If you are using MKS, Export your Car to a INI, then open the INI to Line 440. You can enter any number up to 65,000. THen save the Ini, and IMport the car into MKS.
I prefer doin it by usin CB codes, works for me, just Japanese a diff code in if I need more or less HP depending on the track, ran a lap yesterday of 9,679.0 mph on the Test Track...