Insert reduces maf area. Flex tune will not work with this system due to design causing maf turbulence and idle variation. Straight 91/93 or straight e85 only. I have first hand experience as I tested early on and went back to stock in order to keep running flex tune. Sold mine to a member wanting to run a straight 93 tune and it worked well for them. So as long as not running flex it's a suitable setup. I will say iats with it were close to stock as well.Do you know what the "insert" is and what it does. I want to do a flex tune with this intake and I heard that it will only work with either straight 91-93 or E85, but lund has an option for this cai with or without the insert. Not sure what the difference is.