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Why would you want to change from the delco? Its has more flexibility than most after market ECU's.
What he says ^^^^
Short of a wild motor the factory delco efi unit and a tune performs fantastically well. Same goes for the factory ignition setup (in good condition). The biggest single gain I have found is replacing the coil (often the original one) with a MEC transformer style one from Bosch. Obviously make sure the leads, plugs, rotor button/cap are all in top condition too. Reco injectors also help if the originals have never been professionally cleaned.
Reaper