May I Suggest you something?
http://www.gtainside.com/en/gta4/cars/56096-2013-porsche-911-gt3-991/ go to the link, download a mod. check the readme file and then get the handling line from there. if you want to change the drivetrain awd fwd rwd, then use edit iv, if edit iv is not in english language then select it's english language, then go to handling then change drive train bias or drive bias by respective values between 1.0 which is for front wheel and 0.0 which is for real wheel or 0.5 or awd. it didn't flip the car. not necessary to use the mod only i gave the link of car for handling only.