Thank you guys for your help! I bought something around 10-15 cars, boosted and tuned up to 420 PP, but no one did very good (on Tsukuba, I mean). Some of my best were the Skyline GTS-R (R31) '87, Izuzu 4200R Concept '89 and the Accord Coupe '88, the Skyline helped me getting the 2nd place with a deficit average of 4 secs behind the 1st, unfortunately... now let's see what the Mugen Motul Civic is able to!
Also, if you guys want a hint, when an event limits the performance points, it is useful to add a ballast to your car, if your car is not too weighty (it would be okay to add 150-200 kg to a 700-800 kg car, but not for a car that weighs 1300-1500kg and up...) it lowers the PP while it does not affect the horsepower, it allows you to add more power to your car!
EDIT: The Mugen Motul Civic really helped me getting the first place. I lowered the engine power first, then I changed the gearings to have a top speed of 179 km/h. I also added a stage 3 engine, thanks for everyone's help!