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?

[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

[zpiloto]

Originally Posted by theclencher

My own opinion is that blocking a grille behind the grille will provide most if not all of the benefit that blocking in front of grille would. It probably depends on the surrounding areas of the front end geometry too.

I agree. I used both methods. Blocked the lower opening on the outside and the upper from the inside, works great and is clean looking.

[rh77]

Originally Posted by theclencher

Once upon a time the Ford EXP was the most aerodynamic car sold in America. It has really "scoopy" headlight buckets. The tech article I read on it said those scoops didn't hurt aero as they "fill up" with air and the oncoming flow doesn't see a "scoop".

My own opinion is that blocking a grille behind the grille will provide most if not all of the benefit that blocking in front of grille would. It probably depends on the surrounding areas of the front end geometry too.

...not to mention a more "stock" look, and easier installation. Both of which I'm about with my car. Lately, I haven't had a problem with losing too much heat once it's generated in the engine bay.

I still have that problem with the TC not engaging in cold weather. It has been in the 'teens quite a lot lately -- which is probably a protective feature of the system to not put too much direct pressure on the components. The coolant temp reaches 192F and the stat does its job, so the engine is fully warmed. I may need to develop an insulating shield around the trans-axle housing. When it does lock-up, FE jumps up (higher load, lower RPM). Winter driving

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.

[jwxr7]

Just my 2 cents

grill blocking looks like a good idea , and I want to eventually try it. My only concern is about allowing enough cooling air to still get thru to keep the electric fan from coming on more than usual. On my little car that fan really drags the engine down thru the alternator when it comes on. I could see a point where a grill block's aero gains could be canceled out by a larger electrical load. This would probably happen at lower speeds where the aero would have less effect anyways. If it is really warm out it could happen at higher speeds. Each type of car would be different, and in each type of climate. It would be interesting to test FE loss from the fan running vs gain from grill block aero. I like the idea of movable louvers for the grill block, like I saw in another thread here. Heck, my 55 allis chalmers could come with an optional movable louvered front grill for cold weather use (not too much for aero since it only goes 14mph ).

[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.

[Ryland]

I have a grill bock on the outside of my bumper, (I did it in black plastic, and you don't notice it) I left a 4x4" hole right in front of the radiator and the only time I noticed the fan come on was when it was 100+ degrees outside, and I was doing alot of stop and go driving.
I have a feeling that I might have quicker warm up times if I made a smaller hole, but then I would have to remove it in warmer weather.

the main arodinamic reasoning is that the underside of your car is not arodinamic, and the less air you have going under there, and the more you can evacuate that air that is already there, the less drage you will have, some people drill holes in their rear bumper to let it "vent", but that is already really far back, at that point you are better off, in my mind at least, to just keeping as much air from going under in the first place.
and as far as having the grill block behind the grill rather then in front, the leading edge of any shape you want as smooth as possible, you can make the trailing edge to creat turbulence, but the smoother the leading edge the better, but if it's a choice of a grill block behind the grill to prevent air flow thru the engine compartment, and then under the vehicle, or not having one at all, I say go with a grill block on the back side.

[JanGeo]

Didn't use the stretch plastic - instead tonight I filled the grill openings with white 1/2 inch shipping foam - seems to work great - engine reached 190 in 19 degree temps with the heater running on low. Outside temp according to weather is 10 degrees.

[hotroddr]

This is how I blocked my grill. I made a block off plate out of aluminum and covered the center section. The car ran hotter than stock so I extended the air dam an inch and a half or so, you can see that under the bumper. I noticed an improvement of about 5-10 mpg. I am sure this is all aerodynamic because ultimately the temperature is not any hotter than stock. The car felt a little more stable on the freeway as well judging by the increased steering effort.

[jharbert]

Nicely done, hotroddr. That's very clean.

[SVOboy]

Hey hotroddr, don't forget to introduce yourself to the forum!

[LxMike]

Originally Posted by hotroddr

I noticed an improvement of about 5-10 mpg. I am sure this is all aerodynamic because ultimately the temperature is not any hotter than stock. The car felt a little more stable on the freeway as well judging by the increased steering effort.

do you use acetone also??

[Ryland]

Originally Posted by hotroddr

this is the kind of grill block that I like seeing, make it blend in to the look of the car, keeping it clean, and simple!

[JanGeo]

My traffic and highway driving produced higher temps yesterday - peak at 205 degrees and later found out that 202F is when the fan kicks in. Had to open a small area behind the licience plate and that was still not enough so today I will open it up a little more. Definately PLENTY of heat inside the cabin.

[red91sit]

Originally Posted by theclencher

****ing cops would have you pulled over in about 2 seconds here for failure to display the front plate.

They're not required in Florida :-) just back ones.

But yeah, the cops in your towns are quite dickish. BTW i think i might have seen you the other week. I was in a Greenish Plymouth, and you were coming from by the Hospital?

[JanGeo]

It got too warm today - 40 degrees - so I had to open up the bottom grill area and ended up getting over 50mpg on my return trip from home to office.

[JanGeo]

xB grill Block - works too well - will check thermostat for proper total sealing when cold next but the aero seems to be better.

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[rh77]

I have to say that these are very impressively clean-looking designs.

Just thinking out loud -- perhaps a controlled air-inlet could be operated on the fly (vaccum, servo, or what-have-you) if things get too hot...

RH77

[JanGeo]

Yeah Just a piece of foam and since the air builds up in front of it anyway it just flows around and it is easy to pop out when it gets warmer like yesterday when it reached 40 degrees - I pulled the lower one out and still have enough heat from the heater and got some better aero from the top piece. The cutout behind the bottom of the plate helped a little but when going uphill it tends to heat up too much so I finally had to pull it out. I still want to make it adjustable from inside probably with a cable. That way when I park it I can close it up to retain more heat in the winter to keep it warmer longer before I go out again. Something automatic would be good though in case I forget to open it up. It was very apparent that fuel burn rates over 0.7gph made it warm up a lot and when the rate got to 1.7gph you could see the temp climb and required full speed on the heater fan to keep cool. Still need to check to see of heat/coolant is leaking through the thermostat when cold. One other thing that was nice is that by running the engine temp 15 degrees hotter it warms up the tranny more too.
Also want to make a lower air dam that can be lifted when going over stuff that would hit it to get even less air going under the chassis.

[red91sit]

Originally Posted by theclencher

Dude's from Arizona ain't he?

It is a very nice looking grille block; nice workmanship. But 10 mpg gain from it???

Probably was me- I have the only multi-colored Tempo in town.

Ha ha, well hmmm, errr, haha doh!