The butterfly valve actually controls flow rate. The volume of air/exhaust gas mixture is constant based on cylinder volume but the mixture ratio can vary dependant on the butterfly position. The main purpose of introducing exhaust gas to the combustion chamber is to reduce the oxygen content thereby reducing combustion temperature. This in turn reduces NOX or cleans up emissions and at the same time reducing miles per gallon.

Just wanted to clarify the thought process of installing it in the first place.