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Ecomodding! Making my XJ overland rig drive on-highway just as well as off-road

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Ecomodding! Making my XJ overland rig drive on-highway just as well as off-road

Old 01-03-2018, 11:03 AM
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Default Ecomodding! Making my XJ overland rig drive on-highway just as well as off-road

I'm going to start modding my XJ overland-rig to be more aerodynamic on the highway. This will allow me to get to my off-roading destinations (most are 8+ hours away) with less overheating problems, with less gas used, and possibly at a faster speed.

My vehicle is a 1997 Jeep XJ 4.0L auto. 2" lift and 31" AT tires installed. 250,000 miles on the odometer.

*Is this guy serious? He found the least aerodynamic and fuel efficient vehicle he could find, lifted it and ruined the gearing with larger tires, and now he thinks he can make it a gas sipper?*

Yes I'm serious! There is lots of room for improvement! Check out my main inspiration, this guy got his XJ to go from 16 mpg to 25 mpg, and other claim to get near 30mpg: http://ecomodder.com/forum/showthrea...mods-2614.html

*Yeah but if you're overheating it's because you've neglected your cooling system.*

Well I wish that was true but it's not. I've replaced the radiator, radiator hoses, water pump, thermostat, and fan clutch. I've also flushed the coolant passages many many times, including once with oxalic acid to eat away the rust inside the engine, and using WaterWetter in the coolant. After all of that the engine can still overheat if the outside temp is too hot. However, if I stick to 65mph or less, it doesn't overheat. I'm going to replace the copper core radiator with an aluminum one and get all electric fans, but I think part of the problem is that the larger tires (31") with stock 3.55 axle gears took the engine RPMs out of the torque range when in overdrive at 75 mph. Slowing down to 65 mph helps because the drag at that speed isn't as bad, but this takes the engine RPMs even lower when in overdrive.

So there will be three parts to my ecomodding quest:
  1. reducing drag
  2. decreasing rolling resistance
  3. increasing torque at lower RPMs

Reducing Drag

Here's how to get the air to flow around the XJ without dragging it down so much:
  • air dam
    • the job of the air dam is to deflect as much air to go around the vehicle and tires as possible, so that it doesn't go under the vehicle
    • keep the stock air dam! it's the plastic thing under the front bumper. You can even extend it and make it bigger
    • Removing the air dam will probably mean a drop in 1 or 2 mpg on the highway
  • skid plates
    • this mod is great because it's awesome for both off-road and on-highway
    • the air that is traveling under the jeep can get sucked up into the negative pressure zones under the body and swirl around and cause drag. Skid plates can help prevent this
    • The skid plate that protects the front suspension and steering stuff also does a great job of deflecting air to go under the front axle and engine (see the youtube vid below)
    • transfer case skid plate helps as well, make it as flat and wide as possible
  • roof rack delete
    • yeah I don't like this one either but, depending on how it's designed it can really really slow you down

Check out this video I found on youtube. It shows the effects of the front skid plate on air flow around the front axle. Pretty cool to see how it works!



Decreasing rolling resistance

Surprise, surprise! Larger tires can potentially increase your MPG. And they increase your off-road fun! Win win!!

Taller tires changes the final gearing and reduces the rolling resistance. Read: http://ecomodder.com/wiki/index.php/Taller_Tires This is kind of flying in the face of the old rule-of-thumb that is "for every inch of tire size you go up, you loose a MPG." So which is it? I'll deal with this question in the next section.

2nd part to decreasing rolling resistance: inflating the tires to 40+ psi. Quick story with my jeep that runs hot: I was driving for a few hours to Wichita, Kansas, and the jeep started to overheat bad although it was only 70 degrees out. I had to slow down to 65 mph until I got to a rest top. There, I inflated the tires to from 30 psi to 40 psi. For the rest of the trip I could go 80 mph without over heating! (again, a better radiator will fix the running hot problem, but it illistrates how much more efficient the jeep goes down the road with higher pressure in the tires)

Increasing Torque at Lower RPMs

I emphasized "can potentially" in the above section because, when you increase the tire size without correcting the axle gears, you change the final gearing and this changes the RPM that the motor will run at with respect to vehicle speed.

For example, with 31" tires replacing my stock 28" tires, the transmission is now in overdrive and cruising at 75 mph at a lower RPM. I used http://www.grimmjeeper.com/gears.html to calculate the my RPM at 75 mph:

With stock 28" tires: 2400 RPM
With new 31" tires: 2160 RPM

Now take a look at the power band of a 1997 4.0L I-6 motor:



Notice that the power is significantly lower at 2000 rpm than it is at 2400 rpm. You're going to have to push the go-faster peddle a little harder now with 31" tires than you would with 28". Therefore you MPG will suffer.

But if we can spend a little time and effort to add more power, especially in the lower RPM range, then we can get our fuel efficiency back.

At this moment I'm still researching the best/easiest ways to increase power on the lower RPMs. It may mean adding power across the whole rpm range, or it may mean moving the power band south.


Final Thoughts

With the XJ and it's stock aerodynamics, any speed beyond 65mph is going to make you lose MPGs, but when in overdrive and with larger tires, the engine is out of it's power band at that speed, so you will lose MPGs this way too.

By improving the aerodynamics, you can go faster than 65 mpg without loosing (as much) fuel efficiency. And by improving the power band, the engine is much more comfortable and efficient when in overdrive, which allows for larger tires.

Last edited by mannydantyla; 01-03-2018 at 01:18 PM.
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Old 01-03-2018, 11:43 AM
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I understand you are probably serious here, But right out of the box I'm having trouble taking it seriously because of the idea you linked to there. Anyone who would think blocking off part of the grill opening will make any difference is off a bit. lol
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Old 01-03-2018, 01:20 PM
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Originally Posted by Bugout4x4 View Post
I understand you are probably serious here, But right out of the box I'm having trouble taking it seriously because of the idea you linked to there. Anyone who would think blocking off part of the grill opening will make any difference is off a bit. lol
Try to keep an open mind http://ecomodder.com/wiki/index.php/...artial_or_full
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Old 01-03-2018, 03:44 PM
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OK here's my XJ.



As you can see I've already got a good start. I've removed the fender flairs and trimmed the fenders just a tiny bit. However I kept the front bumper and air dam all intact. I'm really glad I did that now.

One of the things I did when trimming the fenders was I had to deal with the plastic inner fender shield things. Instead of tear them out like most people, I trimmed them and then "stitched" them to the plastic bumper covers using zip ties.



Last summer I added some hood vents, and I added them to the lower pressure zone at the front of the hood, not at the high pressure zone at the back.



At the time that I added the hood vents, I was 100% concerned about the best way to get hot air out of the engine bay. Thankfully that was also a good design choice for aerodynamics! Negative pressure (vacuum) zones, especially when behind the vehicle, slows the vehicle down. When on top of the hood, it's like the lift of an airplane wing. My hood vents will actually help reduce this! Maybe..

It's now winter and I've added a few things since thing..



Aside from all the road salt (gross), the XJ is now sporting TJ mirrors and window deflectors.

The TJ mirrors are probably less aerodynamic than the original ones (but they look cooler!) so I may or may not put the old ones back on. The window deflectors are there because, in the summer, I can cruise with the windows open (I have no AC) more comfortably and efficiently - windows down ruins your aerodynamics.

One more thing I've added recently is a transfer case skid plate. I made it myself.



This is great because not only does it protect the TC it also keeps the air flowing smooth down there. I'll probably extend it a little more by welding on new material, not for protection but for air flow.
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Old 01-03-2018, 03:56 PM
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Here's my plans for the near future:
  • remove the stock cargo rails, and possibly make custom ones that will look like those found on a Nissan Xterra of FJ Cruiser. I think that will be much better
  • add a skid plate in front of the front axle - Rusty's Offroad has one for $130, or I can try to find a factory one, or I can make one myself
  • add little tiny skid plates in front of the lower control arm mounts on the front axle, like these ones. It protects and it makes it more aerodynamic. Win win!
  • extend the transfer case skid plate
  • remove the rear bumper end caps and possibly do a cut and fold to the rear lower fenders and build a new rear pumper or something
  • seal up the rear bumper so no air can swirl around inside of it
  • air up the tires to 40 psi
  • get some engine mods (yet to be determined) such as cold air intake, full length headers, neon fuel injectors, etc. that will boost power especially at lower RPM
  • replace the mechanical fan with electric fans
  • get a new radiator

Too bad the front hubs aren't selectable. Although you could remove the front driveshaft and put it back on when you're ready to go wheeling but that's not for me

Last edited by mannydantyla; 01-07-2018 at 10:25 AM.
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Old 01-03-2018, 03:58 PM
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May I suggest one of the few mods that will help with drag on that? I'm actually going to do this to mine because I am tired of my back window and hatch dusting up. Some way to deflect air from above the hatch or on the sides behind the window, to behind the vehicle and reduce the drag spot behind it.
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Old 01-04-2018, 02:54 PM
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Serious question, do you have actual experience in fluid dynamics, statics, and general dynamics?

Much of this seems to say not so much.

You wouldn't be the first person to try this, and will most likely find out it's not worth much time.

By the time you make an XJ, a large, square object much more aerodynamic, you'll have added enough weight to make the effects negligible.

1.) You mention larger tires and torque curve. While yes, proper gearing and such will help, you still have AT tires in a larger than OEM size. This means larger rolling mass and more friction. Just because it's geared, and the RPM's are lower, doesn't mean the vehicle is exerting the same or less power to achieve the same goals. If that were the case, my buggy on 40's would get up to speed a lot quicker.

2.) Airing of tires. You're running a C Rated (my assumption) AT tire. The max inflation on that is most likely 35psi, possibly 40. Sure, you might improve mileage, but you're going to wear out your tires a bit quicker. So, the cost in that case might break even, but most likely a rather large negative.

3.) "Well I wish that was true but it's not.", in regards to your cooling system. Something is still wrong, whether you want to accept it or not. The cooling system is rather simple overall, and is plenty capable of cooling yours. I was able to drive a '98 XJ on 35's with 4.88's across country, and at one point hauling a Uhaul enclosed trailer. Never once did I have temp issues. There's still something wrong with your's, and getting it fixed will help you run more efficient.

There are ways to achieve more towards what you're looking for, but you might want to step back and ask yourself how much time and money you're willing to spend, for negligible benefit.

Sorry, but those 30mpg claims I call BS on. Without completely retrofitting the vehicle (and thus making it look like something out of a sci-fi movie), it ain't happening.

I've gotten 21 mpg twice, but that was with a stock XJ with a 5 speed manual, 3.07's, and OEM tires, and at the magical 65mph.

Last edited by Freerider15; 01-04-2018 at 04:11 PM.
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Old 01-04-2018, 05:29 PM
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I don't think you can put a TDI or another Diesel motor in them and make 30+mpg so how are you going to do it with gasoline engine? I'm going to sit back and enjoy the show which I have so far and would love to see how this pans out. Good luck though my friend!

Then again I could be wrong on the diesel swap MPG but still if your going to go as far as you are to make it eco friendly then I would just dump a TDI in it and get way better MPG than you do now. Sure it will require some modding and money to get there but would be better suited and easily achieved that way.
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Old 01-04-2018, 08:34 PM
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Guys, woah woah woah! Slow down! I never said I'm going to try to get 30 mpg, or even 25 mpg.

If I could get 20 mpg then that would be great!

There's been a new development today, however. I'm inhereting my Dad's '09 GMC Sierra 2wd work truck, he passed away yesterday. Maybe I'll sell it and invest the money in my XJ. Maybe I'll trade it in for a tiny hybrid and my daily driver XJ can now be transformed into a crazy rock crawler buggy, or maybe I'll keep the sierra and sell the XJ (unlikely). Hell, maybe I'll take the motor out of the GMC and put it in the XJ (very unlikely).

I got a lot to think about (not just about the trucks) and I don't think I'll make any progress on this jeep for a few weeks at least
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Old 01-04-2018, 09:00 PM
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Originally Posted by XJIrish4x4 View Post
I don't think you can put a TDI or another Diesel motor in them and make 30+mpg
There is a gentleman on this board doing damn near this exact thing. VW TDI pushing right at a consistent 30 mpg or slightly over last I read.

Getting there with a 4.0...not gonna happen (in an XJ at least) but as the OP has already said they're not shooting for that goal.

Originally Posted by mannydantyla
I'm inhereting my Dad's '09 GMC Sierra 2wd work truck, he passed away yesterday.

I got a lot to think about (not just about the trucks) and I don't think I'll make any progress on this jeep for a few weeks at least
Very sorry to hear about your father passing, as they say time heals all wounds.
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Old 01-04-2018, 10:31 PM
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Very sorry to hear about your families loss, my condolences go out to you and your family! Hope for what it is worth you do get to spend some more memories in your Dad's GMC or make use of it to fund your project.
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Old 01-05-2018, 07:59 AM
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[QUOTE=mannydantyla;3452566]Guys, woah woah woah! Slow down! I never said I'm going to try to get 30 mpg, or even 25 mpg.

If I could get 20 mpg then that would be great!

There's been a new development today, however. I'm inhereting my Dad's '09 GMC Sierra 2wd work truck, he passed away yesterday. Maybe I'll sell it and invest the money in my XJ. Maybe I'll trade it in for a tiny hybrid and my daily driver XJ can now be transformed into a crazy rock crawler buggy, or maybe I'll keep the sierra and sell the XJ (unlikely). Hell, maybe I'll take the motor out of the GMC and put it in the XJ (very unlikely).

I got a lot to think about (not just about the trucks) and I don't think I'll make any progress on this jeep for a few weeks at least[/


my cherokee gets 20 mpg.......when its on my trailer being towed by my f150

Last edited by tinytrax78; 01-05-2018 at 08:00 AM. Reason: spelling
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Old 01-05-2018, 08:25 AM
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Originally Posted by tinytrax78 View Post
my cherokee gets 20 mpg.......when its on my trailer being towed by my f150
How are you getting 20 with an F150 pulling a trailer??? lol
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Old 01-05-2018, 11:33 AM
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Switch to a 5-speed and problem solved. I get 20 mpg in mine doing 70-75. My display says I get 24 mpg but we all know how accurate those are. I did about 4000 miles in road trips last year and the mpg was always +/- 2 of 20 mpg.
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Old 01-05-2018, 03:30 PM
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Alright, I got an initial MPG reading today. I drove 71 miles, mostly highway with maybe 10% city, and I used exactly 4.25 gallons (I fueled all the way up before leaving, then fueled up again and after it was over and however much I could fit into the tank the second fuel up equals exactly how much gas was used).

71 miles / 4.25 gallons = 16.7 miles per gallon. 90% highway.

Not horrible but a lot of room for improvement

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