While defamation is always a concern, it could only apply if your work was presented non-fiction and made false and damaging statements about Porsche that were factual in nature and not opinion....this is highly unlikely in your situation. You also need to consider trademark issues, but based on your facts, you are permitted to use the name under the fair/nominative use exception to TM law, just make sure you are not doing anything to create consumer confusion i.e. giving a false/confusing impression that Porsche is sponsoring/producing the book...which should not be a problem. Also, use the name only as is necessary to tell your story (as you have state it is). From what you have presented you mostly likely have nothing to worry about. That being said, I wouldnt put a recognizable picture/logo of a porsche on the cover without permission, that would be risky.
Lawyer
5 years experience in intellectual property law