Big beer efficiency and fermentation question

Having problems hitting efficiency and SG targets consistently with big beers. AG brew 5 gal batches with 8 gal pot, 10 gal MT and batch sparge. Know efficiency will drop with big beers. Wondering if as a general rule increasing the mash from 60 to 90 min and increasing the sparge from 30 to 45 min will increase efficiency without adding off flavors. Know SG can be increased with DME but would like to try longer mash / sparge times first.

Typically ferment in primary for 14 days and in secondary for 7 days, both at 68 F. Just started making a yeast starter. Thinking of increasing primary to 21 days, especially for big beers but wondering if the longer primary time is needed with a yeast starter.

10-14 days should be good enough for primary assuming enough yeast is pitched.  I sparge for a minimum of 60 minutes and use water between 170-175 degrees.  If you are using wheat double-crush or set mill to .030 inches.  Wheat beers take longer to convert the starch so 90 minute mashes for those is good.  Also, make sure your mash pH is between 5.2-5.5 after 10-15 minutes.

Mark

Wondering if as a general rule increasing the mash from 60 to 90 min and increasing the sparge from 30 to 45 min will increase efficiency without adding off flavors.

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You should verify full conversion with an iodine test, and your sparge should last most of an hour.

I lose about 3% efficiency per 10 pts of sg. That is normal, and will not be impacted by mashing longer. Sparge time is irrelevant for batch sparging. 

What kind of gravity and efficiency are you calling big? What efficiency areyou ggetting?