There are a lot of "tricks" that you can do to on a saloon car to get rid of unnecessary high speed lift without resorting to air dams etc. I experimented with this a lot and found a fair bit of lap speed time at long fast circuits like Silverstone and Spa.
Outwardly there is no way that the car looked anything other than "normal" and would take a very good eligibility scrutineer to spot anything "untoward"
