Hi, you wrote:-
>The project that we are developing is a time control of a kart racing.
>The first option was the use of transponders for Kart identification,
>but the problem of more then one kart may pass through the finishing
>line at the same time exists.
I beleive this problem has already been solved in normal motor racing (F1
etc.),  so perhaps you should ask in a motor-sport related list.  I think it
took them a long time to get right.

>The other option was the use RF emiters in the kart's.
>I appreciate that you send me your opinion to this problem, and we
>are open to other sugestions.
How about fitting an IR emitter to each Kart, pointing UPWARDS, possibly
driven by a PIC, then have a pole across the track, maybe 10 feet above the
ground, with multiple IR receivers on it.  Transmit a unique frequency
and/or code per Kart and decode the receivers using a laptop (or big PIC).

Just a thought.

All the best.

Dave.