Weight, Aerodynamics, and Power Considerations
 

Let me start by acknowledging that I drive a gutless, heavy, brick of a vehicle, and that most of my concerns here won't affect much. That being said, I'm curious what the collective here has found in building their JKs that I might best use to move forward with.

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I just purchased my 2009 JKUR with 6-speed, hardtop, Airaid Intake/TB Spacer, Jet Chip, Magnafow cat-back, and it currently has about 76k miles. It came with WARN rock sliders, a Garvin Expedition rack, and a Tuffy Security deck. It drives great (for a Jeep) and seems to be in decent shape mechanically.

I bought the Jeep for overlanding, hunting and camping, as opposed to rock-crawling or mudding. I have a very short commute to work, so it will be my daily driver. I have a 5'2" wife and a young daughter, so functionality with smaller folk has to be considered. I don't plan on lifting it much, but want to keep on-trail ability in mind as well.

I'm listing my plans here for feedback, suggestions, and posterity for others that may be in my shoes.

Weight
At what point does weight start really affecting performance?

I am considering selling my WARN sliders and buying something with a lower step that still offers protection. I am considering the Body Armor Rocksteps, which weigh about 25 lbs more total, and they can still be used as a Hi-Lift point.

In the short time I've owned the Jeep, I've started to hate the Tuffy Deck, so I'm probably going to uninstall it, sell it, and lose about 80 lbs in the back.

I do plan to buy a front bumper and winch, so there is weight gain to be expected there, as well. I am considering a stubby bumper and a winch with synthetic rope to cut weight. I like the look of the Rugged Ridge XHD stubby for its fog lamp option and relatively low winch mount. The bumper weighs less than 70 lbs, so I'd only gain 30 lbs or so over stock. A winch will add about 60-70 lbs more.

Eventually I might even add a rear metal bumper, solely for an additional place to stand and hi-lift from.

Aerodynamics
How much of the JK's stock components increase aerodynamics?

In considering a new front bumper, I've wondered how much the design of the stock bumper helps air flow around the wheels. If I go with a stubby, the entire front of the wheel with be exposed. Do we assume that would amount to some decrease in flow? Would a mid or full-width bumper make a difference?

I mentioned the winch mount height for the XHD bumper. Has anyone had negative experience with a winch mounted too high, causing overheating problems due to radiator blockage?

The Garvin rack will come in very useful when I need to carry my kayaks, haul gear on long trips, mount lights, and possibly get a roof-top tent. Right now, though, it's noisy on the highway, and it's got to be losing me some MPGs. It's not difficult to remove and install (I've not tried it, but from looking at the setup), so I might just hang it in my garage and only use it when I need it. Of course, I'll lose the cool factor of sporting a nice rack, so I may want to keep it on more than not.

Power
I sold a BMW 330i to buy my JK, so I am well aware that I no longer have a quick, sporty ride. I am ok with that. As it stands, I can accelerate sufficiently for my needs and don't really feel the need for more power. However, I would be disappointed if I felt a decrease in that power. Moving forward with my plans, between weight gain and loss of aerodynamics, I'm curious what my butt-dyno will detect.

I will eventually want to add a new suspension, even if it's just to even out the sag from additional weight. I don't plan to upgrade past 33" tires, so a 2-2.5" lift should be fine. Everything I've read says I will be "fine" with 33's and my stock 6-speed and 4.10 gears, but will that noticeably affect my power and gas mileage?

I noticed a slight increase in mileage with my last tank of gas, simply from cleaning my Airaid filter. The setup I listed above, from intake to exhaust, was obviously an attempt to squeeze mileage and performance from the 3.8. Short of a supercharger or V8 swap, I've got to assume that's about all I can do in this department, right?


All in all, I am very happy with my JK, and look forward to my (very slow) build. I'm hoping to get some insight on my questions and concerns here, and I am very open to suggestions. Thanks, y'all!

Nice jeep...welsome to the jk world.

Honestly there isn't much you can do to get more performance out of these mnotors sans maybe a turbo or supercharger system. At that point I would be concerend with mpg and reliability....Its a big chunk of change to throw at an offroad vehicle.

Personally I don't think MPG will be effected greatly by bumpers,armor and air dams. I added a ton of weight to my 2013 along with 35" tires and a 3.5" lift and my mpg went from 16.5 to 14.5. I wouldn't say 2 mpg is a make or break point for what you're trying to do......Unlimiteds are heavy and blocky and just get bag MPG.

I do think you will see a bigger loss in fuel economy if you load the rack up....I have towed a heavy boat and a lighter box trailer and the aerodynamics of the boat always equal better mpg vs. a wind block of a trailer.

Biggest quick thing you can do is lose the rack. Aero problems even when empty. Pack smarter on the inside.

The RR XHD is easily at or above 70lbs and the stock bumper cannot weigh 30. My route for the weight issue was to go aluminum. For your overland ideas- aluminum is the way to go. Check out the weights for sliders/ steps in aluminum vs steel and start adding up the savings. Making things removable can also help with your daily gas mileage but still keep your capability for overland adventures.

You shouldn't have a problem moving the tuffy deck but I would suggest maybe putting it to the side for a little while and then sell it if you still don't like it. Lockable storage is a good thing....particualrly in a jeep.

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I have a 2.5" lift with 34" that weigh around 75 pounds each plus the rugged ridge xhd front and rear bumpers with skid plate. I'm sure I easily added upwards of 500 pounds from stock. With my superchips and a regear I will be doing just fine with my pos 3.8. You can always add a supercharger if you have the budget. That would significantly add all the power you need.

I figured taking the rack off would be a relatively easy way to get better mileage in my day-to-day driving. Slap it back on when I need to haul stuff and go from there. I love the challenge of packing and figuring out storage for long trips, so I'm all about packing smarter!

I was getting my weights from posts here (for the stock bumper), and shipping weights from Amazon. I've been surprised, while researching parts for the JK, how difficult it is to determine actual weights of things. So few manufacturers or retailers list that information, and I feel it's an important specification. Funny thing is, Amazon lists the weight of the XHD steel bumper at 68 lbs, but lists the aluminum version at 62.5 lbs. Not a huge difference, especially considering the higher price.

The Tuffy deck has just become annoying. With hunting and camping, and even shopping (you know, when I'm mall-crawling) I am carrying bulky items pretty often. I've already had a couple of instances where I needed to place an item in the back that was too tall for the lid. I end up having to rest the lid on the item and watch it wobble around as I drive. Heaven knows what would happen if I got in to a wreck with that heavy lid unsecured. I can appreciate its value for security, but it just doesn't seem to work for me.

For the $5000-8000 supercharger upgrade, I could probably sell my JK and buy one with the newer engine, so that's probably not an option. I'd like to avoid re-gearing, which is why I targeted the 6-speed/4.10 combo. As I add parts and drive more, I'll see if anything further needs to be done in that department.

'08 auto with trans cooler. No issues for me, plenty of airflow. Here is an old pic from back in mid '08 when I still had the caps on and hadn't re-routed the battery cables.

Attachment 505098

(waayyy back. Before the trans cooler. And before the little blue sticker even fell off... :cool: )

I'll offer up my aero knowledge. Was a US Marine aircraft tech and a model aircraft hobbyist. I've also studied aerodynamics as a topic of fuel economy on vehicles.

There are two primary aerodynamic concerns as far as a Jeep is concerned. Frontal area and the boxy shape at the REAR of the vehicle.

Anything that adds to frontal area, like a roof rack, will increase drag and result in parasitic losses.

But the biggest reason Jeeps are so bad on gas is the REAR of the vehicle terminates in a box shape.

When an object (vehicle) moves through a fluid (air) it creates a high pressure zone in front and a low pressure zone in the rear. The high pressure zone is of less importance than the low pressure zone, as the later creates a vacuum effect, literally pulling (drag) the object back. If you look at the shape of an aircraft fuselage, wing, raindrop or even a Toyota Prius, you will notice they are largest at the front, and slowly taper off to a point in the back. This design is efficient because there is no way to avoid the high pressure zone in front no matter the shape of the object, but if the fluid can be slowly or gradually merged back into the low pressure area at the back, you can greatly reduce the vacuum (drag) effect.

Theoretically, a Jeep would get better mileage if you could drive it backwards. But probably not much better considering the airflow would still be very turbulent.

So with that logic, wouldn't some form of fast back top be a possible improvement?

I feel the front could be improved with a rear opening hood and possibly that hideous wild boar grille. This is so that air can more freely enter and exit the engine compartment.

Neither of these would pay themselves realistically, but might add the convenience of an extra 20miles between fill ups. Which is what I look for in when shopping intake exhaust etc. Trying to get your money back will be less painful if you just shop for light wheels and tires when you move up. Those weight losses tend to be exponential and closer to worthwhile, but still don't expect to double driving distances.

If I were you, I'd ditch that Jet chip and replace it with something that is a little more programmable.

No. A fast back top doesn't slope gradually enough to make any difference.

Generally speaking, chasing fuel economy in a Jeep is pointless. If you can't afford to lose 4mpg in a vehicle that already sucks gas, then you probably need a different vehicle. Only practical solution to extend range in any meaningful way is to carry gas cans.