The key for the S397 Oreca is ride height. In the default setup, the car drags, which limits top speed. The problem is, the current version of rF2 does not show sparks from dragging and also does not have any road surface contact sound. So you don't really KNOW the car is dragging. The new Q2 release adds sparks and lots more sounds so you can now see & hear when the Oreca is dragging. You can test this by using WooChoo's Le Grand Circuit, which features the 1967 Le Mans layout (long long Mulsane straight, no chicanes) Take the oreca out and watch the top speed. Then back to the pits, stiffen the springs raise the ride ht a few cm and drive again. you should see a marked difference. Then it's a matter of testing to see where the car, at max velocity, lifts off the pavement.
Also, at the customize screen, The giant hangar that displays the full car, ensure you have the Le Mans Aero package selected and that you force the setup for other AI cars.
