Grille block: front or back and top or bottom?
 

I'm considering a grille block and have read lots of threads but can't find definitive answers on these questions:

1) Does it make a difference if a grille block is in front of or behind the grille? I read that a stagnant air bubble would form in front of the block like the the "tailgate up" mythbusters that found tailgates hold stagnant air in a truck's bed that helps aerodynamics. A grille block would obviously look better behind the grille and emblem and I would rather do that.

2) Does blocking the top grille or the lower bumper grille work better for aerodynamics and/or engine cooling (assuming both are the same size)?

Before MetroMPG yells 'It depends!'(tm), let me say that generally it's better to block the upper intake. The opening above the bumper is often a styling point, and not a function-driven design. Blocking the upper opening helps deflect smoother air up and over the vehicle. Still speaking in broad generalities, the lower opening is often oversized and can be at least partially blocked. I would recommend to first install an accurate ECT gauge or use a Scangauge to monitor coolant temperature. Then progressively block your grill until your comfort zone is violated.:p
What flavor of vehicle are we talking about here? Got pictures?

Its a 2007 Mazda B2300 pickup. I hear you that the top grille would likely be better to block to direct air over instead of under ( especially with it being a truck), but unless the block could go behind the grille it would probably look better to block the bottom one. So it leads back to my question above--can the block go behind the grille? Does a "stagnant air bubble develop" that helps?

I'm afraid I have no hard opinion on the frontside/backside grill block. Personally, I would put it on the front, as I just don't have that much faith in the 'stagnant air bubble' theory. If you hold a Dixie cup in a stream of air, which way pushes harder-open end forward, or open end back? I would love to see some documentation if someone has researched anything similar to this. Maybe someone could tell us if Hucho has an opinion in his book.

Great example. I have to say that I'm with you. I can't imagine the dixie cup's open end to the airstream would be better than the closed end. Even the stagnant air in the back of the truck w/ tailgate up is not complete and blows forward. As a kid I remember sitting in the back of a dirty pickup behind the cab and having all that dirt blowing into my eyes. So if the dixie cup is sideways to an air stream and there is still some drag, it seems like there would be even more drag if the open end of the cup faces the air stream.

Documentation
 

The only thing that comes to mind is a patent that Dr. Paul MaCready got for a Jeep windshield fairing.MaCready created a surround for the flat frame that incorporated pretty generous leading edge radii not present in the Jeep's frame.Even though the air would be "captured" within the bucket created by the fairing,those radii were such, that the free stream would flow around the vertical puddle of air,attached to the top and sides of the Jeep,something impossible without the fairing.Its all counter-intuitive,that attaching a "parachute" to the front of the windshield could do what it did.It was a carefully designed parachute! So it looks like,that if the area around the grille has sufficient radius,that the air will simply flow on over the stagnant "puddle" created by the block,whether in front of or behind the grille.Then its just a matter of appearance.

No, I definitely would have suggested blocking the upper as well. :p

The logic behind the choice is it's better to deflect air that's already above the stagnation point over & around the vehicle than it is to permit it to enter the aerodynamically dirty engine compartment (and exit contributing to turbulent flow under the car).

As for in front/behind the grille, there may be practical reasons for choosing one side over the other in case you need to remove it (in the event of temps getting too hot).

And I like the idea of "outer" because it draws attention. Maybe it'll help get people thinking...

You have way more faith in people than I do.
Most folk around here do as little thinking as possible.

Sad but true...I just watched Idiocracy last night. :p

It will get some people thinking and some people saying, :confused: because they don't think!

Good point MetroMPG, I think it is a good idea to have it on the outside to advertise even if people at first just think "WTF?" People need to start thinking. And not only about MPGs but also air pollution in our urban centers and global warming.

Thanks for the advice. I am going to certainly do a partial top grille block and likely adding a lower valance that actually goes as low as the vehicle or a plastic skid plate type thing that angles from the bumper to the cross member.