Copied this off Car and Driver website:

And the surprising thing is the Prius handles superbly on the track. Don't expect the batteries and the electric motor to last more than one lap, though. With the accelerator floored constantly, the battery soon runs out of puff, leaving the car to run on its 1.5-liter four-cylinder engine. The power unit has incidentally been changed from an Atkinson unit with specially tuned valve timing and class-leading fuel efficiency to a less earth friendly but more conventional 1.5-liter engine, straight from the company's Echo small car, making 145 hp. On the track, the race version has 10-percent-poorer fuel consumption than the stock model. "This engine switch was necessary to achieve the results we wanted from the car. Otherwise," says chief engineer Masao Inoue, "we have done little else under the hood." The batteries are the same, and the THS II hybrid system is identical. Toyota considered upgrading the batteries and hybrid system to increase power output, but that would have meant totally recalibrating the THS II unit and battery pack, and the powers that be were not ready to do that just yet.