS) absorbs some of these pressure waves and evens out induction. This back flow of fuel/air mix causes richer than ideal running conditions and power losses. Power gains can be had as a result of adding a boost bottle on a 2-stroke and where the power is gained can be pushed higher up or lower down in the rev range depending on the volume of the bottle. The YEIS system was set up to improve (or boost) low down and mid-range power and torque when a 2-stroke is least efficient (out of its power band).



The XT resonator is upstream of the throttle body and is there just to even out the pressure waves in the air flow, and the main reason for this is silencing induction roar on a big 4-stroke single. It makes no difference to power at all.

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