text: More advanced forms of supercharging with 
  higher pressure ratios can produce very large 
  engine airflow. 
  In fact, the typical case of 
  altitude supercharging will produce such high 
  airflow at low altitude operation that 
  full 
  throttle operation cannot be utilized within 
  detonation limits. Figure 2.17 illustrates this 
  two-speed engine driven 
  case for a typical 
  altitude supercharging installation. 
  At sea 
  level, the limiting manifold pressure produces 
  a certain amount of BHP. Full throttle oper- 
  ation could produce a higher MAP and BHP 
  if detonation were not the problem. 
  In this 
  case full 
  is unavailable 
  because of detonation limits. As altitude is 
  increased with 
  at low speed, the constant MAP is maintained 
  by opening the throttle and the BHP increases 
  above the sea level value because of the re- 
  duced exhaust back pressure. Opening the 
  throttle allows the supercharger inlet to re- 
  ceive the same inlet pressure and produce the 
  the increase of altitude 
  same MAP. Finally, 
  will 
  to produce the con- 
  require full 
  stant MAP with low blower and this point is 
  termed the “critical altitude” or “full 
  throttle 
  height.” 
  If altitude is increased beyond the 
  critical altitude, the engine MAP, airflow, and 
  BHP decrease. 