The chip needs to be burned for the injectors. If it has a certain percent, that can vary the delivery by changing the size of the injector. If the chip is calling for 50% at a certain rpm with the stock(approx 30lb) this will be at 15lbs, then you go with the 42 at the 50% you get 21lbs. This can cause idle and other probs. This is just a simplified example using easy numbers. The bigger problem comes when you get to needing 100% and you only get the 30lbs but the car needs the 39 or 42 or 48lbs. Again just example, your car you will need to tune. Scanmaster or some other scantool numbers become necessary to get where you want to be.
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