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Old 06-22-2020, 03:26 AM   #1 (permalink)
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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:


Now:


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:


Last edited by AdrianD; 06-22-2020 at 03:54 AM..
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Old 06-22-2020, 10:12 AM   #2 (permalink)
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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.
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Old 06-22-2020, 10:50 AM   #3 (permalink)
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Too many variables to draw any conclusion, IMHO.
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Old 06-22-2020, 11:20 AM   #4 (permalink)
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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.
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Old 06-22-2020, 11:48 AM   #5 (permalink)
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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):


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.
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Old 06-22-2020, 01:31 PM   #6 (permalink)
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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?
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Old 06-22-2020, 02:18 PM   #7 (permalink)
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I can do it manually, I've only owned it for two years.
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Old 07-20-2020, 03:24 AM   #8 (permalink)
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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.

Last edited by AdrianD; 07-21-2020 at 04:31 AM..
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Old 07-21-2020, 11:34 PM   #9 (permalink)
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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.
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Old 07-22-2020, 09:48 AM   #10 (permalink)
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The thought crossed my mind, but I don't think I can get a set of hubs for free, like the transfer case
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.

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