Boil Volume Question for First PM

Hi all,

I'm interested in trying Partial Mash for the first time and I found this cool recipe (http://menuinprogress.com/2009/02/partial-mash-brewing-and-ipa-recipe.html) that's tailored for a 3 gallon stove top boil. I recently had the good fortune of acquiring an 8 gallon stock pot and would like to do a full boil. Is there anything wrong with following these directions exactly but topping off the brew pot before the boil instead of waiting until I'm done and topping off the finished wort? This tutorial also does a late extract addition. Is it still advisable to do that if I'm not doing a partial boil? Also, would I top it off to 6 gallons to account for evaporation? I apologize for the load of rookie questions in advance.

Here's the ingredients and the instructions on the mash. The article is pretty awesome, full of helpful pictures and detailed steps. It really made me want to give this a shot.

Piggish IPA

Total batch size = 5 gallons; Partial Mash in 3 gallon beverage cooler; ~3 gallon, 60 minute stove-top boil; very late malt extract addition; dry hopped for aroma; target abv of 6.4%.
Additional helpful procedural details can be found in "Countertop Partial Mashing" by Chris Colby in Brew Your Own magazine.

Ingredients:
4 1/2 lb 2-Row Pale Malt
1/2 lb 40L Crystal Malt
1/2 lb Carapils/Dextrin Malt
1/2 lb Wheat Malt
4 lb, 1 oz Briess Golden Light Dry Malt Extract (DME)
2 oz Columbus Hops (12.3% AA)
2 oz Cascade Hops (6% AA)
3/4 oz Simcoe Hops (13.2% AA)
1 oz Centennial Hops (8% AA)
1 tablet Whirlfloc
White Labs WLP001 California Ale Yeast
4 oz corn sugar (for bottle priming)

Hop Schedule:
1 oz Columbus - 60 minutes boil
1/2 oz Cascade - 30 minutes boil
1 oz Cascade - 15 minutes boil
1/4 oz Columbus - 2 minutes boil
1/2 oz Simcoe - 2 minutes boil
1/4 oz Columbus - Dry Hop in Secondary Fermenter
1/2 oz Cascade - Dry Hop in Secondary Fermenter
1/4 oz Simcoe - Dry Hop in Secondary Fermenter
1 oz Centennial - Dry Hop in Secondary Fermenter

Heat 8.25 quarts water to 165 degrees for a target mash temperature of 150-153 degrees. Place the 6 pounds of crushed grain (2-Row Pale, 40L Crystal, Carapils and Wheat) into a large mesh bag. Pour the hot water into the beverage cooler, then lower the grain bag into the water very slowly, pushing and prodding with a large spoon to ensure all the grain is wet (this can take several minutes). Put the lid on the cooler and allow to rest 60 minutes.

While the grains are mashing, heat another 4-5 quarts of water to 180-185 degrees for sparging (rinsing the grains). Near the end of the 60 minutes, heat 2 quarts of water to a boil in your brew pot.

After the first mash is complete, remove the cooler lid and open the spigot to draw off about 2 quarts of wort into a large pitcher. The first draw will likely be cloudy with grain particles; pour it gently back into the cooler over the grain bag to help filter it. Draw off the remaining wort by the pitcher-full and carefully pour that wort into the boiling water in your brew pot; continue until only a trickle of wort leaves the spigot.

Pour about 4 quarts of your hot sparge water over the grain bag in the cooler. Gently lift the bag up and down to thoroughly re-wet the grains (but don't slosh). Cover and let sit about 5 minutes. Use the spigot and a pitcher to draw off all of the second wort and add it to your brew pot. You should have about 3 gallons of wort.

That recipe is totally feasible for you. Yes, you'll need to start the boil with about 6 gal to account for evaporation. And there's no problem with topping off with water before beginning the boil, but I have a better suggestion:

So after mashing, you'll have all these grains sitting in a sugary liquid. This is when AG brewers sparge as best they can to rinse all those sugars off and out of the grain. But inevitably, we can never rinse ALL the sugars away. You reach your pre-boil volume before you get close to separating all the sugars from the grain.

But with a partial mash, you have the option of sparging a little more than an AG brewer is able. As a consequence, you'll experience higher efficiency (higher amount of sugar extracted out of each grain). HOWEVER, you shouldn't keep rinsing your 6# of grain until you get up to 6 gallons. What will happen is you will sparge so much that eventually the pH of the mash will increase and you'll begin extracting harshness from the grains. The common wisdom is to stop sparging when the gravity of the runoff equals around 1.008 or less. I bet you'll be able to sparge until you have at least 4gal of wort, if not a little more.

I hope this made sense.

Ahhh ok - that makes a lot of sense. Thanks for your help. The sparge is what threw me off. The 4 to 5 quarts of sparge water in the recipe are just to get it up to the 3 gallon pre-boil volume. But really, I can keep going with the sparge water until I get to 1.008 or less. I was thinking there was a water to grain ratio I was trying to keep in line with.

Many thanks