Hi and welcome. Look at sites like howstuffworks.com to find out the basic mechanics and how an engine works. I would do this first because you need to understand how and why an engine works and does what it doe before you can think of tuning it. For the
mods, the best place to look is probably good tuning magazine. I think Haynse do a good manual for tuning and
mods, that is probably worth a look.
