The project is interesting but how receive the time of each lap? I saw some applications before using transponders (but on rc-cars). I suppose alerady exist some hardware that keep log of each 'transponder lap' and the tablet would 'just' act as monitor/visualization/ranking. I guess on nascar kind of race a manual lap buton for each car as you suggest is precise enough but seem a bit hard to handle 10 (hypoteticalnumber) cars timing. Ill take a look at the mfortner link to better understanding the problems