If you want to limit the boards to specific automotive discussion only - do so - it probably won't be recieved well by everyone, but since when can you please everyone.
I belong and have belonged to several boards - invariably, there are friendships made (net-friends, netships?) and people begin to open up more and discuss their 'real' lives. Once this occurs, the 'casual' boards begin to get much busier and the tech boards will slow down.
Another aspect of this, is the intimidation factor. No one wants to post something in a tech forum and get called out for saying something incorrect - but in an open forum, you can be free to post your opinions at will.
Finally, this being a hot rod site there is not going to be a real frequent development of new material. After all, what is the average time to finish a project car - few years to a decade sound about right....
The reason people come to this site to discuss politics, religion, drugs etc - is because they want to discuss these topics with the members of THIS board, whom they know, trust, like, dislike, whatever.
Why not have an experiment - make 2 new boards:
1. Car related non-tech only
2. PG off topic open discussion
See which one accumulates the most posts in 30 days.
Also, I don't see what the big deal is regarding off-topic posts - no one is forced to go into any board or any specific post. Are they agitated by the mere existence of these threads? Does the very sight of an OT thread threaten the very fabric of HR.com?
As a final note, if a thread is getting overly large - 10 or more pages - just lock it and delete any posts that are out of line with board standards.