I'm going to try and ask this question as simply as I can, although I suspect it's a complex answer....
When brewing with extract, and using only a partial boil, what affects the potential hop utilisation, the volume of water in my boil, or the gravity of the water in my boil...or both?
Now for my example...let's say i'm doing a 2 gallon boil. I steep my specialty grains for 30 minutes in say 1 gallon @ 160f, then remove the grain, bring my volume up to 2 gallons, and bring that 2 gallons to the boil.
As soon as I reach the boil, I turn the heat off, and add enough extract to bring my gravity to around 1040. Mix well, re-start the boil. When boiling add the hops.
It's at this point i'm curious. If my gravity is 1040 during the boil, will my hop utilisation be the same as if I was doing say a full volume/6 gallon boil at 1040? Or is the amount of bitterness extracted from the hops during the boil affected by how much water i'm boiling in?
To be really clear, I fully understand that if I added ALL of my extract at the start of the boil, as it seems brewers used to do, making my boil gravity really high (say 1080), that would reduce my hop utilisation. But i'm not doing that, i'm happy to get my BG to 1040, boil my 2 gallons, add the remainder of my extract at the end of the boil, cool the wort and add it to the fermenter, then top up with cool water to 5 gallons and ferment.
Question is, do I need a little extra hops in a partial boil to get a similar IBU as i'd get from less hops in a full boil?