Basically at 4000rpm's when you floor it boost comes on almost instantly.. When that happens, your FMU which is mechanical, cannot raise fuel pressure fast enough. Thats why you lean out for a second or two.

If you had a stand alone programmable computer, this would never happen because all fueling is controlled by the ECU.
with the fmu however there is a small amount of lag before it can pressurize enough fuel the specified boost..


Hope that helps !