Limited, Full-Time 4WD vs AWD Differences

[aggrobot]

What’s up 4runner fans? After successfully and completely derailing another thread that had nothing to do with the Limited’s Full-Time 4WD system, I figured I’d start a new thread so we can establish the differences in the system for probably the millionth time. I own a limited, and like many others, have wanted to learn more about the Full-time 4wd system. LOTS of mention of full-time AWD, and I get it. I’ve referred to it as such in the past. Wanted to lay down the differences to help eliminate some confusion.


First, let me say I’m not here to discuss which system is better or worse for this or that driving condition. They all have merit.

Second, if you hate the limited, great! Thats fine. Please start your own thread about it. I’m here to talk about and enjoy the differences of our 4runners, not kick you the balls for them. Please avoid kicking mine if you can.

Diving right in: all systems send power to all 4 wheels under normal operating conditions. It’s how they behave when grip is compromised that exposes the distinctions.

AWD, 4WD (part time), and Full-Time 4WD

AWD is a system in which each wheel can gain traction independent of the other wheels and their behavior. All wheels are capable of being driven simultaneously, spinning at different speeds, in all grip conditions. Power is sent through the center diff (often with a rear power bias) to each axle, which both contain limited slip differentials. If traction is lost on both axles, all four wheels will continue to receive power and turn regardless of the condition. No 4runner possesses this system. These are our Audi and Subaru friends.

Part Time 4WD system: When engaged, part-time 4WD system sends power 50:50 (front:rear) to each axle, and simultaneously to all 4 wheels. In the event of a loss in traction, power is sent to the wheel with the least resistance. If you have a loss of traction on each axle, power is sent to the wheel offering the least resistance (on each axle), meaning you're basically stuck. This is due to having open differentials on each axle. All 4WD 5th gen 4runners have open differentials on each axle. Only our 2WD friends get an LSD on the rear axle.

Im pretty sure most of us agree on these two systems and their differences, but if not, please chime in!

Now where things get confusing:

Full-Time 4WD is just like it says on the box. It’s what we have in the Limited. It’s the full-time version of what I’ve just described for our part time friends (specifically open differentials), but with a major distinction: The system employs a 40:60 torque split in most driving situations and alters that in response to slippage. In addition, if the front wheels are slipping while the vehicle is turning, the  center  differential changes the torque split to 30:70. If the rear wheels slip while the vehicle is turning, the split changes to 53:47. This info is taken directly from Toyota: Toyota 4Runner Celebrates Historic Run with 40th Anniversary Special Edition - Toyota USA Newsroom
*note the section, “4x2 and Two kinds of 4x4.”

In execution, it’s sort of a mix between AWD and part-time 4WD. Power is still delivered to both axles via a center diff, but each axle has open diffs. You have all the benefits of a part-time 4WD system, full-time. The center diff allows for each axle to spin independent of one another and still offers the rear-power bias/adjustments. In a loss of traction, it adjusts dynamically. If there’s a loss of traction on both axles, youre still stuck, just like in a part-time 4wd system. If youre stationary, this is a much bigger problem than if you’re moving, where grip is changing constantly throughout your drive.

 

[Humble Leader]

I think this is a good description. Many people view AWD and fulltime 4WD as the same thing. Tbh I didn't know there was any difference, but I don't know that many things anymore. Also generally fulltime 4WD will have a HI and LO range as well, where you wouldn't see that in the audi set up for example

while there are some technical differences in how power is delivered to the wheels in both the AWD and fulltime 4WD as described above, for the most part, the limited full time 4WD with traction control is going to produce the desired "AWD" results. AKA the full time 4WD set up will be just fine in bad winter weather. in fact I've never had to engage 4HI on my limited ever during any snow storms in daily driving

 

[aggrobot]

I think this is a good description. Many people view AWD and fulltime 4WD as the same thing. Tbh I didn't know there was any difference, but I don't know that many things anymore. Also generally fulltime 4WD will have a HI and LO range as well, where you wouldn't see that in the audi set up for example

while there are some technical differences in how power is delivered to the wheels in both the AWD and fulltime 4WD as described above, for the most part, the limited full time 4WD with traction control is going to produce the desired "AWD" results. AKA the full time 4WD set up will be just fine in bad winter weather. in fact I've never had to engage 4HI on my limited ever during any snow storms in daily driving

Thanks for having this conversation with me! Ive learned a lot and its from these sorts of chats.
For the purposes of this conversation, I completely ignored the gearing ratio changes afforded by the transfer case (high/low) and yes - a major distinction between AWD and 4WD.

I also agree that traction control can go a long way to help mimic the behavior of an AWD system, and I also agree that for the sake of conversation, its easier to call it AWD and be done with it! Getting into the technical differences though, there are some major distinctions. It's similar to comparing ATRAC to a diff locker. ATRAC is great, it works with the drawbacks of an open diff to send power to the wheel that needs it, but its no diff locker. Similar outcomes, different ways of getting there.

 

[Humble Leader]

Thanks for having this conversation with me! Ive learned a lot and its from these sorts of chats.
For the purposes of this conversation, I completely ignored the gearing ratio changes afforded by the transfer case (high/low) and yes - a major distinction between AWD and 4WD.

I also agree that traction control can go a long way to help mimic the behavior of an AWD system, and I also agree that for the sake of conversation, its easier to call it AWD and be done with it! Getting into the technical differences though, there are some major distinctions. It's similar to comparing ATRAC to a diff locker. ATRAC is great, it works with the drawbacks of an open diff to send power to the wheel that needs it, but its no diff locker. Similar outcomes, different ways of getting there.

that's why I come here - to learn and offer advice. learned a lot from various posts and even videos over time. I don't pretend to know everything and I don't pretend to be right all the time lol. I ask a lot of questions and try to learn a thing or two. the TFL video on the slip test really does show how well ATRAC does work though. couple videos below for anyone interested

https://www.youtube.com/watch?v=LUdQ4WgRbgE

here's another one showing different vehicles

https://www.youtube.com/watch?v=6x2vXItn4TA

 

[stiles_s]

Thanks for starting this! I want to learn more here as well.

Let's take Audi longitudinal torsen vs 4runner Limited
Both use a Torsen T3 center diff at 40:60 "static" torque and high variability
Therefore, both send torque fore and aft all the time
Many would call that AWD

AFAIK none of the Audis nor the Limited have a "mechanical" LSD front or rear.
I believe some of the Audis have brake-driven "limited slip" in that if it senses slip on one wheel, that wheel is braked and it sends power across that axle to the other wheel at that axle. I believe this is what A-TRAC does and it is in the Limited, SR5 and Land Cruiser to name a few.

Another advantage of the 4runner Limited's implementation of the Torsen is it allows Locking the center diff. this forces 50:50 and in this mode it would operate just like an SR5 in 4 High.

There are so many different implementations of AWD these days. MOST of the newer ones are front or rear wheel "first" and transfer power based on sensing slip on the primary axle. These can be effective in extreme conditions but do not give the sense of real "go anywhere" security that a "true" AWD does. Again all IMO.

BTW does anyone know definitively that the SR5 and TRD 4runners utilize the Torsen center for 4wd mode? It would really surprise me because it's an unnecessary expense if you aren't using it for AWD.

 

[chrisridebike8]

Thanks for starting this! I want to learn more here as well.

Let's take Audi longitudinal torsen vs 4runner Limited
Both use a Torsen T3 center diff at 40:60 "static" torque and high variability
Therefore, both send torque fore and aft all the time
Many would call that AWD

AFAIK none of the Audis nor the Limited have a "mechanical" LSD front or rear.
I believe some of the Audis have brake-driven "limited slip" in that if it senses slip on one wheel, that wheel is braked and it sends power across that axle to the other wheel at that axle. I believe this is what A-TRAC does and it is in the Limited, SR5 and Land Cruiser to name a few.

Another advantage of the 4runner Limited's implementation of the Torsen is it allows Locking the center diff. this forces 50:50 and in this mode it would operate just like an SR5 in 4 High.

There are so many different implementations of AWD these days. MOST of the newer ones are front or rear wheel "first" and transfer power based on sensing slip on the primary axle. These can be effective in extreme conditions but do not give the sense of real "go anywhere" security that a "true" AWD does. Again all IMO.

BTW does anyone know definitively that the SR5 and TRD 4runners utilize the Torsen center for 4wd mode? It would really surprise me because it's an unnecessary expense if you aren't using it for AWD.

Only the Limited has a torsen center diff. All other 4WD models only have a transfer case, no center diff. If they are in 4WD, it's a set 50/50 split. That's why you can't drive on grippy surfaces in 4WD and turn.

 

[CutthroatSlam]

Is it truly a center diff or clutches in a transfer case?

 

[aggrobot]

Thanks for starting this! I want to learn more here as well.

AFAIK none of the Audis nor the Limited have a "mechanical" LSD front or rear.
I believe some of the Audis have brake-driven "limited slip" in that if it senses slip on one wheel, that wheel is braked and it sends power across that axle to the other wheel at that axle. I believe this is what A-TRAC does and it is in the Limited, SR5 and Land Cruiser to name a few.

.

This is not my understanding at all. Im wrong all the time, but It's my understanding that quattro systems *ALL* have three differentials: A center diff and an LSD on each axle.

Forgive my ignorance here, but Im unaware of a break-driven limited slip. What you've described is what people used to have to do manually to overcome the challenges of open diffs in the first place. its how many traction aides work to overcome the obstacles introduced by a open diffs. this as more a function of the braking system to regulate power to your wheels. NOT a function of the drivetrain to deliver power simultaneously to all 4 wheels at once. These systems can work side-by-side with drivetrains of all kinds, and often do. But again, they are brake-assisted traction control systems.

Another advantage of the 4runner Limited's implementation of the Torsen is it allows Locking the center diff. this forces 50:50 and in this mode it would operate just like an SR5 in 4 High.
.

Totally agree! I also think its silly to see folks with part-time systems bag on ours. its goofy. ours is a very elegant evolution of the system they enjoy. Toyota REMOVED this system from all but the Limited for the 5th gen. All 4WD 4th gens had this system. As already explained, its also in the LC, and the GX. its very nice. We get all of the benefits of keeping it in 4hi all the time without drivetrain binding. We get a driver-centric RWD bias on the road. We get dynamic torque/power to front and rear axles on the fly. win/win.

BTW does anyone know definitively that the SR5 and TRD 4runners utilize the Torsen center for 4wd mode? It would really surprise me because it's an unnecessary expense if you aren't using it for AWD.

only the limited has the center diff and this is an important distinction. its what allows the front and center axles to operate at different speeds. The transfer case in the other models is locked 50:50 100% of the time.

 

[02SE]

Traditionally.

AWD = Power sent to all four wheels with differentiation between front and rear driveshafts, with no gear reduction ability as in a traditional two-speed transfer case.

Full-Time 4wd = Power sent to all four wheels through a transfer case that allows differentiation between the front and rear driveshafts, and has high and low range gearing.

 

[aggrobot]

Traditionally.

AWD = Power sent to all four wheels with differentiation between front and rear driveshafts, with no gear reduction ability as in a traditional two-speed transfer case.

Full-Time 4wd = Power sent to all four wheels through a transfer case that allows differentiation between the front and rear driveshafts, and has high and low range gearing.

Help me understand, because maybe we are saying very close to the same thing.
Using your explanation, the Limited does not qualify as full-time 4WD. The limited does this through the center differential. The transfer case is primarily handling gear ratio for the limited. Toyota calls the system Full-Time 4WD. Id also argue that the transfer case has two functions in non-limiteds: changing your gear ratio, and locking the front/rear driveshafts so there's no differentiation at all. I think i get what youre saying about how its allowing the front/rear to spin freely but this is really not a function of the T/C. Its just a RWD 4runner until engaged. What you've otherwise described is the Limiteds behavior, so we are nearly saying the same stuff.

Edit: Id also like to add that if the "traditional" distinction between AWD and 4WD is simply the gear reduction element, I can get down with that. Ive never heard of that being the big one, but this is fine by me. Appreciate your input. Its certainly far more simple and straight-forward.

 

[Captain Spalding]

A few notes:

“AWD” is an umbrella term that applies to a lot of different technologies from many different manufacturers. It’s a very imprecise term. What AWD systems have in common is that they work by detecting wheel spin, and then applying brakes or activating clutch packs or viscous couplings in order to send torque to the wheel with traction. The downside of AWD is that it is reactive, and in an off-road setting it engages only after wheel spin has been detected and precious momentum has been lost. We will leave the excellent Audi Torsen based system out of this discussion, so Audi fans stand down.

The Limited is indeed true full-time 4WD. The beauty of full-time 4WD is that it is always engaged. Note that the transfer case differential can be locked. When in this state it behaves just like the part-time 4WD versions of the 4Runner, with the same 50/50 torque split but also the caveat that with the center diff locked the car shouldn’t be driven on dry pavement, to prevent undesirable stresses in the drive line. The huge benefit of full- and part-time 4WD systems over AWD is that they are proactive instead of reactive, thus the driver is able to “read” the trail and engage these systems preemptively and traverse obstacles without a loss of momentum.

It’s this difference between proactive and reactive which is salient point. Even AWD systems like the Subaru, which are sending some power to all wheels all the time, operate on a reactive basis.

The reason threads like this exist - the thing that confuses many 4Runner owners - is that 4 Runners are redundantly equipped with AWD-like ABS-based traction control systems. VSC, TRAC, A-TRAC, and MTS all leverage the ABS system and work by sensing a spinning wheel and applying brakes to it. These systems are of great assistance to less experienced drivers who might not be sure when to engage a diff lock (or when to disengage it!) I would contrast the nanny nature of those systems with the downhill assist (DAC) or Crawl Control which can be of great help on tricky descents even for talented drivers.

Finally, while I don’t know of any real part or full-time 4WD vehicles without a low range, I don’t consider the low range to be a defining feature.

Friends with 4Runner Limiteds: rest assured that your Limited, with real full-time 4WD and center and rear locking differentials, represents the peak of off-road capability and convenience. Anyone who tells you different is ignorant. Be proud!

 

[stiles_s]

This is not my understanding at all. Im wrong all the time, but It's my understanding that quattro systems *ALL* have three differentials: A center diff and an LSD on each axle.

Forgive my ignorance here, but Im unaware of a break-driven limited slip. What you've described is what people used to have to do manually to overcome the challenges of open diffs in the first place. its how many traction aides work to overcome the obstacles introduced by a open diffs. this as more a function of the braking system to regulate power to your wheels. NOT a function of the drivetrain to deliver power simultaneously to all 4 wheels at once. These systems can work side-by-side with drivetrains of all kinds, and often do. But again, they are brake-assisted traction control systems.

I'm unaware of any of the late-90s onward Audi quattros (we've had a4s, s4s sq5s, etc) with front or rear limited slip. They all have torsen center diffs and open front and rear using the brake-assisted setup to mimic LSD.

FWIW I'm nearly certain that our Limiteds have the same setup. If I'm accelerating sharply-turned on wet surfaces and the front+inside slips the ABS reins it in and ideally retains the front (or rear) end's ability to take torque from the torsen & apply it to the ground.

 

[aggrobot]

I'm unaware of any of the late-90s onward Audi quattros (we've had a4s, s4s sq5s, etc) with front or rear limited slip. They all have torsen center diffs and open front and rear using the brake-assisted setup to mimic LSD.

FWIW I'm nearly certain that our Limiteds have the same setup. If I'm accelerating sharply-turned on wet surfaces and the front+inside slips the ABS reins it in and ideally retains the front (or rear) end's ability to take torque from the torsen & apply it to the ground.

Thank you for sharing, and I stand corrected! I cant speak to this system being on the limited, but clearly, I cant speak to a lot of things.

 

[aggrobot]

A few notes:

“AWD” is an umbrella term that applies to a lot of different technologies from many different manufacturers. It’s a very imprecise term. What AWD systems have in common is that they work by detecting wheel spin, and then applying brakes or activating clutch packs or viscous couplings in order to send torque to the wheel with traction. The downside of AWD is that it is reactive, and in an off-road setting it engages only after wheel spin has been detected and precious momentum has been lost. We will leave the excellent Audi Torsen based system out of this discussion, so Audi fans stand down.

The Limited is indeed true full-time 4WD. The beauty of full-time 4WD is that it is always engaged. Note that the transfer case differential can be locked. When in this state it behaves just like the part-time 4WD versions of the full-time 4WD, with the same 50/50 torque split but also the caveat that with the center diff locked the car shouldn’t be driven on dry pavement to prevent undesirable stresses in the drive line. The huge benefit of full- and part-time 4WD systems over AWD is that they are proactive instead of reactive, thus the driver is able to “read” the trail and engage these systems preemptively and traverse obstacles without a loss of momentum.

It’s this difference between proactive and reactive which is salient point. Even AWD systems like the Subaru, which are sending some power to all wheels all the time, operate on a reactive basis.

The reason threads like this exist - the thing that confuses many 4Runner owners - is that 4 Runners are redundantly equipped with AWD-like ABS-based traction control systems. VSC, TRAC, A-TRAC, and MTS all leverage the ABS system and work by sensing a spinning wheel and applying brakes to it. These systems are of great assistance to less experienced drivers who might not be sure when to engage a diff lock (or when to disengage it!) I would contrast the nanny nature of those systems with the downhill assist (DAC) or Crawl Control which can be of great help on tricky descents even for talented drivers.

Finally, while I don’t know of any real part or full-time 4WD vehicles without a low range, I don’t consider the low range to be a defining feature.

Friends with 4Runner Limiteds: rest assured that your Limited, with real full-time 4WD and center and rear locking differentials, represents the peak of off-road capability and convenience. Anyone who tells you different is ignorant. Be proud!

Appreciate the input, and thank you! Limited’s do not have a rear locking diff, though that would be awesome. Edit 5th gen limited’s do not have a rear locking diff.

 

[stiles_s]

Is it truly a center diff or clutches in a transfer case?

You could be asking two questions. and I'm not sure what you mean by "it".

If you're talking about the Limited:
- I'm using the term "center diff". It's logically in the "center" of the drivetrain and it is a mechanical torque biasing gear set. It isn't "clutches in a transfer case".
- Where is it housed? I don't know. Is it in "the transfer case"? Not sure exactly where it sits but it's "between" the output of the Transmission and it splits the power and sends it to the front and rear axles.

Every time I try to explain how a Torsen works to someone I end up just linking to a youtube video. I'm not smart enough to explain how it works

 

[Captain Spalding]

Every time I try to explain how a Torsen works to someone I end up just linking to a youtube video. I'm not smart enough to explain how it works

Brother you’ve got that right. It takes at least two: one with that guy with the white board, and a second one with 3D X-ray animation to see what he’s explaining.

I always loved this video’s simple explanation of a differential.


https://www.youtube.com/watch?v=yYAw79386WI&feature=youtu.be

 

[02SE]

Help me understand, because maybe we are saying very close to the same thing.
Using your explanation, the Limited does not qualify as full-time 4WD. The limited does this through the center differential. The transfer case is primarily handling gear ratio for the limited. Toyota calls the system Full-Time 4WD. Id also argue that the transfer case has two functions in non-limiteds: changing your gear ratio, and locking the front/rear driveshafts so there's no differentiation at all. I think i get what youre saying about how its allowing the front/rear to spin freely but this is really not a function of the T/C. Its just a RWD 4runner until engaged. What you've otherwise described is the Limiteds behavior, so we are nearly saying the same stuff.

Edit: Id also like to add that if the "traditional" distinction between AWD and 4WD is simply the gear reduction element, I can get down with that. Ive never heard of that being the big one, but this is fine by me. Appreciate your input. Its certainly far more simple and straight-forward.

The Limited 5th gen 4Runner is Full-Time 4wd.

What gives it that ability is that there is a differential in the transfer case which allows the front and rear driveshaft to turn at different speeds.

Using the traditional terms, it is Full-Time 4wd because it has a differential, (which can be locked in the 4Runners) in it's two-speed (high and low range) transfer case.

A vehicle that doesn't have a two-speed (high and low range) transfer case, but can send power to all four wheels, would typically be called AWD.

Anyway, that is how vehicles with variations of "Four Wheel Drive" were traditionally described. The terms have been used and misused, to the point where one has to educate themselves if they care to know the difference.

 

[Steve40th396]

Great thread. I still after all these years dont get how brake systems send power from one wheel to the other in an open diff.. I thought you need a LSD to get power to the other side. That GM video is a help.

 

[aggrobot]

The Limited 5th gen 4Runner is Full-Time 4wd.

What gives it that ability is that there is a differential in the transfer case which allows the front and rear driveshaft to turn at different speeds.

Using the traditional terms, it is Full-Time 4wd because it has a differential, (which can be locked in the 4Runners) in it's two-speed (high and low range) transfer case.

A vehicle that doesn't have a two-speed (high and low range) transfer case, but can send power to all four wheels, would typically be called AWD.

Anyway, that is how vehicles with variations of "Four Wheel Drive" were traditionally described. The terms have been used and misused, to the point where one has to educate themselves if they care to know the difference.

Much appreciated for all the input and perspective here! Love these chats with the community.

 

[aggrobot]

Learned a lot, and misspelled axle about a million times. I omitted the gear reduction function, and it being for some, the biggest distinguishing feature of a 4WD, and Full-Time 4WD system. one of the goals of having this conversation was better establishing what full-time “4WD system” means and how for some, these terms are interchangeable with AWD. Thanks to everyone for chiming in and helping work towards that goal! We may not completely see eye to eye on what exactly is featured in the limited’s system. Im speaking specifically about my differing understanding of the way power is distributed (or not) via the brakes on the limited. I know ATRAC does this under 5mph, but not above that, as I feel you;re implying - just as the audi would. im not as certain that this system is on the limited. All the same I appreciate the education on the Audi’s [MENTION=4236]stiles_s[/MENTION]. i still think we got much closer to fleshing out what full-time 4wd means, or is meant to mean vs AWD, and the nuance, (or maybe just my cluelessness, lol). Thanks!