Thank you. When you mean apply pressure can that be consider in the siphon as I assume it increase the flow as it is being pulled by the siphon correct?Gary, if you increase the size of the bulkhead and ALL the pipe in the drain to 1 1/2" it will flow 1500 gph vs 960 gph with a 1" pipe.
If you increase the drain to 1 1/2" pipe but still have a 1" section of pipe or bulkhead or union, then the entire system will run at the 1" diameter flow rate of 960 gph. The only way you can get more flow through a 1" pipe is to apply pressure. If the drain were in the very bottom of the tank with 2' of water pressing down on it, it would flow a little faster (the water pressure behind the weir/overflow on even a bulkhead at the very bottom would be small. The other way is to use a pump, which is impractical when trying to get water from the tank to a sump... not impossible, but I don't know anybody who does it.
Does that help?