A Fuel Management Unit (FMU), also known as a rising rate fuel pressure regulator, is an alternative to an ECU upgrade. This is *not* the same thing as an adjustable fuel pressure regulator (AFPR). The AFPR will not change "on the fly", it's "set it and forget it". The FMU is used in boosted applications. The problem is simple: the ECU from the factory will not send enough fuel for a proper air/fuel mixture under boosted applications. The common solution is to upgrade the ECU to one that has been pre-programmed for your injectors, or one that allows you to adjust the programming for your injectors. The FMU, though, can "trick" the system and correct the fuel delivery. It has a vacuum line that measures boost, and as boost increases, it increases the fuel pressure, forcing the injectors to over-flow by a corresponding amount.
The FMU is not considered a reliable solution for high power applications, but for a low boost (under 7psi) situation, you can get by with this method. It is generally cheaper to get an FMU than an ECU upgrade.
As always, be conscious of the somewhat agressive stock timing on the 240SX. An FMU will not adjust timing, and under boost you can end up with the dreaded detonation. The MSD Boost Timing Master is a popular solution to this problem. However, it should be noted that the price of an FMU and an MSD BTM will be close to that of a CalumSult ECU setup, which pretty much resolves all ECU concerns. The only catch is CalumSult does not work yet on OBD II cars (96-98 240sx). This is a work in a progress and by the time you read this it may be resolved.