Jeep transfer case swap - preliminary fe data
 

I've replaced the full-time transfer case in my WJ with a part-time one from a KJ, which allows me to run in 2wd mode and the entire crossmember and transfer case are now tucked in higher (40mm) for better off-road clearance. That might also help with aero drag under the car.

Stock:
https://i.imgur.com/bCc8KF0.jpg

Now:
https://i.imgur.com/QNDDf5D.jpg

The stock transfer case had way more "drag" between the front and rear output flanges than I imagined and the fuel consumption data is interesting:

Average fuel consumption (city, two-lane roads, some light offroading), with airdam, street tires and standard transfer case, over 750 miles:
21.09 MPG

Average fuel consumption (including about 50 miles of offoading), with the new transfer case, 32 inch mud tires, no airdam and chopped bumper, over 300 miles:
21.22 MPG

If I account for the tire size difference, which is 9.3%, it would mean I traveled more than what the odometer reads, which brings the fuel consumption to 22.55 MPG

That looks like there were some solid gains from the transfer case swap, enough to offset the aero differences and drag differences from the mud tires, or am I interpreting this wrong?

Mud tires vs street tires:
https://i.imgur.com/t3FHr8e.jpg

Yikes! That's a lot of drivetrain drag. Nice mod!

In other words, it looks like that old transfer case its burning around 1 gallon of gasoline per 320 miles, assuming the tires added no extra drag.

Too many variables to draw any conclusion, IMHO.

Nice work! On your question.... I also think there are too many uncontrolled variables in your test, BUT there is no way to A-B-A test such a mod AND the data you have is suggestive of what we might expect, so keep tracking your fuel economy. If it's working nicely you should usually see better FE in similar driving conditions. Keep a fuel log here at EM so we can see your progress.

I'm tracking my fuel consumption by an app called Fuelio and when I filled up yesterday I was expecting to see 18-19 MPG at best. I wonder if I can add all the historic data in the log here.

An extra improvement was losing 70 lbs of weight (left is new, right is old):
https://i.imgur.com/SIMp6wf.jpg

I'm curious to compare with the street tires and aero mods, but it will have to wait until August for data, until then I have offroad trips planned.

As for comparison, this WJ has shown slightly worse highway mileage than my old one, same engine and same aero mods. Maybe the transfer case on this new WJ one had a lot more drag than the old WJ one.

Not even a close comparison, but in my old WJ I had a trip which was 200 miles highway, 80 miles offroad and I averaged 18.66 MPG. Smaller mud terrain tires and no aero.

There has got to be a way to upload an FE data file, but I just checked and did not see a way. Anyone else know how?

I can do it manually, I've only owned it for two years.

Not exact A-B-A data but here is some info following recent data-logging:

Same stretch of road, same tires, same speed and it's in a straight line, so the transfer case shouldn't make such a difference.

May 20 test:
Old transfer case
55F outside, light rain, no wind
22.08 milligrams / stroke average injection quantity
4 psi average boost

July 19 test:
New transfer case
64F, cloudy, no wind
20.53 milligrams / stroke average injection quantity
3.26 psi average boost

That shows a 7.27% difference, seems pretty big, I can't wait to get run through a full tank and see actual numbers :)


Usually the car's trip computer shows between 33.6 and 32.6 MPG (it's optimistic, I've compared it in the past with actual fuel consumption) on national roads. This includes inclines, passing slow cars, speed limits in villages, not exactly good for eco driving.

This time, I reset the trip computer when I left home and at the end of the trip it showed 36.75 MPG, over 231 miles. I've never seen it so low before the transfer case swap, not even on long distances on highways.

Next mod, already ordered: straight through muffler, because it allows me to tuck the exhaust higher for off-road clearance. The stock muffler is baffled and I hope less restriction post-turbo will reduce pumping losses, hopefully some FE gains there too.

Interesting results to say the least. BTW maybe you could consider to fit your Jeep with locking hubs to fully disconnect the front wheels, reducing the drag even further while on 2WD.

The thought crossed my mind, but I don't think I can get a set of hubs for free, like the transfer case :D
A new conversion kit is $1000 in the US, which means around $1300 after shipping and tax here.

This fill-up will have some mixed driving: 231 miles with the street tires, so far and 116 miles with the off-road tires, including mountain trails and fooling around to test the new locking differentials. Previous data shows 20.4 MPG average with the off-road tires so I could take a stab at estimating my street tires real average consumption.

So, besides getting this transfer case for free, what were other reasons that led you to do this conversion? Was your stock transfer case needing repairs or you just wanted to do it for the sake of experiencing?

Adding efficiency to the drivetrain while adding clearance underneath a Jeep is one hell of a mod in both directions!

And it sounds like another good excuse.. err.. good reason to get a Jeep :D

It was a mixture of less drag and more clearance. There were reports of decreased fuel consumption but I always took them with a grain of salt.

My car is the smallest in the club, no lift and only 32 inch tires, so it needs every bit of help off-road. It would always drag the crossmember on obstacles. I gained 4cm clearance with the new transfer case and the new crossmember.

It needs to be able to do this and then switch to road tires and drive as economically as possible :D
https://www.youtube.com/watch?v=XJhIggV2v7M

Even though you'd take it with a grain of salt, switching from full-time 4WD to part-time improves fuel-efficiency nearly everytime.

New Parts / New Venture promised that the georotor design in the transfer case would direct 100% of the torque to the rear wheels under normal driving.

I guess after 16 years of wear and tear and possibly incorrect fluid, the clutches inside start to add drag.

I'm anxious to get some real fill-up data and see the difference.

Interesting, but I wouldn't hold my breath for that.

New muffler is on and tucked almost 1.5 inches higher:
https://i.imgur.com/wLooFR9.jpg

The exhaust no longer hangs lower than the crossmember:
https://i.imgur.com/JzjCaBl.jpg

Due to the madness in Europe we will most likely skip going to Greece this year, so long-distance fuel consumption data will have to wait a while.

Any gains from the drivetrain mods should still bring down the average fuel consumption, so I will update this as I get new data.

Turning the output yokes by hand once it was removed showed quite a bit of resistance.

The old transfer case funded the exhaust mods :turtle:

Seems like the decoupling only works with the engine on. Well, I never held my breath for those "automatic 4WD" systems, unless they had a proper center differential which IIRC some SUVs from the '90s didn't resort to and relied mostly on sensors input to engage and disengage the front drive. And once the clutches wear out, it may become worse to drive on pavement.


That sounds good.

There's no electric involvement in the center differential. It's a set of clutches with a gerotor pump. When there is a speed difference between the front and rear output shafts, the gerotor spins to pressurize the clutch pack.

There's no preload on the clutch pack when it's new, but I suspect incorrect fluid usage or water contamination makes the clutch pack drag. I've seen an identical transfer case with even more resistance than mine.

Same system was in the front and rear axles (now replaced with air-locking differentials).

Pressure is regulated by a simple metal tab over the pressure relief orifice. Subsequent version used a solenoid to control pressure in both front and rear clutch packs and could act almost like lockers if the solenoids held pressure high and long enough.

It's a good system for mild off-roading but it requires wheel speed difference to work and it still allows for slip between sides. I couldn't have made it up that obstacle in the video I posted with the standard traction system.

Another bonus with the transfer case swap is that fluid changes (every 20k miles) cost $8 instead of $30. No need for special fluid.

I'm more used to Ford's Control Trac II and other similar viscous-coupling systems. Well, considering clutch problems due to the usage of an unsuitable oil may plague even a cheap 125cc motorcycle, it wouldn't surprise me if this was the reason for the damage to your previous transfer case.

Pump to pump results are in!

27.705 MPG (8.49 l/100km) over 265 miles (427km) of national roads. Including some roads with construction work and a trip in town with mild traffic. All driving done with A/C on.
Trip computer showed 34.59 MPG, which is in-tune with the differences I've noticed in the past.

Previous results in similar conditions:
25.59 MPG (9.19 l/100km) over 318 miles (512 km)

All aero mods were in place in both situations.

I've never had such a low fuel consumption on national roads. Not even on the old Jeep. This is a personal record for me and it looks like the transfer case used up an extra 0.7 l/100km. Factoring in around 15.000km per year, that's 105 liters of fuel saved every year.

What would be the next mod without making my car look weird and maintain the current ride height?
A rear spoiler to improve the airflow at the rear of the car? The rear window gets dirty instantly in bad weather, so I assume some gains could be had there.

https://ecomodder.com/forum/showthre...tml#post599010

I am very happy with my rear kamm back spoiler on my 99 4runner. If this motif does not pass your looks criteria, You could make something of this basic shape out of steel or aluminum integrated into a rear swing out style carrier with tire bumper carrier.

It's pretty big :D
I was thinking about starting with a first generation Porsche Cayenne spoiler as they are cheap and good base to start modifying to fit instead of building from scratch.

I think a good side picture of the car and an overlay of the optimum aero shape would help with ideas and show improvement points.

The first thing that came across my mind is the spoiler of the Opel Corsa B GSi, which was also a quite common aftermarket feature for other versions of the Corsa back in the day.
https://1.bp.blogspot.com/--SzOtMzQP...Photo10221.jpg
https://3.bp.blogspot.com/-IISZz8G4G...Photo10222.jpg

Found one locally and I'll ask for measurements. Eye-balling it, it might fit nicely and remain as a permanent addition but I need to figure out a good way to test if it makes any improvement.

You say the one out of a Corsa or out of a Cayenne? I'm sure both might need some adjustments anyway.

Corsa. You're right, both would need modifications and the Corsa one would have to become a 3-piece, as the side plates would mount to the D-pillar instead of on the hatch, like on the Corsa.

All in order to avoid clearance issues while opening the hatch? Would it be totally out of question to mount it as a single-piece on the hatch instead?

it would be possible but I don't know if there would be benefit from the side plates being too far inward from the edge of the D-pillar:

https://upload.wikimedia.org/wikiped...r_20071102.jpg

Maybe, even if the side plates get closer to the edge of the D-pillar, you could still make some attachment which would allow the entire spoiler to lift up with the rear hatch. How to get it supported by the rear glass might not seem easy, but I'm sure it's not rocket-science.

The front left wheel looked like it pointed way out compared to the right when the steering wheel was straight. So I rigged up a toe-measuring tool and it turns out there was a massive amount of toe-out, enough that I had to thread in the left side tie-rod 5/32".
Normal specs call for 1/16" toe-in on each side and by my measurements, toe-out was 5/16".

I've been driving like that for all the tests after the transfer case swap since almost at the same time I replaced the left side tie-rod.

Now I'm curious to see what effect that has on the fuel consumption reported by the trip computer.

I took the car for a professional alignment after my diy alignment:
0.005" of toe-out on the left side
0.04" of toe-out on the right side.
The guy at the shop said it's green on the machine and did not want to touch it any further. I'll build a couple of steel plates and then I can dial it in to a little bit of toe-in.

And we were lucky and managed to go on the summer vacation, which means fuel economy data.
I've had some sections identical to last year.

Section 1 (293 miles, 40 miles of highway, similar traffic):
25.59 mpg in 2019
27.22 mpg in 2020 - this is very close to what I noticed testing earlier

Section 2 (232 miles, 70 miles of highway, worse traffic in 2020):
28.51 mpg in 2019
27.47 mpg in 2020

Total average fuel consumption over the entire trip:
2051 miles / 25.34 mpg in 2019
1608 miles / 27.57 mpg in 2020 - this includes 2 hours of driving in torrential rain, and even more with very serious wind and 400 less highway miles.

This is a sample of national roads. Every time I'm driving under 40mph, the torque converter unlocks, so that's wasted fuel.
https://i.imgur.com/NUwsAhn.jpg

Driving in the torrential rain I must've hit a puddle fast enough to break the airdam away from one mounting bolt, so I will rebuild it out of sheet metal and I will have the option of adding a shelf on the bottom if that could reduce drag.

Total average fuel consumption over 11.250 miles: 23.9 MPG.

I take it that this did not give you a low range like the truck transfer cases?

IIRC they still had a low range as standard.

It's still the same 2.72 low range as the old transfer case.

Not much happened in the past year car-wise, as having a baby that doesn't love sleeping in the car means all trips were under 90 minutes :)


However, in the idea of improving the rear aero, I have found the factory spoiler, which is said to reduce the dirt build-up on the rear glass. On rainy days the build-up is very severe.

https://i.imgur.com/qDkQM4J.jpg

I remember similar spoilers used to be a quite common accessory for economy cars in my country, even though people got them mostly for the look than for the actual function.

Odd side statement: way late into our travels, we found that my son was extremely sensitive to travel and got car/motion sick about 1/2 hour into trips unless he was sleeping.