Ayrton Senna himself said that karting is the purest form of motor sport and you won't find any arguement from me on that one!
Karting1 have found some rare footage of Senna racing karts against Prost and other F1 drivers from 1993 in Paris Bercy. Other Drivers were Panis and Herbert. Also I've put together some analysis of Senna's karting techniques so you can get a feel for what made him such a fantastic karter. Senna was a multiple Brazilian kart champion and came runner up in the world karting champs a few times.
He was so keen to win the world karting title that he made several attempts even during his car career but never actually won it
here is the link to the senna karting video
