If the FMU is ramping up the fuel too fast, then yes... it can cause a backfire. Are you sitting stationary and revving the motor up to redline time and again? If so then stop doing that. lol The FMU is designed to increase fuel pressure for load conditions not no load conditions. If it's backfiring when you're moving and under power then the ratio of the FMU is too high for your injectors and application.
i have 24 lbs injectors in it now. would you reccomend stepping up to 30's or 36 pounders ? its has a vortec s-trim pushing 9 lbs. of boost. My friend suggested larger injectors. Also if i keep the gas pedal at a certain position it wont do it. never at idle only at speed when i let off the gas pedal....
i bought the car second hand but i do know it has a cam, bored 30 over ( 306 ), shorty equal length headers, 4.10 gears, stock t-5, cat-less h-pipe that flows into flowmasters then dumps... also has jr heads . it sounds killer until it backfires and people start running fo rther lives... lol
Alan is right. Properly sized injectors and a custom tune are infinitely more adjustable and accurate than an FMU and under size injectors. The higher you press the fuel pressure up, the more inefficient the spray pattern of an injector becomes. FMUs also have no regard whatsoever of air/fuel ratio through the power range. They do what they do and AFR is either within range or it's not. The only way to correct that (if it's not) is with a (wait for it)... custom tune! If you in a spot where you need that tune anyway then it may as well be with an injector that correctly sized for your application.