I wondered this too. The idea of course would be to force more air through the radiator to aid cooling. Before I bought ah new aluminum radiator to solve over heating problems I tried adding one to my 66-289. It didn't solve my problem ,but that dosen't mean it didn't help. As to why small block V8 cars didn't have them I've never heard a difinite answer either. My crazy theory is this, Big block cars tended to have hood scoops or even ram air to feed cold air into the engine. Small blocks (if equipped with open element air filters) could get cold air into the top of the engine compartment if they deleated the rad support seal. This is just my crazy theory 

Last edited by Derek (3/04/2014 3:34 PM)

I've often wondered about this, but have no answer. It strikes me odd too that the radiator is only eighteen inches wide when other fords of the period with the same 289 had twenty and twenty four inch wide radiators. Leave to a guy named Shelby on the GT350 and Mustang Steve to figure out a way to add larger radiators!

I think it was to aid in cooling. There must have been an issue with the 6 cylinder that was not with the V8's.  I know that I've felt hot air rush OUT of that gap which then would be easily sucked back through the radiator.

I've noticed that newer cars with electric fans seal off everything between the engine compartment and the front.  I've been told that it's to force more air thru the radiator (an interesting concept), but maybe it's really to keep any hot air from recycling back throught the radiator at stand still.