It's still nice sometimes for user friendlyness to have relevant words in a URL, and in the description but I don't think any longer they have any relevance to serach position.
But once a user sees the search results, they may click on something that looks more relevant, whether it's the URL having a useful word in it as opposed to lots of random letters and symbols, or whether it is a well cratfed meta description.
Google no longer wants sites to be "optimised" for search engines, it is more likely to go for sites with good well written content (or big brands, but that's a different discussion!)
In other words, in the old days you site might be well optimised for SEO, but not as useful for the user. Latest Google algorhythms are more after quality inbound links and good content.
In your case I would say if engineheads alone attracts users then leave it at that. If you think the words car and/or forum need to be there then do so, but I think it is best something like this:
engineheads.com A friendly forum for car enthusiasts interested in discussing all aspects of mechanics, tuning, customising, racing and rallying
(or whatever its about) is going to be the best option.
That would be the home page. By default, as Brogan says, any listings for the forum or threads within will have /forums/ anyway:
It depends how plausible that is, though. AndyB, for example, has a non-XF landing page so he has no choice unless he does his landing page in XF. Obviously anyone who wishes to use a third party front end won't be able to install XF into the same directory.
But yeah, if you don't have any sort of front end, I'd move everything up to the root.
Failing that, a good example is exactly what XenForo does. They solve the forums/forums issue by installing XF into the community directory.