Never tested with or without on the dyno but my results are opposite of what you would expect. With both a 1" open and then a 1" taper 4 hole I lost 2/10ths and 4 MPH. Now we have determined this is due to the lack of space between the carb and the hood of my car. I need to go up, but hood choices that are functional and look good for a 3rd Gen Charger are far and few between.