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'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:

reducing drag
decreasing rolling resistance
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.

Quote:

Originally Posted by Fat Charlie (Post 558469)

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.

Quote:

Originally Posted by mannydantyla (Post 558449)

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.

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!

Quote:

Originally Posted by Baltothewolf (Post 558500)

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

Quote:

Originally Posted by oldtamiyaphile

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.

I found a very good write-up on how one guy got his Jeep XJ to get 25 mpg (at 60 mph, he says) - mostly by tuning the engine. http://www.jeepforum.com/forum/f11/m...5-ats-1551782/

Maybe I should focus on tuning the engine

Here's all the tuning he did:
My goal is to get 25+ mpg from my 99 XJ with 30 x 9.5 R15 ATs - Page 221 - JeepForum.com

Quote:

I used to get 15 to 16 mpg highway before I started trying to improve. Now I'm getting 24 mpg at 60 mph highway on a good day. On a bad day 21 mpg. Most days 22-23 mpg. I consider that darn good for an XJ with 1.25" lift, 30 x 9.5 R15 Cooper AT3 tires, and 200 lbs of skidplates. My XJ is super peppy because several of the mods improved torque and-or HP, as well as gas mileage.

Though it should be noted that I'm using 89 octane gas (see explanation of why I'm using 89 octane gas in the Kolak ignition section later in this post).
I've been driving it with these mods for 1.5 years. It's time to post an update to this thread.

I've learned which mods are helpful and offer the most bang for the buck, and which are not worth it.

I was thinking about this today when I was helping my cousin with his new-to-him old 98 XJ that needs fixing up, and his kids 89 Comanche that needs fixing up. They want to know which of my mods I recommend for them, and which they can or should skip. That's a valid question since several of my mods helped my gas mileage, torque, and/or HP, but some mods made no difference, and one or two mods reduced my gas mileage (and I had to undo those mods).

Here are the mods that gave me the most bang per buck by improving gas mileage, and some of these mods also improved torque, HP, easier startup, and smoother idle.

1) 205F Stant Thermostat (part # 13352) is great for Northern climates year round. Cost $5. It helps efficiency because gas burns more efficiently when engine is fully warmed-up to full op temp. Very nice side benefits are a warmer heater and defroster for more comfort and faster defrosting in Winter.

If you've ever driven in a really cold damp blizzard, you may have experienced that your defroster can't keep windshield thawed out, defrosted, and defogged. The solution is a 205F thermostat. Your defroster (and heater) will easily keep your windows thawed, defrosted, and defogged in any weather.

This is a great improvement (warmer engine) in colder climates, especially in Winter, but also in Spring and Fall. In Summer it won't hurt anything because it still runs the same temp as with the 195F thermostat that I used to have. With either stat (195F or 205F thermostat) my XJ water temp is 210F to 212F in Summer (which is perfect for efficiency) and 208F to 210F in Winter (near perfect). A fully warmed up engine runs cleaner and more efficient (as long as it's not warm enough to engage the mechanical fan clutch or electric fan). My fans engage-activate around 214F or 215F (so running at 208F to 212F does not engage-activate my fans).

For hot Southern climates and year-round use, I don't know whether a 195F or 205F thermostat is best. I'll leave that to Southerners to decide what they think is best. If you choose a 195F thermostat, then I suggest a Stant Superstat because it does a great job of maintaining a steady engine temp, especially during warm-up, and Superstats are capable of very high flow rates on hot Summer days.

2) Adjusted auto transmission cable for better shifting at appropriate rpm. My engine was lugging and not shifting when it should. After I adjusted the transmission cable, the shifting and engine efficiency improved noticeably. I adjusted it myself the first time, with some success, but still not as good as I wanted it to be. I wanted it to shift at a slightly higher rpm so my engine could have more torque for my 30" tires on hilly highways. So then I had my local transmission shop adjust it and they got it shifting really nice at the perfect rpm and they didn't even charge me for it.

3) I changed my rear diff oil from 75w140 synthetic (tow package rear diff oil) to 75w90 synthetic. This reduced my rolling resistance noticeably (especially on cold days) and helped my gas mileage enough to be a worthwhile mod, especially in cold weather. Cost $20 for two bottles of 75w90 synthetic gear oil. I'm using Valvoline Synpower 75w90 and like the results. According to its spec sheet it has a better viscosity index than other brands of 75w90 (thinner when cold, but just as thick when hot - compared to Lucas, Mobil One, and other brands I compared). Newtons recommended this to me earlier in this thread and I've found it helps.

4) I removed my stock roof rack. That probably helped highway gas mileage a little, but I don't know how much since I didn't do a before and after test for this mod.

5) I removed my winch and winchplate to reduce weight and wind drag. This helped city and highway gas mileage and saved my nerves. My winch plate used to whistle at highway speeds and drove me insane. Glad to be rid of it. A tow hook is good enough.

6) I removed my AC compressor and replaced it with an idler pulley. Cost $20 for idler pulley of Dorman brand from Oreilly's. My AC didn't work anyway, and I live in a cool enough climate to not need AC except once every 2 years or so. The AC system is very inefficient in a cold wet climate like mine. The AC compressor keep cycling on and off when I use the defroster to defog my windshield, and each time the AC compressor cycles it causes the electric radiator fan to come on too. Removing the AC compresor saved a lot of weight under the hood and it means my compressor and electric fan no longer cycle on and off when using the defroster. This increases my Winter-time efficiency. My windows still defog and defrost as fast as ever. Faster actually (thanks to my 205F thermostat).

7) Motorvac cleaning of topend of engine. Motorvac is a trademark name for a professional cleaning process done by some auto repair shops. It's a similar idea as Seafoam, but I think it's more effective than Seafoam. Motorvac cleaning removes carbon deposits from the topend of an engine, and is a great place to start for a tuneup, IMO. I suggest changing motor oil afterwards (just in case Motorvac chemical contaminated the motor oil).

8) Modern 4 hole fuel injectors. Which injectors you should use depends on your model year of XJ (or TJ). See post 209. Cost $190 purchased from Motorman on Ebay, but that price might be out of date by now.

8) Kolak Ignition Kit (it's both a tuneup kit and an upgrade kit). See OP for more details on this. Includes new cap and rotor with brass contacts, new Autolite Single Platinum plugs, Magnacore upgrade racing plug wires, and an upgrade MSD coil. Gap plugs to 0.045" (for my 99 XJ model year anyway) for a longer, hotter spark for better ignition. Cost $230 last time I checked, but that price might be out of date by now.

Note: After installing Kolak ignition, my XJ now requires 89 octane gas to run well. It purrs beautifully on 89, but on 87 it runs so rough it shakes the whole car like an alcoholic who ran out of booze. So be aware that if you install a Kolak ignition you'll have to use 89 octane gas, which partially negates the financial benefit of gas mileage improvement from the Kolak ignition. So money-wise the Kolak ignition is a break-even deal, but is well worth it for the increase in low-end torque and easier cold starts.
10) Jeepers and Creepers 00 battery cables. These are very high quality, 00 thick, and give the engine improved grounding via the huge 00 ground wire, and a 00 thick positive wire as well. Cold starts are vastly improved, and I suspect these battery cables help me get the most from my Kolak ignition. These battery cables help my engine start easier, idle smoother, and might even improve my performance-torque-power (along with Kolak ignition). It may even help my gas mileage, but I don't know since I didn't test gas mileage right after installing these battery cables.

If you use flat washers as part of installing your battery cables and ground wire (to engine), use brass flat washers because this helps conductivity, IMO.

I don't know how much of my ignition improvement is from Jeepers-Creepers battery cables and how much from Kolak ignition. The combination of the two is great. I suggest doing both since IMO they work together as a team for easy starting in any weather, smooth idle, increased lowend torque, and a slightly more mid-rpm torque. Gas mileage improved slightly because of these electrical mods, but fuel cost savings of Kolak is partially offset-negated by having to use 89 octane gas.
11) Dynamax Cat Back and Dent-Free Downpipe help enough to be worthwhile, but I'd do these last since the prior mods help gas mileage and performance more per dollar spent, IMO. The Cat Back and Dent-Free downpipe work well together as a team. You can't fully benefit from either of these mods until you've done both these mods.

The best prices I've seen on Dynomax 2.25" Cat Back is at Summit Racing. I paid $150 with free shipping (but that price is probably out of date by now). Make sure you get the 2.25" diameter CB, not the 2.5" diameter CB. I tried both.

The 2.5" CB decreased my gas mileage and reduced lowend torque compared to stock exhaust. The 2.5" CB is recommended for strokers, but is terrible for a stock(ish) 4L engine, IME.

The 2.25" CB" improved gas mileage, retained torque, and improved my horse power compared to stock. The 2.25 CB is excellent with a 4L engine.
SummitRacing.com has the best prices on Dynomax cat backs and other Dynomax products.

I like the Dynomax muffler (included with CB) because it sounds slightly mean at idle, but quiet when cruising.

You can't buy a dent-free downpipe online. You have to make you own or have one made. My local muffler shop made me a dent-free downpipe and installed it at the same time they installed my Dynomax CB. Cost of my downpipe was $90 to make and install it.

12) Edited in Later: I should have listed this as number 1. I got my Jeep aligned and monitored my tires pressures carefully. Don't under estimate how much good alignment and tire pressure help gas mileage, and also help tires last longer and improve handling. Alignment and tire pressure are the first things we should do for our Jeeps to be more efficient.

In addition to the above mods, I did several other mods that didn't make much difference to gas mileage or performance. So I'm not even going to mention them here.

It's worth mentioning that during the 2 years of modifying and testing gas mileage, I added 225 lbs of skidplates under my XJ to protect it off road. This extra weight probably reduced my city gas mileage slightly, but I didn't test city gas mileage. I tested highway gas mileage. The skidplates did NOT change my highway gas mileage. The skidplates add weight, but make the underside of Jeep smoother (less wind drag). So IME skidplates don't affect highway gas mileage.

Note that he says at the end that the underbody skid plates did NOT help is fuel efficiency and may have hurt FE in the city due to their weight. Hmm....

Quote:

Originally Posted by mannydantyla (Post 558516)

oh damn!

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

The auto actually has a higher top gear than the 6 speed, so I'd say no, it's not in the gearbox.

I cruise at 55 though where traffic permits, mostly because TJ's are pretty unstable at speed and my tyres are as old as the car (no cracks or anything- but I don't trust them 100%).

Do you have instrumentation? That's always the biggest one.

From that list of mods, the only thing I'm planning is the injector swap (to 12 hole). I like the idea of the Banks turbo kit though.

The skid plates sit higher than the axles so they're in turbulent airflow which is why smoothing them won't help. Nor will smoothing the fuel tank transition. Unless you can somehow control the airflow from the axles anything more than an airdam/ tyre deflectors is a waste of time.

I've only got 18K miles on my TJ :eek:

Put a 2.8 VM Motori turbodiesel out of a Liberty in it
Jeep Cherokee 2.8L Turbo Diesel Conversion - Motor Mounts - Diesel Power Magazine

We got 24+ Hwy dead stock from our ‘01 XJ. 2WD. Option-loaded. Three adults plus luggage. Consecutive round trips to Chicago from Dallas.

Your other boat tail option is the beer cooler trailer U-Haul used to rent. Get something similar and build appropriately.

But you simply aren’t going to get “good” mpg much above 60.

“Thinking” is part of the problem. No trip takes time, they take miles. And those miles are understood as types of roads and traffic.

So long as time is your enemy, you’ll not find satisfaction.

.

Quote:

...the beer cooler trailer U-Haul used to rent.

[citation needed]

I've ordered some 4-hole fuel injectors, stock ones only have one hole. I've also removed the roof rack and I'm in the middle of fabricated a new one that will feature removable cross bars. It might make a difference, it might not, but it's cooler looking! So that counts.. Also, stick welding thin-wall galvanized steel is a terrible beginner's project! Pics coming soon

Quote:

Originally Posted by Hersbird (Post 558728)

Put a 2.8 VM Motori turbodiesel out of a Liberty in it
Jeep Cherokee 2.8L Turbo Diesel Conversion - Motor Mounts - Diesel Power Magazine

Good find! I'll consider it, but I have a feeling that the TDI diesel would be the better engine. In either case I would want to replace the auto trans I have now as it sucks too much power. It looks like if I go with the Liberty engine I'll need the liberty motor, but I think if I go with the TDI motor I can find an XJ manual trans to swap in there (AX-15 I believe).

But TBH an angine/trans swap isn't in the plans at the moment.

Quote:

Originally Posted by slowmover (Post 558745)

Your other boat tail option is the beer cooler trailer U-Haul used to rent. Get something similar and build appropriately.

Huh?

Quote:

Originally Posted by oldtamiyaphile (Post 558658)

Do you have instrumentation? That's always the biggest one.

Yeah I need to get on that.

Quote:

Originally Posted by oldtamiyaphile (Post 558658)

The skid plates sit higher than the axles so they're in turbulent airflow which is why smoothing them won't help. Nor will smoothing the fuel tank transition. Unless you can somehow control the airflow from the axles anything more than an airdam/ tyre deflectors is a waste of time.

That kind of depends on how much lift you have between the frame/unibody and the axles. I only have 2" lift. And wranger TJ is probably very different, I'm assuming. Also, if I'm fabricating my own then I can make it as big and low-hanging as I want.

And you're correct about the fuel tank transistion, I'm afraid. An interesting/crazy option might be to lift the fuel tank up into the body. Check this out:

http://www.pirate4x4.com/forum/attac...picture053.jpg

Quote:

[citation needed]

Quote:

Huh?

Made me look:

https://www.ls2015.com/wp-content/up...L-TRAILERS.jpg
https://www.ls2015.com/farming-simul...ers-v-1-0.html

...as interpreted by the Hanomag Deutsch Modding-Team.

The U-Haul “Weekender Sport Trailer”.

I believe we had a few over 300 bottles of beer upright and iced. Plus quite a bit of other stuff.

Ok guys, I need your help.

Which do you think is more aerodynamic? My original XJ mirrors, or the TJ mirrors I put on?

Original XJ Mirrors:
http://dannix.net/sites/default/file...nal-mirror.png

TJ (wrangler) mirrors:
http://dannix.net/sites/default/file...e/IMG_2374.JPG

The TJ mirrors can also be folded in while still being usable as side mirrors (not just folded and pointed at the cabin)

Too late to make an adapter plate and reuse the original mounting holes :(

Quote:

Originally Posted by freebeard (Post 558935)

Too late to make an adapter plate and reuse the original mounting holes :(

If I did that the TJ mirrors would sit too high and would block my view out of the winow

I can put the old mirrors back on but would have to deal with the holes I drilled in the door. Not a big deal just not something I really want to do. The TJ mirrors are better looking but worse in every other way (they're manual, old ones were powered; they don't reflect as well; etc) and if it's agreed that they're definitely worse for aerodynamics then I might just have to swap them back out.

BTW I haven't yet but I am definitely covering those exposed holes in the door where the old mirrors were bolted to

I'm not an expert in aerodynamics, but those TJ mirrors seem to be bigger, also that "arm" is quite big.

As an former off roader i still would keep the TJ mirrors, would suppose that they are better for reversing?

I’d pay the penalty for the TJ mirrors. In fact I’d add a 4” tall or taller convex also the width of those TJ mirrors.

Convex is for what’s next to you. Higher importance.

On big trucks the “aero money” goes into the arms. Otherwise, the housing is rounded.

Anybody have an opinion on whether or not a rear spoiler like this would help with aero and FE?

http://dannix.net/sites/default/file...cs/s-l1000.jpg

They're on EBAY right now for like $152. I would just make my own. https://www.ebay.com/itm/JEEP-CHEROK...%257Ciid%253A1

Quote:

Originally Posted by mannydantyla (Post 558556)

Note that he says at the end that the underbody skid plates did NOT help is fuel efficiency and may have hurt FE in the city due to their weight. Hmm....

I found another ecomodder who claims that a front skid plate (he calles it the aero-skid, I like that) moved his 6.5-inch-lifted (that's a lot!) Jeep Cherokee XJ from 15.8 mpg to 22+ mpg at 65 mph ...

WHAT?!?! :eek::eek::eek:

http://ecomodder.com/forum/showthrea...tml#post292536

I was looking for a quote on Faschenfeld tearing edges when I found this:

http://ecomodder.com/forum/attachmen...1&d=1499046088
http://ecomodder.com/forum/showthread.php/random-wind-tunnel-smoke-pictures-thread-26678-17.html#post544437

This is a similar shape. The vertical edges are 'hotter'. two triangular fins from the outer edges down toward the taillight would be a good start on a boxed cavity. Here — according to the text is an explanation, if you can see Photobucket images: http://ecomodder.com/forum/showthrea...tml#post542668

^^^ thanks!

New idea: indentation of the fender right behind the front wheel, and air curtains (not to be confused with skirts!) for the front wheels! The indentation is easy; the air skirts will have to be fabricated along with a new front bumper (maybe one day far into the future). Ecomodding is an amazon thing. So many activities! So many enhancements to be made! Can you tell I'm excited!

http://us1.webpublications.com.au/st...4/2455_2mg.jpg

(source: http://ecomodder.com/forum/showthrea...s10-35043.html )

An extreme version of the fender indentation:
http://i.wheelsage.org/pictures/f/fe..._spyder_3.jpeg

And here's how air skirts work to reduce turbolant air around the front tires: http://ecomodder.com/forum/showthrea...tml#post496612

I feel like a BIG improvement that can be made with my jeep, with the huge openings for the tires, is at the front wheels. BMW agrees, they say on their official blog that 30% of the drag is at the wheels. http://www.bmwblog.com/2014/02/26/bm...n-engineering/ (the article is really really good btw)

http://cdn.bmwblog.com/wp-content/up...20-750x272.jpg

Also, notice the articulation of both front and rear wheels, I need every inch of the wheel well when offroading:

http://dannix.net/sites/default/file...92999424_n.jpg
http://dannix.net/sites/default/file...76606464_n.jpg

Lastly, here's a nice visualization of my jeep's aero:

https://robrobinette.com/images/S200...cs/CFD_Cow.jpg

Note the blue pressure zone under the rudder.

(lol ok ok I stoll that joke from here: https://robrobinette.com/S2000Aerodynamics.htm )

http://ecomodder.com/forum/member-fr...gt-one-tm1.jpg
ToyotaGT One TM1

The relief you show requires major surgery, extend the wheelbase or re-hinge [suicide] the door. Less radical is the Honda Insight and others with a rolled lip.

I think a good start for an air curtain would be a splitter under the front bumper with upturned wingtips, like modern air liners. It needs to be a converging duct that exits at the outer edge of the tire.

Quote:

Originally Posted by freebeard (Post 559028)

The relief you show requires major surgery, extend the wheelbase or re-hinge [suicide] the door. Less radical is the Honda Insight and others with a rolled lip.

Ok, let me make something clear...

That's not going to happen. :)

I'm not going to do anything too radical! I'm not going to have a NASCAR air dam that scrapes the ground. I'm not going to build a giant boat tail that another driver will drive into.

First and foremost, this is an off-road rig. Period. I really do appreciate your advice but please stop promoting silly ideas, this isn't aerocivic. (I really hope that doesn't come across as too rude)

As for the fender indentation to promote airflow reattachment, I'm just going to smooth the transition behind the front tires by bending the sheet metal a little, maybe a little welding, but it will stop at the door. It wouldn't be as sculped as the older Honda Insight's.

Basically, I would just flatten and smooth this section here, pulling the lower corner to the inside.

http://dannix.net/sites/default/file...reo-fender.jpg

And for the rear wheels, I'm still thinking. Right now the plastic bumper end caps are still on, but the ends facing the rear wheels were cut open when trimming the fender flairs so now they're like big air scoops. They should probably be removed, and the sheet metal behind it "cut and folded." What do you think?

http://dannix.net/sites/default/file...nder-scoop.jpg

http://dannix.net/sites/default/file...dceb68eab2.jpg

Just for fun, check out how I drove ~1,500 miles on an overland trip last year:

https://cimg9.ibsrv.net/gimg/www.che...a379ff6aa3.jpg

I think this year I'll figure out a better way to store the spare tire, lol

Quote:

I really do appreciate your advice but please stop promoting silly ideas, this isn't aerocivic. (I really hope that doesn't come across as too rude)

Come at me, bro. :)

Silly? It was intended as Reductio ad absurdum. I guess it worked?

Quote:

Originally Posted by freebeard (Post 559087)

Come at me, bro. :)

Silly? It was intended as Reductio ad absurdum. I guess it worked?

I guess it did work. And yes I had to look up Reductio ad absurdum.

This is my daily driver, it can't be a concept vehicle, I'm just trying to get reasonable fuel economy

It's not going to get me better MPG, because I've already disabled the A/C (still need to pull the compressor and replace it with an idler pully), but I've decided to paint the roof white to keep summer temps down inside the cabin.

Quote:

Originally Posted by mannydantyla (Post 559119)

but I've decided to paint the roof white to keep summer temps down inside the cabin.

http://ecomodder.com/forum/showthrea...oof-35655.html

Quote:

Originally Posted by oldtamiyaphile (Post 559134)

http://ecomodder.com/forum/showthrea...oof-35655.html

Hmm, maybe in the very center, like between the roof rack rails

You could look at the movement of your tires as they rise into the wheel wells. If you can close any of that gap without interfering with the movement of the tires, it's worth doing. Otherwise those wheelwells are just big pockets full of turbulence, making noise and eating energy that could otherwise keep you moving down the road. In your flexed photos, it didn't look like there would be too much interference if you brought the lips of the rear wheel wells down a couple of inches.

The rear spoiler above the liftgate could help, but experiment with the angles. So long as airflow stays attached it's helping.

An option for aero would be a hitch mounted tailgater with an aerobox built onto it. Build it right so it's collapsible when you get to the trails, move it inside the truck for wheeling.

An air dam would probably yield quick, cheap results, but since you want to keep this thing good for the trails you'll need to do some fabbing to make it easy to remove and stow inside the vehicle.

Like others said, instrumentation. You can't know what works if you don't know how it's working. Having instant feedback also makes it a lot easier to adjust your own driving style. A Scanguage 2 should plug right in to your OBDII port and allow you real-time info on your fuel burn rate, engine temperature (important to know if you're experimenting with grille blocks), etc.

Quote:

Originally Posted by elhigh (Post 559202)

Aerobox? I do plan on fabing a hitch and/or rear bumper mounted spare-tire carrier. So if it could carry the spare tire then win win. I still need to see out the rear window though as my side mirrors are not very good for that.

I already have an air dam. Keeping the factory air dam is kinda like a mod since 99% of cherokee owners who offroad them will take the air dam off first day they buy it. Maybe I could extend it but it would have be be with a softer material then plastic so it can hit rocks and sticks and stuff and bounce back.

Would you recommend the ScaneGauge^e if I wanted to save a little money? It's like $50 cheaper.

If the E says it'll talk to your XJ then it should be 95% of everything you could ask for. It'll leave a few features off that you might use if you were a serious ecomodder, but you sound more like a serious weekend wheeler. The E should suit you. It'll provide:
- water temp (you want that for grille blocks),
- avg and instant MPG,
- MAP,
- Intake Air Temp (very useful if you want to try a WAI for highway pseudo-lean burn cruising),
- TPS (also useful for WAI)

I think an E would do you pretty well.

For a softer but durable air dam, conveyor belt material if you have an easy source. If you don't have an easy source, find your nearest Rural King or Tractor Supply and pick up some baler belting. You may need to double it up to make it stiff enough to hold up against the wind of driving down the highway, but sturdy? Shoot yeah: it's designed to work inside a hay baler. It's tough enough. And if you have some left over you could extend sides down from your rockers, side skirts that, like the air dam, will give when you bump into something, but bounce back from it.

The hitch carrier can come right up to under the window and stop. All it has to do is fill as much of the wake behind your rig as you can make it, while tapering in from the sides (and up from the bottom) gradually enough to prevent turbulence from forming. Look up the Aero Template to see what an "idealized" aerodynamic profile looks like. You don't have to apply the entire template to your entire car, improvements can be made just be tweaking here and there. It's what the automakers have been doing this whole time anyway.

Also read up on what Aerohead has done, I think the guy is a literal rocket scientist. Even if he isn't, he's done what looks like Basjoos-level aero work on an otherwise stock T-100.

If you can lure aerohead into the conversation, great. I'd point to the NASA van and Mair

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

Something the size of the spare tire in the middle of the back is the opposite of what you want, a box cavity. Consider the Trailer Tails on the back of big rig trucks. They are flat and collapsible because reasons, but something that is curved to follow Mair on the top and side, and flat on the bottom would outperform a half-height boat tail.

Could you put the spare tire on the front bumper?