Low Effeciency for Russian Imperial Stout

This was the biggest beer I have ever brewed and I'm freaking out about the lower mash efficiency than my system typically produces. I mashed 23# of grain at 1.30 quarts per pound of grain (just like I always do). 19lbs of British Pale and the rest included chocolate, pale chocolate, special B, Black Barley and caramunich.

Mash temp: 154 for 60 minutes

I fly sparge with 168-169F water and always at a rate of about 1L per minute until I achieve a pre-boil volume of 8 gallons, boiling down to 6 gallons over a 60 minute period (total sparge time of 35-40 minutes). I keep about 1" of water on top of the grain to help get even distribution and rinsing of the grain.

My system typically comes in at about 72-74% but this time I calculated 64%. The SG I was shooting for was 1.095 and it came in at 1.086.

Any thoughts as to why these results are off from that of my systems typical performance? Am I just freaking out over nothing?

Thanks for any feedback you might have!

Lower efficiency is typical with very large grain bills. Someone more knowledgeable can explain why, but just know that it probably wasn't anything you did or didn't do, it's to be expected. This exact thing happened to me on an imperial stout a few weeks ago. Calculated the recipe for 75% and got 66%.

Efficiency always suffers for high gravity beers.

Sugar moves from where there is a lot (grain) to where there is less (sparge water). The difference between the amount in the grain and the amount in the water dictates how fast the sugar moves from the grain to the water. The first portion of your runoff will be extremely sugary. The more you sparge, the more dilute your water will become. If you want a high gravity beer, there are two options:
1. stop when the wort you have collected has the sugar you want (leaving a lot behind in the grain)
2. Use more sparge water and boil off longer to get the sugar concentration up.

I always did #1 above (like you did). If you taste your mash after you sparge a big beer, it will still taste sweet. This always bothered me--a lot--, and then I read about parti-gyle brewing.
http://morebeer.com/brewingtechniques/library/backissues/issue2.2/mosher.html
http://***********/component/resource/article/2021-parti-gyle-brewing-techniques
http://www.brewboard.com/index.php?showtopic=62046

Make your Imperial Stout, then sparge with another 8 gallons and make a brown ale or porter.

Here's an imperial stout, porter, braggot I have fermenting.
British Pale 33
Wheat 0.5
Crystal 2
Roast Barley 2
Choc. Malt 1
Black Patent 0.25

Sparge with 7 gal. - Imperial Stout (O.G = 1.099)
Sparge 2 with 7 gal. - Porter (O.G. = 1.072)
Sparge 3 with 7 gal. - Brown ale/Braggot. For the braggot, I added 5 lbs of honey at the end of my boil. I'm not sure if that makes it a real braggot or a honey brown. O.G. = 1.057)

That is just the way brewing goes. The more malt you use, the lower you efficiency will be. Conversely, if you brew a Mild, you'll get "great" efficiency.

The basic homebrew options are

1) use more grain and just adjust the recipe based on lower efficiency.
2) Sparge with more water and increase your boil length. (you capture much of the remaining sugar and then evaporate the water out)
3) Add sugar after the mash - a pretty common route for many high OG styles

Sounds like you got your answers already. What I do is try to max out at 18 pounds or so and then add DME to the boil. It just works easier and you adjust based on your gravity during the boil to make sure you get it right.

I took a 10% efficiency hit with my imperial stout as well... now I know!

Sparge more, boil longer also works.

Whenever I do a Large beer I automatically assume a hit on efficiency. In Beersmith I adjust the recipe to 65% total efficiency. If doing a Mild I adjust to 75%. For everything in the middle I use 70-72%. Probably not the best method but it keeps my brew day the same each time.