Boost Bottle

This article has been plagiarized

Much of the information and images for this article has been plagiarized from various sources. It is kept here only for reference.

In a naturally aspirated engine, on the intake stroke, the piston
drops creating an area of low pressure in the combustion chamber that is less than atmospheric pressure and as the intake valve opens, the air from the outside is set in motion down the IM runner.

Once air (as a sound wave) has been set into motion down an IM runner, it does NOT simply stop when the intake valve is closed shut and wait for the intake valve to re-open.

Instead, when the intake valve closes shut, this air sound wave bounces off the backface of the valve and travels at the speed of sound back up towards the IM carb throat (rarefaction wave). This reflected wave has a frequency, amplitude, and negative pressure associated with it.

Once the wave reaches the carb, the resonance wave is isolated and the carb chamber behaves like a resonance chamber. What is a resonance chamber?

The analogy used by most mechanical engineers to explain how a resonance chamber works is that it acts like an oscillating spring (i.e. imagine the carb acts like the spring) with a block attached on the end of the spring (imagine the air wave in the IM runner to behave like the block) . As the block compresses the spring, the spring builds or stores up energy and when the spring uncoils, the block is given a push or energy as it travels away from the spring's compressed position.

Like our block and spring, the air resonates (or compresses the spring) at a certain frequency (spring bouncing back and forth) inside the intake tract and gains energy (pressure) . The air wave is then bounced back at the speed of sound down the IM runner towards the intake valve again. But this time it has been given an extra "push" from the resonance chamber. The new sound wave going to the intake valve has a positive pressure and is travelling at a higher tone or energy (higher sound frequency).

The bouncing back and forth of sound waves from the closed intake valve to the plenum and then back down again occurs over several intake valve openings continuously. Why does this happen?

These reflected resonance waves don't reach the intake valve when it re-opens and therefore continue to reflect. This continues until several reflected air sound waves (or columns) stack up (amplified) at the closed intake valve. The energy (or pressure) of these amplified ( or stacked up ) reflected waves build up until they reach a maximum energy (and pressure).

The trick to resonance (or sound) tuning of the IM is to have these maximally amplified waves arrive at the intake valve just as it opens . The basic mechanism of intake manifold "tuning" or design is to provide high pressure at the intake valve so that the mass flow rate into the cylinder is boosted at a given engine speed or rpm. We do this "tuning" by changing the IM runner length and diameter (area).

By building up pressure from stacked resonating (or reflected) air sound waves (or columns) and releasing this "boost" at a specific rpm, you can get higher cylinder filling [ i.e. achieve a volumetric efficiency (VE) greater than the cylinder swept volume. The engine breathes at a VE > 100% ] . The reflected positive pressure waves from the plenum, when it arrives at the right time, actually pushes in more air into the cylinder beyond the effects of the piston sucking in air. Not only do you control the location of where peak torque occurs by varying runner length and diameter, you get a gain in power by using the plenum's resonance effect. This is what we call " acoustic supercharging".



Y.E.I.S is a very efficient invention. A small bottle, often called "boostbottle", is installed between the carb and the intake port connected with a branch pipe to get the engine to respond quickly from bottom end without flat spots.
It saves 10-15% in gas consumption and makes the engine wake up at lower rpm.

Like many of Yamahas inventions. When it first came out there were many doubters. Just as they doubted the wisdom of a separate oil pump and the 7th port. Or Torque Induction. But if done properly this does work.

The following diagram shows a typical Y.E.I.S setup. The most common mistake people make is not placing the boost bottle above the cylinder, not getting the size right and using a common bottle for two cylinders and failing to make sure that the bottle drains back without collecting fuel.


Since Carburators can carburate both in and out, when the charge is out of
phase with the engine it creates a double dipping intake charge (passing the
jets twice). This kills the engines ability to make Torq due to this RICH
condition, thus creating the Torq\HP DIP just before the engine starts to
take in the charge and use it all. This Power dip can be seen in both 2 and
4 stroke engines. The ~BOOST~ Bottle will help cure the problem by providing
a storage tank, of the required volume and distance to resonate at this RPM.
It stores the charge that once reversed back through the carb and returned
as a DOUBLE RICH condition.

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