Roughly the number of horsepower at the rear wheels is the same number of kW's at the flywheel.
so in butthead's case 245rwkw = 327rwhp
therefore if the car has 327rwhp, the engine produces 327kW (436hp) at the flywheel (roughly)
i dont think this would apply to all cars but seems to be pretty close on rwd skylines.
hope that helps....