help with the math

Need a little help crunching the numbers here...

I'm switching over from extract to all-grain, but I have one last extract kit sitting around that needs to be brewed. It's the Midwest Brewing Bourbon Barrel Ale. I have a shiny new 10-gallon kettle, so I want to try this one out as a full boil just to practice going through the motions for the upcoming AG brews.

Here's my issue: I've been running the recipe through the hopville beer calculus to adjust my hop additions for the increased utilization, but according to that I wouldn't even hit my target IBUs if I added everything.

I know bitterness isn't an exact science, but maybe someone with more familiarity running the numbers can tell me where I'm going wrong. My pre-boil gravity should be ~1.075. The full hop additions called for in the recipe (assuming a partial boil) are:

- 1 oz. of Northern Brewer pellets (8.5% AA) @ 60​

- 2 oz. of Willamette pellets (4.9%) @ 30​

- 2 oz. of Fuggle pellets (4.6%) @ 20​

That gets me ~70 IBUs using Tinseth's equation if I added everything. The recipe calls for ~78 IBUs with a partial boil. How can I be coming out less bitter using all of the hops and a full boil?

 

You don't give us very many details about the recipe. What's the boil gravity of the partial boil supposed to be? If it's a late addition recipe, then it's quite possible the boil gravity of the partial boil is lower than yours, and will therefore extract more alpha acid.

 

Well...I guess you've exposed a flaw in me thinking I know what I'm talking about. I'm getting the ~1.075 boil gravity from the recipe's target OG. I'm not sure what the boil gravity is supposed to be for the partial boil.

The fermentables are:

9.3 lbs. amber LME
8 oz. Crystal 50
8 oz. Brown malt
4 oz. wheat malt
4 oz. chocolate malt

The grains are steeped and the LME is to be added at the start of the boil. Anything else essential I'm leaving out for someone to help figure out the partial vs. full boil hops additions??