I added a Detroit Bypass Valve to my mini a while ago after the spring in the stock one broke and wasn't closing all the way....bleeding off most of the boost.

I decided to install a Detroit BPV with a stiffer spring instead of a stock BPV. It seems to work great....came into the boost a little sooner (stiffer spring), but I have noticed that my overall average fuel MPG has went from around 29mpg to around 25mpg. I'm guessing that this is because the stiffer spring is building more boost at any throttle position cmopared to the stock unit.....so even when I'm easy on the gas, I'm building more boost with the Detriot BPV. Right? They even say in the description that you are going to loose some MPG.

For those of you who have went with the Detroit BPV....how much MPG did you loose? Have you ever thought about going back to a stock BPV? As long as it's adjusted right, a stock BPV should make the make the same boost as the Detroit unit at WOT.....but it won't make as much at park throttle...which is better for MPG.