Even with a "normally aspirated" car (i.e. uses a carburetor) I would want to check for something that may be causing gasoline to seep or leak--heavy float (common after a few years), hardened gaskets, loosened screws).



Once I was to meet my wife so we could go someplace and she had not eaten. I wrapped a frankfurter in aluminum foil and rested it on the exhaust manifold for the 30 minute drive. It was nicely cooked when she ate it.



Back in the mid-1960s I saw an article in the Minneapolis Star about a family that cooked roasts and potatoes on their engine when taking family vacations. It was the days before the Interstate highway system was widely in use and roadside rest parks were easy to find. They drove for a period of time, stopped, and turned their food so it could cook on the other side. When the food was done, they stopped at a rest park and ate it while seated at a picnic table.



My daughter was interested in this and I checked into the book "Manifold Destiny." It is out of print and available copies were quite expensive. I expect a person could do what you are doing--experiment and make notes.



I thought it would be interesting to make a basket or enclosed box that would absorb heat from the engine, but would also protect it from air drafts and from falling down to the road, as well as from tearing or opening of the foil so the food was contaminated by dirt or oil.