Well 8 PACK there are many variables to consider along with HP in trying to reach a certain performance goal. Some of these are, in no particular order:
Traction
Auto or stick
If auto, what converter
Chassis set-up
Rear gear ratio
air density, humidity, altutude
Example: Take a 500 HP car with an auto tranny and a 2200 converter and 3:23 rears gears with DR's on it. Let's say for the sake of this discussion that the car runs 13.80 with that set-up. Now change the converter to a 4000 stall and put 4:10 gears in it and you will see the ET drop considerably. Both passes were with 500 HP engines, but the ET's are way different. Make sense?
I ve seen well sorted out low HP cars run very fast, and I've high HP cars run kinda slow for what they had in the engine. There are lots of considerations, so those calculators only help out so much.....