Originally Posted by MrFatbooty
The VAFC is perfectly capable of adjusting the a/f ratio in both directions, through changes to injector duty cycle. You don't need to touch the fuel pressure.
There is a severe limit to the amount of fuel that can be added with a piggy-back such as a V-AFC or an Field unit, especially in the upper RPM ranges when the MAP signal is approaching its max value (leaving very little or no room for manipulation via the piggy-back). Most tuners (including one of the most capable on the east coast, employ the method of raising fuel pressure (and thus adding fuel) with an adjustable FPR, then leaning out where necessary using the piggy-back.