hmm... found this in johnny palmers book,
""Manifolds are less likely to allow the mash to become compacted during lautering, resulting in a stuck sparge, in which water will not flow through the grain bed. This brings us to the question-- what is the optimum outflow rate? There is a trade-off: if you lauter too quickly you will collect a lot of wort but have a low extraction, if you lauter too slowly you will have great extraction but you will take all day to do it. Most homebrewers use the rule of thumb of 1 quart per minute. If your extraction is low, i.e. less than 28 points/pound/gallon, you should try a lower flow rate. The best way to control your flow rate by using a ball valve or stopcock on the outflow.""
found here:
http://www.howtobrew.com/section3/chapter17-2.html eleventh paragraph down...