Boeing Laptop Tool
My company operates B737-700 with a Boeing Laptop Tool to derive performance calculations. This is a self dispatch tool that covers takeoff and landing performance as well as weight and balance.
With regards to V1, there are five selectable options:
-Optimum
-Balanced
-Min
-Max
-Balanced Plus
Our company uses Optimum because it allows the highest takeoff weight and also matches the FMC derived V1 (assuming no other factors such as MEL performance degradations).
The BLT also allows the option of Improved Climb as previously discussed in this thread. As a point of comparison a takeoff on a longish runway, using Improved Climb, gave an increase in weight of 9T and a V1 25 kts higher.
Effect of Obstacles
I plugged in a limiting obstacle that reduced the MTOW quite markedly (by 15T) and used a wet runway to force a larger V1/Vr split.
When I cleared the obstacle, the V1 at minimum thrust remained constant. Minimum thrust dropped by about 4% N1.
The obstacle reduced the MTOW but had no effect on V1 for the equivalent weight at minimum thrust. Interestingly enough, for a fixed thrust and weight, clearing the obstacle forced the V1 DOWN by 4 kts.
Effect of Thrust
A decrease in takeoff N1 using either derate method produces an increase in V1 and Vr. V2 remains the same regardless of takeoff thrust.