Back fire EFI?

[Tom.A]

I took a ride on a back road Today and got on the car pretty good up through forth gear. I let off and it back fired through exhaust. I recall it doing once before but at that time I was not sure if it was a back fire or the exhaust bottoming out. I am now sure it was a back fire also. The engine is bone stock fuel injected LS3 with Speartech E38, I tried to call John Today but he was closed for some race event. I have approx 500 miles on it and it is a crate engine. I have not had it tuned yet as I am researching what cam I want to go with. Car runs fine other than a little rough idle ~750 RPM. I need to work getting the breather system done but not sure if that played into it at all. I know Speartech puts a baseline tune but suggest that you get it on a dyno. Anybody have any other clues other than just needing a tune?