'97 Jeep Cherokee XJ 4x4 overland rig
 

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 gas used, less possibility of overheating (that's an issue with these hot-running motors), and possibly at a faster speed. Goal: find mods that are helpful to both off-road performance and aerodynamics/fuel-economy.

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

http://dannix.net/sites/default/file.../pics/myxj.jpg

There is lots of room for improvement. Check out my main inspiration, this guy got his XJ to go from 16 mpg to 27 mpg highway, and others claim to get near 30mpg highway: http://ecomodder.com/forum/showthrea...mods-2614.html

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 some ideas on 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
• rear diffuser...?? It could also double as a gas tank skid plate!
• rake the vehicle by lifting the rear only a few more inches? I doubt this will help, it seems to me that a raked vehicle is only a benefit because the front is lowered rather than the rear raised.
• tire spats! for the rear tires
• Soften the aft edge of front wheel arches

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!

https://youtu.be/a8PUBB99opo


Decreasing rolling resistance

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: Taller Tires - EcoModder 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)

3rd part to decreasing rolling resistance: replace all the gear fluid to a lighter synthetic. Will be expensive but will help a lot as the front axle is always spinning even if in 2wd mode, because the front hubs aren't selectable.

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 Gear Ratio Calculator 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:

https://cimg6.ibsrv.net/gimg/www.che...78e5583b4d.jpg

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.

Another thought: swap in a VW TDI engine. There's swap kits available for putting TDI into Jeeps!

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.

OK here's my XJ.

https://cimg7.ibsrv.net/gimg/www.che...309c274585.jpg

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.

https://cimg2.ibsrv.net/gimg/www.che...5a6aa16d87.jpg

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.

https://cimg5.ibsrv.net/gimg/www.che...8fc1c880fe.jpg

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..

https://cimg0.ibsrv.net/gimg/www.che...04fe6f511b.jpg

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.

https://cimg1.ibsrv.net/gimg/www.che...8a0e3f8f5c.jpg

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.

---------------------------------

A few days ago I purchased my first welder. It's a DC stick welder. Looking forwarding to fabricating and modding!

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. 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
• build a gas tank skid plate and connect it to the rear bumper with a diffuser
• 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

oh yeah, base MPG figures...

A few days ago I figured the highway numbers to be 16.7 MPG.

I simply fueled up before leaving town, and fueled back up when I got home, and on the second fuel up I noted how many gallons was used. Then I divided the number of miles driven by that number. Because the bigger tires throw off the speedo a little (I'm actually going 75 mph when it says I'm going 70) the odometer is also off a little. So I used Google Maps to find out exactly how many miles it was between gas stations

Next I'll get some city data, though it will be less accurate as I won't be able to use google maps, will have to rely on odometer

Use a tire size calculator to find the difference in revolutions per mile from your OE tire size to your actual tire size. Then just multiply your indicated miles by that when you calculate it and you're in.

I'm sure this is just like every other ecomodder nube, but I'm looking into vortex generators. They seem to be hit or miss. But the back of the XJ is a big giant box, not unlike a tractor trailer, and maybe a vortex generator "tuft test" (I'm already getting into the lingo!) is worth the $20 and 5 minutes. Perhaps.

Alright let's see how smart I am.... or how correct you are.... *after some time has past* ...ok now that I have realized that revolutions per mile is a tire thing and revolutions per minute is the engine speed and the two are not related...

28" tires revolutions per mile: 742
31" tires revolutions per mile: 670
difference: 742 - 670 = 72

I have a feeling multiplying the trip odometer reading by 72 is not going to get me my actual trip millage....

Maybe you mean get the ratio. So 742:670 = 1.1074:1

So my trip odometer reading of 66 miles, multiplied by 1.1075 equals 73 miles which is what Google Maps also said the millage was. Yay!

I got my first city MPG reading, but it's only half a tank of data:

12.7 MPG

Together with highay (16.7) make it a combined 14.4 mpg.

Not very good. Worse than I expected, tbh. Maybe I should just get a TDI motor and auto tranny...

Be careful with that skid plate and no external cooler for the trans. My buddy had an xj that he fried the transmission in because he had full skid plates and the radiator cooler just wasn't enough to keep the trans cool.

If you are overheating at highway speeds, you need to check your water pump. My little brother has a YJ (same motor) and going out to Glamis we overheated at anything above 60mph (4 people and full of gear). When we got home the radiator checked out, so we checked the WP and sure enough, the blades were eroded enough to not flow as much water under prolonged heavy load. Changed it at 195k miles, haven't had a problem since.

I've got 31" A/T's and a six speed and get 20mpg (off road only), and the XJ is far more aerodynamic than a TJ. Still gets around 26mpg highway. Stock other than tyres.

The 4.0 is over powered so dropping highway RPM can only help, even with the aero penalty of the bigger rubber.

Maybe I shouldn't be surprised. EPA figures for my vehicle are 13 city, 18 highway. http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=29386&id=29388]

And I'm getting almost exaclty that on a motor with over 250,000 miles, I should be thrilled!

good advice, thank you. I do, however, have an external transmission oil cooler. Also, I replaced the water pump a few months ago

oh damn!

how the **** do you do that? Is it all in the 6-speed? That would be a worthy swap

The single biggest aerodynamic gain is with a full boat tail. So how would one do that?

Here's the science:

http://ecomodder.com/forum/member-fr...1215134663.jpg

Here's a model:

http://ecomodder.com/forum/member-fr...pe-ii-boat.jpg

This example uses a Gothic arch, but a truncated boat-tail could be four rigid curved trapezoids. The main thing is a rigid floor. With a high hinge line and short arms, it could swing down and align with the diffuser you're planning.

It's like the Trailer Tails on big-rig trucks, except it's not cut off to clear a loading dock.