What do I need to look at adjusting in order to modify a 2.7T file for a returnless fuel system?
In regards to ECU tuning? Nothing.... or am I misunderstanding the question?
You'd just need the parts kit (pump, filter/regulator, fuel pressure sensor, manifold pressure sensor, and PCM).
Beware, many of the less expensive kits (like the LS1 kits) don't have the ability to either:
1. Sense and respond to boost pressure and increase fuel pressure on a 1:1 ratio w boost, or
2. Reach an overall operating pressure at least equal to desired fuel system pressure plus your max boost. Say, 58 psi fuel system pressure (~4 bar) plus 22 psi peak boost requires 80 psi system pressure in order to maintain 58 psi differential.
Some of those kits get pretty costly.... like over a grand.
