How fast the fastest car in the world?

The most powerful race car is 1973 Porsche 917/30 from Can-Am series. Its twin-turbochared flat-12 5.4 litre engine delivered 1580PS in qualifying tune with maximum boost and around 1100PS in race mode to provide more reliability.

You might be also interested in F1 cars. In the second half of 1980s, before 1989 regulations which banned turbo engines, the most powerful F1 cars delivered up to around 1400PS in qualifying tune and something around 900 in race mode. They utilised turbocharged V6 1.5 litre engine. It is hard to be more precise as F1 teams rarely reveal output output figures of their cars and actual settings varied from track to track.

However, if you consider drag-racing, which I personally would not as in my understanding race car should be able to go around the corners, you can think of something between 8500-10000PS for Top Fuel dragsters. This power output is an estimate because engines cannot maintain peak output level of revs for sufficient period of time to be measured on dyno-machine.