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ls1 edit can change many things, a few that i know of are
1. Delete codes
2. Increase timing
3. increase line pressure to the tranny (for autos)
4. Adjust for gear size and also tire size so speedo will be correct
5. Adjust A/F all across teh board
But make sure your "tuner" know what he is doing, the "tuner" can very easily blow the motor if they have no experience and just start adjusting too many things.