Blow-Off Valve: A blow-off valve is used to relieve boost pressure. Boost is created as throttle input is increased, and the engine’s RPM begins to increase. If throttle input is decreased, the boost pressure must be relieved. A blow-off valve allows this pressure to be vented to the atmosphere, and not enter the engine’s intake when it’s not needed. Sometimes with smaller boost installations of 5 psi, a blow-off valve may not be used. As boost pressure increases, however, the ability to relieve the boost pressure becomes increasingly important, as engine damage could result.
Bypass Valve: The blow-off valve is sometimes incorrectly referred to as the bypass valve, however, there is a difference between the two. To increase the efficiency of positive displacement superchargers, some manufacturers will allow some air to bypass the supercharger’s rotors at lower engine RPM speeds. This equalizes the pressure on both side of the rotors and increases the efficiently of the supercharger when boost is not required. This comes into play at cruising speeds or slow acceleration; basically, times when very little throttle is being used.
One of the side effects of the way superchargers work is that, when it’s compressing the incoming air, it also heats that same air being compressed. That lessens the density of the air some, and can potentially increase the heat in the engine. To combat this, an intercooler is often used. The intercooler is typically placed after the supercharger, and the compressed air is forced through it. Intercoolers are fed by engine coolant most of the time, although some race setups have the ability to facilitate the use of ice. While the fact that the air passing through the intercooler will cause some minor loss of boost pressure, the benefits of cooling the incoming air far outweigh the disadvantages of an intercooler.
A centrifugal supercharger uses an intercooler that is placed into the piping that connects the supercharger to the intake. In the case of a positive displacement supercharger, the intercooler is placed between the supercharger and the motor. The air exits the underside of the positive displacement supercharger, and is moved through the intercooler and then into the intake. Sometimes the intercooler is also referred to as a heat exchanger.
As a side note, a typical intercooler in use today would be more appropriately referred to as an “aftercooler”, since it cools the air after it has been compressed. Early intercoolers were placed before the supercharger/turbocharger, and used normal airflow to cool the incoming air charge. The name "intercooler" has stuck through the years, and persists to this day.