I want to split an 8.5 gallon batch of IPA to compare a couple of different yeasts. Is there any problem with fermenting 4 gallons of wort in a 6.5 gallon bucket?
No problem at all. Extra headroom will fill with CO2 so no worries.
I don’t think you will have a problem in the primary. Active fermentation producing carbon dioxide will form a protective blanket over the wort. If you would be bulk aging or dry hopping in a secondary, with only residual carbon dioxide released from the beer, there might be an oxidation problem with that large of headspace.
I would be dry-hopping since it’s an IPA. I usually wait 7-10 days until fermentation seems to be over, then add the dry hops to primary.
Should I use another dry hopping technique with this much headspace?
I would not be concerned with dry hopping with that much space. You are only going to have it in there for a week or so.
After fermentation there is a lot of dissolved CO2, and a lot comes out of solution regularly and blankets the beer, so no concern at all. Just don’t go out of your way to splash the beer when introducing the hops :-0
