I got worse fuel mileage with a grill block
 

I put a cheap grill block on my jeep(plastic sheeting and duct tape.) I wasn't able to do a formal A-B-A test on it, but it was mostly a proof of concept to see if the vehicle would over heat. I left it on for a trip to Montana recently. On the first two legs, I posted 14.9 and 16.1 mpg. This is the worst hwy mileage I've posted since I began hypermiling the jeep. Both of these were level stretches of freeway with some rolling hills. I removed the grill block for the rest of the trip and mileage improved to 18.4 on the next leg. The next leg was windy mostly two-lane, and a had an approximately 3 mile 6% grade in it. I know this isn't a very scientific test, because the roads changed between runs, but I would think the third leg would be worse for economy than the first and second. Also, the fourth leg was very similiar to the first and second and I again was over 18. I have two hypotheses for these results:
-The grill block decreased airflow too much, so the fan worked more and the extra drain on the engine canceled out any aerodynamic savings.
-Either myself or the road introduced enough variation that the test is just invalid.

Has anyone here had experience with grill blocks on 97-2006 Jeep Wranglers(TJ series)? If you have, what effects did you notice on fuel mileage? I'd like to gather some more info on this vehicle before I run more tests of my own. If I cant get anything, I'll probably run the block again with a larger opening too see if I get different results.

I think that's it, in a nutshell.

Not saying the idea that over-blocking the grille couldn't reduce mileage, but the driving you did isn't a "test".

There are a number of Jeep owners who have grille blocks. Hopefully they'll chime in with their experiences.

honestly I dont think a grill block on a jeep will do much beside overheat the engine. if you have the 4.0 I6 especially, that engine runs hot. the front of the jeep is essentially a brick, the holes in the grill might even allow enough air to pass through to make it a smaller brick aerodynamically

Hello,

A headwind / tailwind could easily explain the change? Do you have a picture of your grill block?

I didn't have a noticeable change in winds. No I don't have a picture, but it was just clear plastic sheeting stretched across the grill with duct tape:) holding it on. I cut out the plastic over the center grill slot for airflow.

Nope.

I had blocked approximately half the radiator on my Thunderbird in an initial temperature test for aeromods/trying to get it to OT faster. I discovered I couldn't reduce the airflow that much due to the clutch fan engaging frequently and dropping the mileage like a rock.

No data for me or you for sure but I could hear the fan in my situation.

My dakota quad cab looks a lot like the jeep to the on coming air. I took off my clutch fan and totally blocked the upper grille. This netted me about 1-1.5 mpg. I'll probably have to install an electric fan after this winter, but so far I have had no overheating problem.--It's a 3.7L

I've got all but about 1 of the openings blocked right now. Great for winter time warm up. So far I haven't had any problems with it getting too hot. Summer time I had 4 of the openings blocked and the only time it kicked in the fan was when we saw temps over 95F with a lot of stop and go in town driving(then again what car won't). Haven't seen a drop in mileage. I've done several mods at once so I'm not sure how much it improved it. But I do remember after putting them in I noticed it pick up speed faster going down hills. So I guess my butt dyno says its helping.

I did a simple cardboard grille block on a '97 dodge ram (which has a HUGE grille) and saw 2mpg, or about a 10% improvement. I did the same thing to my Chevette, which is shaped like a small brick and saw no improvement. So I think it depends on the vehicle. Grille blocks help some more than others.

Why not? The grill is big and flat, and has huge holes in it. It looks like maybe the Jeep was designed for a lot of flow through the grill, past the motor, and out the bottom of the engine compartment and/or the wheel wells. So blocking the grill would stop that flow, resulting in a big high pressure area sitting in front of a planar surface, which could result in more drag. It isn't like a passenger car, where the grill is smaller, and the front of the car more aerodynamic to start with.

you could try using a flat angled sheet of plastic from the front edge of the bumper to the top of the grill, maybe clear plastic and youd get you headlights to shine through. also smooth out and fill in the gap between the bumper and wheel wells, throw an airdam on there that covers the front wheels as much as possible, and if you have skinney tires and flares, 4 wheel skirting would help.

though, I dont know how much all that would do. your biggest problems are the engine and drivetrain, neither are efficient at ALL, but then again they are rugged and tough

I think people on this forum put too much stock in grill blocks. They think that because the air going through the radiator and engine bay is turbulent, having the airflow slam into even a vertical surface is somehow better. I think if anyone here had a jet engine, they'd try to block the intake "because flow through a jet engine is turbulent".

A grill block may improve aerodynamics on some vehicles, but on a Wrangler, airflow is gonna hit that grill block, and fan out.

To see what I'm talking about, hold a knife horizontally under a running tap. Now hold a fork under the running tap to simulate a grill.

You should try an angled air deflector attached to the bumper. You can extend it downwoard to deflect flow under the front axle, and attach a bellypan to the crossmember to deflect air under the rear axle

a vehicle has to already have some aero properties to benefit from a grill block. sealing the front up helps its aero. with a brick all you are doing is blocking radiator. Helps it reach operating temps faster. Its not doing alot of diverting air. You'd need a nosecone or what others are posting IMO. I would do simple cardboard rad blocking, and also work on sealing up the underside.