Mutt,
Hmmmnn, might be you have so much punch in the 77 that you need a goodly derate for it to be limiting. Have you tried with the max allowable derate of 25%
Starts working straight away in a light twin (like the 73)
...or - maybe I'm not thinking straight
- how about "excess thrust to accelerate from V1 to V2 is less, therefore the V2/V1 ratio must shrink (if you still wanna reach V2 & tell the tale, that is)"?
Empty