why does grill blocking work? aero or heat?

[bzipitidoo]

At first I thought grill blocking was purely aerodynamic. A quick look at our Festiva confirmed that the radiator occupies only the left half, and the condensor coil only the left 3/4s of the grill. It seemed to me that an engine compartment open to the front must be acting like a drag chute, and blocking part of the grill (the right 1/4 on our Festiva) would reduce that effect. Perhaps adding a baffle to shunt air from the back side of the radiator fan to under the engine would complete the effect.

But from what I've been reading, seems maybe the point of grill blocking is to make the engine run hotter, or perhaps both aero and heat. Block off as much of the grill as possible-- all of the grill if the car doesn't overheat. Well, if that's why grill blocking works, why not just use a higher temp thermostat? Are there also seasonal changes? Block in the winter, and unblock in the summer?

Part of what confuses me about the point of grill blocking is where the blocking is done. If it's for aerodynamic reasons, seems the best thing to do is make the exterior of the grill as smooth as possible. If its for temperature, then it's ok to put the block on the inside of the grill. Also, a lot of cars have a lower and an upper opening. If only one should be blocked up, which one is better to block?

[GasSavers_Brock]

I do it more for heating up reasons, mostly in winter. I would think it helps in the areo dept as well. Changing my thermostat wouldn't help since it won't get up to temp no matter what. I have 100% blocked and today at 12F I hit a 164 just when I got to work. Without any blocking I bet I wouldn't ahve hit 130F.

[Gary Palmer]

Well I don't do it for the heating. I am running it on my 89 Civic and I am not doing it to help it warm up any quicker. I am only running it with a 1 by 2 inch opening and it seems to stay at the normal temperature, with just that much. The rest of the blocking is aerodynamic, in two respects.

First, apparently it keeps air from being pushed places it doesn't really need to go and where it isn't doing much more than causing increased drag on the car. Second, in my second attempt, I am attempting to give the air a smoother path to follow, up and over the car, rather than under it.

In either case, I am 100% impressed, that it makes a difference. On the Honda, I am still in the process of running a full tank on it, so all I can tell you is that it seems to have been a help and certainly hasn't been a hinderence to higher mileage, per gallon.

If I were in Wisconsin, I would definitely be doing it for the heat.

[budomove]

I do it on my 91 civic hatch for heat, and changed to a 195f thermostat for the same reason. I have a 3x7" hole, but also have a full block i will install tomorrow...forgot i had it til this thread popped up. I can't tell whether the block helps the temp, but will try and see after i install the full block.

[rh77]

Dan did a nice looking grille block on his old Del Sol -- did a nice write-up/how-to on the subject.

RH77

[JanGeo]

I'm going to take some stretch wrap across the front openings of my xB tomorrow morning. Clear and easy to remove and a plus it will help protect the paint on the front bumper.

[basjoos]

The effect of grill blocking is primarily aerodynamic. It reduces the amount of drag from air flowing through the engine compartment and being dumped under the car, where it adds additional turbulence and drag to the confused airflow down there. It also has an effect on the heat environment of the engine. Without a constant draft blowing therough the engine compartment, a cold recently strted engine can reach normal operating temperature quicker and it also acts to increase the temperature of the intake air for the engine, which makes the engine slightly less powerful, but run with better FE. I adjust my grill block seasonally. The grill block on my car is totally blocked off during the winter, but I open a 4"x4" hole in the grill block when outside temps get over 80F.

[rh77]

Is it true that you can block the grille from the rear, effectively sealing the flow and using the cushion of air in the slots as an aero shield, or should the block be entirely external?

RH77

[skewbe]

I can't disagree that blocking from behind has benefit, but being an avid aviator I can say assuredly that a hard smooth airfoil is far less draggy than air filled pockets. There will be turbulance generated at the edges of the pockets and that spells drag.

However, most home external grill blocking is an inexact science. Really you need to do wind tunnel tests and lots of experimenting to tweak out the aerodynamics. We're just filling in the holes here, not much aerodynamic redesign (in general) so it probably isn't a huge improvement over blocking from the rear.

I actually like a custom look myself, and to be fair that was part of my decision also.

[ELF]

I recently blocked off the lower part of my grill, I already had the upper part done. My last tank was almost all short city trips bad weather, so didn't see any gains.
Now with some better weather and slightly longer trips I am noticing a difference. My engine temps are higher than normal for winter and its helping FE, I just went 30 miles today almost all city, when I got home the SG read 30.8 for the day, This is really impressive for winter in my car in city driving, I noticed my temps were up over 200f at times, this never happened in winter before, so I think most all the FE gain is due to warmer engine temps and not areo improvments.