How does a Boost Bottle improve performance? Air rushes thru the carb on the intake cycle. Ignition accelerates the piston down, forcing the reeds to snap shut. The momentum (inertia) of the air has to go someplace. It goes back thru the carb causing a double rich mixture. The result can be rough idle or hesitation.
Due to the high velocity of the fuel air mixture through the carb boot, we need to take advantage of this momentum/inertia energy. The Boost Bottle and tube provide storage for the fuel air mixture between carb boots. The energy (pulse) forces the pre-atomized mixture in the tube from the previous cycle into the adjoining carb boot where the reeds are just opening and ready to receive the fuel air mixture. The boost bottle and tube provide a "boost" supercharge effect to each cycle.
The boost bottle is most effective at lower and mid-range rpm, and, when you are on & off the throttle. It assists in filling the cylinder (shorter distance) for a fraction of a second until the carburetor venturi gets enough velocity to fully atomize the fuel. The advantage is that it brings up the torque and horsepower in the bottom of the rpm range, up to where the pipe(s) starts to work. There is a small benefit at high rpm's.
What Size? Singles = usually the CC of one cylinder or larger. 700cc Twins = 300cc boost bottle. 700cc Triples = 225cc.
Variables: Engine CC's, Carb Size, RPM below pipe, Number of cylinders. Large carbs like larger bottles/tubes. Triples like smaller due to less time between pulses. I use the TSR computer software to calculate the Boost Bottle for a specific set up.
Yes they work, when the size is right, the inertia of the air is stored like a spring in the boost bottle to help charge the next cycle. Try tuning a Yamaha Phazer without it's boost bottle!
Power valve engines may not have similar performance gains because the valves will keep the bottom end crisp. Testing w/ powervalves will probably show gains off idle and low bottom end. Is it worth it?? That depends on what you are willing to pay for some torque and a couple of horses!