Grill Block
 

On a grill block, does the block have to be in front of the grill or could it be recessed (hidden) behind the actual grill?

Ideally...it should be flush on the front but fer "asthetics" you can put it on the back. ;)

In practical application it makes no difference at all. It's really a radiator block for use in winter. On one of my cars I am able simply to slip two thin sheets of formica in the space in front of the radiator. The minimal clearance is such that they will stay in place at any speed without falling out. If the temp gets too hot I simply remove them. It takes less than 30 seconds to place or remove them. Sweet & easy! :)

In my opinion, the grill block is really more about aero than radiator blocking. Then again I run the grill block year round.

If you want a 'stealth' grille block, you can place it directly behind ( flush with the back ) of the grille slats and paint it flat black.
Some grilles come blocked from the factory this way, and no one even notices.
For example, look closely at the grille on the Mazda 3 - a large portion is plugged :

http://www.fastmotoring.com/oldfiles...ive_la_opt.jpg

The farther that the block is from the actual grille, the more of an air pocket that is created. Placing the block at the radiator will help, but not as much as just behind the slats like in the picture above.

That's sort of what I had in mind at first, a slot system so you could change out a plate with small/few holes for big/many holes.

On another thread I stated where after driving in the rain, I looked at the dry rain marks and the side areas seem to show the most pressure so I think I might do one flush but block it from the sides.

Made my first template for the block, now I need a clear material that is bendable but pretty rigid and a good way to install the blocks, IE aesthetics in mind.

http://i678.photobucket.com/albums/v.../mpgq45114.jpg

see link in my signature box

If it does not matter, the behind the grill would be nice and stealthy.
Making a grooved holder for different plates would be very easy.

So is it the removal of air from getting in the engine bay, or the actual streamlining of the exterior, or is there a real difference in effectiveness between the two?

I used aluminum trim coil to make the grille blocks for my '09 Aveo. It's extremely light, easy to cut and easy to bend; it can be fastened using screws and tinnerman nuts.

The lower grill block is behind the lower grille and is invisible, permanently mounted, painted black. The upper grille block is also painted black and is attached externally by two long screws through the grille. It is only attached in winter as it would be too restrictive for summer temps.

Grille blocks will help the car warm up a bit faster. However, aerodynamically it had no effect on the car's FE, nor did the front belly pan I made.

front bellypan did nothing? thats sad

IMHO aero mods are vastly overrated. I have yet to see things like grille blocks, belly pans, or modified wheel covers and side mirrors improving fuel economy to any noticeable degree.