MD-80 seems to have a speed limit regardless of how much i increase thrust

This is the part of the aircraft.cfg that has engine data. What can I do to make this aircraft function like the default fsx aircaft in that it doesn't have a top speed of around 630 kts? I like to have fun with this aircraft lol. What variables do what?

[GeneralEngineData]
engine_type = 1
Engine.0 = -26.6, -8.5, 0
Engine.1 = -26.6, 8.5, 0
fuel_flow_scalar = 1.75
min_throttle_limit = -1

[TurbineEngineData]
fuel_flow_gain = 0.002
inlet_area = 10.4
rated_N2_rpm = 12245
static_thrust = 18500
afterburner_available = 0
reverser_available = 1

[jet_engine]
thrust_scalar = 1

MD-80 seems to have a speed limit regardless of how much i increase thrust_scalar