Then you're saying the FM online calculator is wrong. Because if you use 4000 lbs, 12.75 ET you get an example 400FW & 339RW.

400 - 18% or (or times .82 if you like) = 328

To work from the other way round...

328 + 18% ( x 1.1[img]/forums/images/smiles/icon_cool.gif[/img] = 387hp ..... just lost 23hp

Percentages aside, I've read the C6 costs about 75HP. I can't see the 9" diff losing an additional 40.