Originally Posted by chanson16
That is what I thought. I have been doing AG fit a while and like the full boil minus the extra time it takes.
Do you think it is right that the IBUs would nearly double just by adding 3 gallons to the boil? Seems like a large jump.
That is definitely correct. Better hop utilization is part of that because with more volume you are able to dissolve more iso-alpha acids into the wort resulting in higher IBU's.
The other part is that when you add top off water to a partial boil you aren't just diluting the wort gravity, you are also diluting the IBU's, hop aroma, hop flavor, etc. So you are getting all of your IBU's in about 2.5 or 3 gallons of wort then you are diluting them with water and effectively reducing them by about half. This can be accounted for by just using more hops in a low hop beer, but in something like an IPA you max out the IBU's in the partial boil (many people think wort can only hold about 100 IBU), then you dilute that by half resulting in the maximum amount of IBU being around 50ish. And 50 IBU or less doesn't make for a great IPA in my opinion.