actually gents the time for VEF prior to V1 is 'established' by the constructor....As you say this can be up to 3seconds in the failure of the Centre eng(slow recognition due to no Yaw)..VEF first issued in the AFM in 1978 due to insistance by the pilot conmunity(it had always been evident to the test /certification pilots),is about 1 second prior to V1..
And as FAR25-107 says must be recognized and reacted to prior/by V!(about 5knots acceleration)
AGAIN the 2seconds reaction time is built into the DISTANCE allowed for the ACC/stip runway case,not in the handling of the Stop case..
JAR25.107(a)(1)
For the FAA case go to FAA.GOV.go to AC25-7a for the flight test information
Aircraft Performance Theory by Stanton is good for JAR stuff
