Many people claim their vehicle runs better on rainy or very humid days. You may have noticed this yourself. Under high vacuum and the resulting temperature drop, very humid air will form a cloud of water vapor. This fine mist will enter the cylinder and provide some of the cooling effect already discussed.

        In fact you may have seen drag car pit crew spraying something into the supercharger intake immediately prior to racing. That is an anti-freeze and is used to prevent ice from forming and jamming the throttle plates.

        The simplest system involves bubbling air through a container of water and passing this humid air into an existing vacuum line. This can be done very cheaply using ordinary aquarium air accessories and plastic tubing. It's even the right size to fit most vacuum fittings. An in line air valve is used to control the amount of airflow. There are two major drawbacks to using this method.

• Modern engines use vacuum from several  locations to control a number of different functions. Ignition advance, EGR, idle up and others all use vacuum. It is important not to upset the vacuum levels to these functions if you tap into these lines.
• The amount of water that can be admitted this way is very low, probably not enough to make a measurable difference to the engine operation.