The first thing to do before starting modding is to learn what actually happens inside an engine and what characteristic each modification has.

This link is a favourite with a lot of people to learn more about engines and modding. It is primarily about RC boats but the same principles are applied to cars also.

http://rcboat.com/past.htm

There is no point watching someone modding an engine and then trying to recreate what they did. For one you dont really know why you made the change and that particular change may not suit your requirements. Different changes can be needed for different tracks (depending on whether you just need out and out speed, really low end punch or a mixture of both).