Difference between 2H and 4H AUTO on dry roads?

[DonH63]

Everything's been answered about mechanicals of 2H vs. 4H auto. Note if you hit a patch of gravel or ice in 4H Auto it may slip into 4WD briefly. I could not find my old notes, but my previous Grand Cherokee and wife's Suburban both lost about 1 mpg IIRC when running 4 Auto vs. 2 H.

My 392 XR gets around 16~17 mpg driving around town (mix of surface and highway roads), hit a high of 20+ mpg driving easy on a road trip (highway miles), and averages around 14 mpg for combined off-roading and road trips to the trail and back. My previous Toyotas (I4 truck, V6 4Runner) and Grand Cherokee (5.2 L V8) were all in the single digits off-roading and the 392 is no different -- not really any worse or better, often see around 6~7 mpg, but I rarely look at mpg off-road (other things to watch). The 392 does have the 4-cylinder mode, same as my 21 GC, that helps on the highway (sometimes -- lot of hills around my place).

What surprised me is that friends running the V6 and 35"~37" tires are getting about the same mileage as I, and the stock Rubicons (33" tires) maybe a hair better (again with the V6). I don't know anyone locally with a diesel (but I only know a small number of local folk with Wranglers), and the couple with 4Xe models I haven't seen off-road though they were getting 20+ easy on the road.

Important caveat: My house sits at 7500' and Colorado Springs (closest city) at ~6000' so power is already down ~20% or more from sea level, contributing to a reduction in mpg.

To me the biggest pain is not the poor mileage, which I expected with a brick on big tires, but having to buy premium gas all the time. Our midgrade is the same as standard in the rest of the country so we're already paying extra for our other cars, and premium carries a stiff price tag in some of the little towns where we gas up for the trail. Last time I was thinking it would have been cheaper to dump in an octane booster and fill up with regular fuel, though that has rarely been a winner IME.

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[6.2Blazer]

Here are my comments:
- That so called fuel economy test video is a joke. I spent 10+ years performing government certified and industry standard fuel economy testing. If my company performed a test like that and presented it to a automobile manufacturer they literally would have laughed at us. At least the guy basically states the results are skewed and probably not accurate. To do this anywhere close to an official test you would need two identical Jeeps running back to back down a stretch of freeway at the same time (just far enough apart to not "draft" the other vehicle). You would run from point A to point B (let's say keep it at 80 miles) with both vehicles in 2 HI and calculate the fuel economy difference between the two. Theoretically they would identical MPG numbers, though in reality there is always a slight difference. You would then need to go back and start again at point A and drive to point B, and again in 2 HI, and repeat the MPG check. You can't run a different stretch of highway or different direction. You then repeat this a 3rd time.....exact same stretch of highway and still in 2 Hi. All 3 test results must be consistent with each other. You then go back to point A again, but this time you keep one Jeep in 2 Hi but switch the second Jeep to 4 Auto and repeat this at least 3 times and compare the results.
So in theory with both Jeeps running in 2 Hi they had identical MPG results. Now with one Jeep in 2 Hi and one in 4 Auto you see if one has better MPG. If they both got 20 MPG running in 2 Hi, but now the one running in 4 Auto only got 19 MPG (and the one in 2 Hi remained at 20 MPG) you know that 4 Auto caused a 1 MPG decrease.

- 392's have 4 Auto both for control and to keep the rear axle and driveshaft alive. 4 Auto allows all of the torque to be split between the front and rear. You would be much more likely to blow up the rear axle in 2 Hi.

- Do the 392's even have a front axle disconnect? There is no reason to have an FAD in an all wheel drive vehicle so would surprised if the 392 had one???? If it does not have FAD anyway then having a 2wd options won't make a difference. The front axle disconnect and 2wd mode are separate things.....the Taser just allows the transfer case to do 2wd regardless of the FAD or not. The disconnect is not what really allows 2wd....if the transfer case is in 2wd you still only have 2wd regardless of the FAD or what position it is in.

- If you are that concerned about fuel economy that you are looking at the difference between 2 Hi versus 4 auto....there are a way more differences than that..........

 

[Zandcwhite]

Dana rear axle has left the chat....

Yes, because the m220 couldn't possibly hold up to the 392 in 2wd, you need that whole extra 10mm in the m230 IRS in the scat packs. If you think the Dana 44 is somehow weaker than the challenger/charger independent rear suspension I think you're confused. The same axle that holds up pretty well to 37"+ tires, locked, bouncing in rocks in 4LO can't hold up to 470ftlbs on pavement? Even the 3.6L 210ftlbs is 840ftlbs in 4 Lo, pretty sure the Dana 44 will be fine with 470ftlbs especially at street pressures where burnouts will happen long before you put peak torque to the ground.

 

[2nd 392]

I just again found that the Hemi 392 is supposed to have MDS which will cut power to four cylinders in order to improve MPG, a "20 percent improvement in fuel economy from cylinder deactivation" as explained here:

https://www.motortrend.com/how-to/chrysler-5-7l-hemi-mds-lifters/

So, even with that 20% working for it, the MPG still sucks on the Rubicon 392! LOL.

I religiously avoid MDS by using manual shift to minimize AMAP- resulting in an ~ 1 mpg loss or ~ 7/7.5%

 

[DonH63]

Here are my comments:
- That so called fuel economy test video is a joke. I spent 10+ years performing government certified and industry standard fuel economy testing. If my company performed a test like that and presented it to a automobile manufacturer they literally would have laughed at us. At least the guy basically states the results are skewed and probably not accurate. To do this anywhere close to an official test you would need two identical Jeeps running back to back down a stretch of freeway at the same time (just far enough apart to not "draft" the other vehicle). You would run from point A to point B (let's say keep it at 80 miles) with both vehicles in 2 HI and calculate the fuel economy difference between the two. Theoretically they would identical MPG numbers, though in reality there is always a slight difference. You would then need to go back and start again at point A and drive to point B, and again in 2 HI, and repeat the MPG check. You can't run a different stretch of highway or different direction. You then repeat this a 3rd time.....exact same stretch of highway and still in 2 Hi. All 3 test results must be consistent with each other. You then go back to point A again, but this time you keep one Jeep in 2 Hi but switch the second Jeep to 4 Auto and repeat this at least 3 times and compare the results.
So in theory with both Jeeps running in 2 Hi they had identical MPG results. Now with one Jeep in 2 Hi and one in 4 Auto you see if one has better MPG. If they both got 20 MPG running in 2 Hi, but now the one running in 4 Auto only got 19 MPG (and the one in 2 Hi remained at 20 MPG) you know that 4 Auto caused a 1 MPG decrease.

- 392's have 4 Auto both for control and to keep the rear axle and driveshaft alive. 4 Auto allows all of the torque to be split between the front and rear. You would be much more likely to blow up the rear axle in 2 Hi.

- Do the 392's even have a front axle disconnect? There is no reason to have an FAD in an all wheel drive vehicle so would surprised if the 392 had one???? If it does not have FAD anyway then having a 2wd options won't make a difference. The front axle disconnect and 2wd mode are separate things.....the Taser just allows the transfer case to do 2wd regardless of the FAD or not. The disconnect is not what really allows 2wd....if the transfer case is in 2wd you still only have 2wd regardless of the FAD or what position it is in.

- If you are that concerned about fuel economy that you are looking at the difference between 2 Hi versus 4 auto....there are a way more differences than that..........

AFAIK, no FAD on the 392.

 

[Shark01]

If you're thinking about buying a 392 and any concerns about "does (whatever) impact mileage?" then you're looking at the wrong engine. The 392's mileage is absolutely horrific. That's not what people buy it for.

Note, however, that you can get a Tazer and enable 2H with the 392. That's intended for burnouts, however, and not for any perceived increase in mileage.

I wouldn’t term 392 mileage as “horrific”. My engine isn’t even broken in yet (2,030 miles) and I get 16.5 mpg on my Houston commute which includes some fun driving when I escape the salt mine for the day.

It’s not like the boring engines are getting twice the mileage….

 

[2nd 392]

@sunset -there are threads on this issue including a recent one titled something like a boom with MDS . A forum search will be informative.

 

[6.2Blazer]

Yes, because the m220 couldn't possibly hold up to the 392 in 2wd, you need that whole extra 10mm in the m230 IRS in the scat packs. If you think the Dana 44 is somehow weaker than the challenger/charger independent rear suspension I think you're confused. The same axle that holds up pretty well to 37"+ tires, locked, bouncing in rocks in 4LO can't hold up to 470ftlbs on pavement? Even the 3.6L 210ftlbs is 840ftlbs in 4 Lo, pretty sure the Dana 44 will be fine with 470ftlbs especially at street pressures where burnouts will happen long before you put peak torque to the ground.

Comparing the Jeep versus Scat Pack axles isn't exactly apples to apples. The curb weight of a 392 Wrangler is 1,000 lbs. (almost 25%) heavier than the Challenger Scat Pack. Now add in the larger tires on the Wrangler.
The other thing to consider when you are talking about 4 low. In those cases the torque is being split 50/50 between the front and rear, so the rear axle is only seeing half of what you state.
I do understand where you are coming from, and I don't think the D44 is going to randomly explode if you ever tried to do a burnout in 2wd with a 392. I've watched videos of people removing the front driveshafts from a 392 Wrangler and performing burnouts and it stayed together (check out WFO Off-road videos on YouTube). The issue isn't ultimate strength but more fatigue strength. Doing one or two burnouts may be perfectly fine, but if you do them all the time it will eventually fail.

 

[Zandcwhite]

Comparing the Jeep versus Scat Pack axles isn't exactly apples to apples. The curb weight of a 392 Wrangler is 1,000 lbs. (almost 25%) heavier than the Challenger Scat Pack. Now add in the larger tires on the Wrangler.
The other thing to consider when you are talking about 4 low. In those cases the torque is being split 50/50 between the front and rear, so the rear axle is only seeing half of what you state.
I do understand where you are coming from, and I don't think the D44 is going to randomly explode if you ever tried to do a burnout in 2wd with a 392. I've watched videos of people removing the front driveshafts from a 392 Wrangler and performing burnouts and it stayed together (check out WFO Off-road videos on YouTube). The issue isn't ultimate strength but more fatigue strength. Doing one or two burnouts may be perfectly fine, but if you do them all the time it will eventually fail.

Like all those ram trucks drag racing? The split 50/50 is incorrect. In reality if the front axle and rear axle has equal traction then yes the torque is split, guess what almost never happens off road? Equal traction at all 4 wheels. Throw it in 4LO with lockers on and remove all traction from 3 wheels, guess how much torque you'll measure at the 1 wheel with traction? Technically a little bit of torque will be used ripening the tires with no traction, so let's say 97%. Of course in that situation of 1 wheel has all the traction and you're making peak torque that axle shaft is likely broken but you get the point.

 

[sunset]

Well, would ya look at this...

A 2024 Rubicon 392 with the red seats and the Dual Mode Exhaust Button.

But, what's this? The 4WD Shift Knob has 2H?

I thought that wasn't supposed to be?

Here's the one I've always seen on the 2024:

Whaddya think? Early production model for sale?

 

[AFD]

Well, would ya look at this...

A 2024 Rubicon 392 with the red seats and the Dual Mode Exhaust Button.

But, what's this? The 4WD Shift Knob has 2H?

I thought that wasn't supposed to be?

Here's the one I've always seen on the 2024:

Whaddya think? Early production model for sale?

Thought somebody said that was a mistake for the '24 reveal? Maybe I'm mis-remembering.

 

[6.2Blazer]

Like all those ram trucks drag racing? The split 50/50 is incorrect. In reality if the front axle and rear axle has equal traction then yes the torque is split, guess what almost never happens off road? Equal traction at all 4 wheels. Throw it in 4LO with lockers on and remove all traction from 3 wheels, guess how much torque you'll measure at the 1 wheel with traction? Technically a little bit of torque will be used ripening the tires with no traction, so let's say 97%. Of course in that situation of 1 wheel has all the traction and you're making peak torque that axle shaft is likely broken but you get the point.

Understand what you are saying. Yes, if you get technical it's never really a perfectly even torque split because of weight transfer, traction, etc, etc.... But in general if you are in any 4wd mode at least part of the torque is removed from the rear axle and placed on the front axle. Again, not saying the rear axle will explode and kill a bus full of nuns the firs time somebody tries to a do a burn-out, but the cumulative load (fatigue life) will build up over time and cause earlier failure or wear. Manufacturers have used this philosophy for years including Chrysler/Dodge/Ram/whatever the current name. They generally used smaller (i.e. cheaper) rear axles on SUV's that were AWD and larger axles on ones that had a 2 Hi option.

 

[Zandcwhite]

Understand what you are saying. Yes, if you get technical it's never really a perfectly even torque split because of weight transfer, traction, etc, etc.... But in general if you are in any 4wd mode at least part of the torque is removed from the rear axle and placed on the front axle. Again, not saying the rear axle will explode and kill a bus full of nuns the firs time somebody tries to a do a burn-out, but the cumulative load (fatigue life) will build up over time and cause earlier failure or wear. Manufacturers have used this philosophy for years including Chrysler/Dodge/Ram/whatever the current name. They generally used smaller (i.e. cheaper) rear axles on SUV's that were AWD and larger axles on ones that had a 2 Hi option.

I'm sure that axle size difference had nothing to do with the vehicle being half the weight with 1/3 the towing capacity? Even if you buy the even torque split argument, 2wd on the street puts 100% of the torque to just the rear axle, in 4LO it puts 137% of the torque to each axle (half the torque time the 2.72 low range). Add in the fact that you're aired down and you'll never convince me that 2wd on the street is going to over-stress the rear axle wether it's a few burn outs or a life time. The axle will see far more load off road period.

 

[6.2Blazer]

I'm sure that axle size difference had nothing to do with the vehicle being half the weight with 1/3 the towing capacity? Even if you buy the even torque split argument, 2wd on the street puts 100% of the torque to just the rear axle, in 4LO it puts 137% of the torque to each axle (half the torque time the 2.72 low range). Add in the fact that you're aired down and you'll never convince me that 2wd on the street is going to over-stress the rear axle wether it's a few burn outs or a life time. The axle will see far more load off road period.

A agree with what you are saying, and to be clear I did not say otherwise. The start of the conversation deals with why 392 Wranglers had 4 Auto and no 2wd mode and the affect on the road. My comments were stating that fact that 4 Auto transfers at least part of the torque to the front wheels in those conditions and thus less total torque and stress to the rear axle.
I will agree you are much more likely to break and axle when in 4 low and bouncing around under full throttle climbing hills and such for reasons stated above. My original comments contradicted the statement that ALL of the increased torque from the low range gearing went to the rear axle but was rather split front and rear. Yes, there is more torque going to the rear axle in 4 low, even with a 50/50 split, when in low range...but it's only around 137% increase and not 272%. Just for reference I have had a trail rig/rock crawler for 20 years with 1-ton axles (Dana 60 front) and have broken several "stock" front axle shafts (both inner and outer). Currently run chromoly front shafts with upgraded 35 spline outers. I have also broken several "1/2 ton" front axles over the years so very familiar with this situation.
In regards to the axle size difference my previous posts was referring to the exact same model vehicle (in this case talking about the 1st gen Durango's to be specific) with the only difference being one had a 2wd mode and one did not have 2wd and only 4 auto/AWD. The version with 2wd would grenade an 8 1/4" rear axle several times during a Powertrain Durability Test. However in the AWD version the same axle would survive the entire test. I worked for many years as a test engineer at an automotive proving grounds. Obviously the the weight of the vehicle and towing capacity between completely different models plays a role in the size of the axles. That is why I made previous comment that comparing a Wrangler 392 versus Challenger Skat Pak rear axles wasn't an apple to apple comparison. The comment was made that the axle was sufficient for the Challenger so it should be sufficient for a 392 Wrangler. I contradicted that statement because the Wrangler is much heavier and had larger tires.
Again, I agree with everything you said above.

 

[Tyler-98-W68]

This clears up exactly how the system works.

https://www.4xeforums.com/threads/m...n-and-monitoring-with-j-scan.2602/#post-31415

For 4xe owners, this is why 2H is much slower

https://www.4xeforums.com/threads/4xes-limit-boost-in-2h-mode-vs-4-high-auto.3208/#post-40270

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