When you go to the original recipe on the website ( http://soup.allrecipes.com/az/MnifldStw.asp ) it does list both times in different places on the page. I suspect the 4 to 5 hours is the correct time.
One of the reviews of this recipe (from 3/15/2005) contains the following:
"No joke, we used to do this when camping but wrapped chuck roast and veggies in alumunium foil, placed on engine, then set off to our destination. When we got there, maybe 8 hours away, our dinner was complete. By the time we (sic) camp was set up, we could sit down to eat. No kidding!"
When I was young (about 60 years ago), my father would put a can of baked beans on the manifold to heat up when we went on a trip, so we could have them hot with the picnic lunch we took. It always worked then, but car engines may well have been laid out differently at that time.