Why should a Piper Seneca not take off after an engine failure below 100MPH? In the Seneca (PA34-200) AFM, in the ENGINE FAILURE DURING TAKEOFF section, it is noted that if engine failures occurs during takeoff ground roll and 100 MPH has not been attained, the pilot must abort the take off and if attained the pilot must continue. Since Vmc is 80 MPH, why was 100 MPH chosen as the cutoff point for go/abort?


  
  ENGINE FAILURE DURING TAKEOFF
  The single engine minimum control speed for this airplane is 80 mph (CAS) under sea level standard conditions.  
  
  a. If engine failure occurs during takeoff ground roll and 100 mph (CAS) has not been attained, CLOSE BOTH THROTTLES IMMEDIATELY AND STOP STRAIGHT AHEAD. If inadequate runway remains to stop, then:
  (1) Throttles - CLOSED.
  (2) Brakes - apply maximum braking.
  (3) Master switch - OFF.
  (4) Fuel selectors - OFF.
  (5) Continue straight ahead, turning to avoid obstacles as necessary.  
  
  b. If engine failure occurs during take-off ground roll or after lift-off with gear still down and 100 mph (CAS) has been attained:
  (1) If adequate runway remains, CLOSE BOTH THROTTLES IMMEDIATELY, LAND IF AIRBORNE, AND STOP STRAIGHT AHEAD.
  (2) If the runway remaining is inadequate for stopping, the pilot must decide whether to abort the takeoff or to continue. The decision must be based on the pilot's judgement considering loading, density altitude, obstructions, the weather, and the pilot's competence. If the decision is made to continue, then:
  (a) Maintain heading and airspeed.
  (b) Retract landing gear when climb is established.
  (c) Feather inoperative engine (see feathering procedure).
  


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I finally found out an answer which makes more sense to me . By test pilot , vmca masured with a lot of citation and one of them Is 5 degrees angle of bank towards the live engine . So it means that we can't have that 5 degrees because we are still on the ground.
for each 1 degree , our speed decreases 3 knots .So 5 degrees = 15 knots which is equalls to 20mph !
 80+20 = 100 mph ! 
This was the best answer I have got until now.