Quick question
 

I am thinking of doing a grille block on a couple cars, but have it behind the grille.

The reasons are:

1) aesthetics
2) protection
3) making it motorized and driven by the radiator temperature to open when more cooling is necessary

My question is if there is an appreciable (noticeable) difference in drag for the block to be flush with a grille that is about 1/2" thick, effectively making pockets vs being flush (and smooth on the outer surface)?

Since it is not an aircraft or race car, but being driven exclusively below 80mph, does it REALLY matter??

flush
 

There's probably plenty of radius around the grille opening to keep the flow attached,since the whole area is in a favorable pressure gradient,courtesy of all the attacking air in front of the car.
The grille opening itself should have some radius itself,or you can have what is called a vena contracta entry loss,which can invisibly choke down the flow.
Here's an extreme example
http://i1271.photobucket.com/albums/...Untitled32.jpg
The yellow region is nothing but turbulence and a real energy loser.Pure drag.:(

No, but don't let that stop you.

1) aesthetics
2) protection
3) motorized

When you say a couple of cars ...which cars?

Putting a grill block behind the grill is a lot more work. Not only do you have to get behind the grill, but it's usually not a flat surface that's easy to cover either. And air pressure on a grill block behind a grill will try to push the block away from the grill and let air in. When mounted in front, the air pressure pushes the block tighter to the grill. I wouldn't expect an aerodynamic difference between the two methods if it's well done.

2004 Ford F-150
2004 Mercury Mountaineer
2006 Dodge Grand Caravan

For the initial test on the F150, I will just take some styro board and ziptie it to the outside of the grille for the reasons you mention...there is a LOT of surface area and it is almost vertical. For the more permanent mount, I would like it behind and motorized as I am getting ready to replace the mechanical fan with a couple electric fans. I want the option to thermostatically control an opening grille block for hotter weather, but for now, I am planning.

A neighbor works at a race shop and got me some race fans for free, and I know the mechanical fan is a real drag given that simply backing up the driveway blows most of the leaves off the driveway in a 3 foot swath. ;)

The shuttering mechanism will have to fight the air pressure one way or the other, either opening or closing. And you will need space behind the grille to match the depth of the shutters.

I propose two sheets of perforated metal. Our local recycler has a lot of stainless steel with inch-and-a-half perforations. When the holes are aligned you'd have a ~50% grille block, but slide the inner one sideways and it goes to 0%.

For sale:

http://ecomodder.com/forum/showthrea...ers-31585.html

Maybe you should consider making a custom grille similar to the one used in the previous-generation Volkswagen Polo BlueMotion.

Here is the quick (and dirty) grille block that I did last week
 

I did notice a quicker warmup time, and the SG didn't show much difference in the operating temps...hovered right around 180-184 (about 5 degrees warmer than normal).

The mpg change over my typical route was a bit more interesting...

My trip to work is about 45 miles each way, and typically by the end of the run with a mixture of city, stop and go highway and cruise control at 75mph, I get an ending average mpg of 18.8 to 19.5 without the block.

With a longer trip (125 miles each way) consisting of a little bit of stop and go, but mostly cc at either 62mph or 70mph depending on the segment, I typically see about 20.5 at the end of the trip.

With the block, it got to the 19.5 much quicker and topped out at about 19.9. On the longer trip I topped out at 19.8.

So...it is mixed results.

Makes me wonder if I either did a flush mounted (smooth to the outside edge) grille block, or a complete block...or both if that would improve??

I am now thinking about doing a tonnau cover for the bed and see what that does. Dropping the tailgate saw essentially no change in mpg (less than .2 mpg for the typical trip).

The grille block was just some cardboard (with paint) zip tied to the outside of the grille (but recessed inside the grille shell) and a lower block that was about 80% (duct tape over styrofoam).