I've been spending a lot of time lately studying Subarus and am starting to see clear through the misleading text by Subaru and others on the subject of AWD and automatic transmissions. Personally I drive a manual but I've been so bothered by the incomprehensible descriptions of the automatics that I wanted to find out what was really going on.
With manual transmissions there isn't much confusion. Most if not all AWD manuals use three conventional differentials : one to split the power front to back, then one at each end to split the power side to side.
There is usually something to block the center differential so if one wheel spins power will get sent to the other end of the car just the same. Most Subarus use a viscous coupling in the center differential which locks up if there's too much of a difference between front and rear.
My Corolla AWD wagon has an electrically lockable center differential which is operated at the push of a button (as does the current Subaru STi). A much better idea, in my mind, in that the spinning wheel doesn't get to dig a hole before the differential locks up.
With automatic transmissions the subject should be as simple, but the literature describes things in such a was as to make things almost incomprehensible.
The only difference between a manual and an automatic should be that the clutch is replaced by a torque converter and the gears and shifted automatically. An automatic AWD Corolla is like this and the power out of the automatic transmission goes to three differentials like the manual. The only difference is the locking of the center differential is automatic if speed differences are detected.
Things are not the same with Subaru automatics. First, there are two kinds : the MPT and VTD. The VTD is on the higher end models like the WRX and Outback H6.
The VTD is true AWD in that it has a center differential. But : the MPT, used in most models, does not have a center differential. As far as I can tell power always goes to the front unless the system detects a difference between wheel speeds (using sensors) at which point it will engage a multi-plate clutch (like in a motorcycle) to send power to the rear. What Subaru doesn't say is that this will only happen in the case that one of the front wheels is spinning.
I've seen lots of complaints about the Honda automatic CR-V being like this, but I haven't seen anywhere that the non VTD Subarus are also like this.
In my mind, then, these automatics are front wheel drive vehicles unless some very particular circumstances are met.
AWD, in my mind, is much more than something to help getting unstuck. It's a matter of balance, both in power application and engine braking, with everything distributed to all four wheels, each doing 25% of the work. There is less chance of a wheel spinning during acceleration, or of a wheel locking up if you engine break in slippery conditions.
If you read texts by Subaru and Honda on their automatics you get the idea that the system is constantly adjusting things front to back depending on conditions. Well, it may be monitoring things constantly, but it isn't doing anything most of the time.
So if you're looking at AWD for safety and fun, either go with a manual, or make sure you get an automatic with a center differential.
Great description, Paul. However, the auto is more proactive than you are describing (so I have been told - anyhow). It is my understanding that it is tied into the engine computer so that it can proactively send torque to the rear wheels when you get accelerate - to reduce wheel spin and in other sitiations. I agreewith you , however, that I would rather have the manual transmission.
Subaru of America (tech dept) and Subaru of Australia claim that my 2000 Forester AUTOMATIC has a default ratio of 60 front/40 rear until wheel Torque difference is detected. The AT is almost always in a state of flux and is always activated unlike the locking center differential on the MT models. The 60/40 default ratio is fixed by the transfer gear set as shown in the Subaru Factory Manual. Power is transferred by a "continuously variable clutch pack" behind the transfer gear set in the transmission. The same gear set is reversed to obtain a default ratio of 40/60 for the Impresa GT ? To quote Subaru literature "to give a more sporting feel in spirited driving" I think the "old" Subarus that were front or all wheel drive depending on whether the system was activated may be where the 90/10 or
100/0 idea came from. The AT system even in the base Subaru is a far more advanced AWD system. In fact Subaru called it the thinking mans transmission for its almost instantaneous reaction to wheel torque compared to the locking differential which is locked until wheels slip and then the silicone oil heats and then power is transferred. Subaru factory racers have a switch to deactivate the locked center differential so as to bypass the much delayed power transfer. Let us get our facts in line before typing.
Yes, you are basically correct here (and this has been discussed several times in this news group). But as Rob says, even Subaru's inexpensive AT AWD is a bit more proactive, e.g., when you start driving, or under strong acceleration. However, it does not have the full set of sensors that the VTD/VCD has, and does not give you the benefits a set-up with ~50-50 default has.
As I have said before here, for technical reasons and given the lack of a center diff, the clutch pack can basically only work in an on-off mode. That is, when it is off, the distribution by definition must be almost 100:0. When it is on, it can be anything because with an (almost) locked differential, the torque goes to where the traction is. That is, the torque distribution then is variable not due to variable clutch pressure, but because traction to the ends varies due to surface conditions.
This is a trick that is used by several manufacturers to reduce fuel consumption. So, Subaru can claim that the AT rquires as little gasoline as the MT. (Wether this is true under practical circumstances, I don't want to get into).
The Suburu Auto AWD is not the same as the Honda and should not be compared. The Subuaru system is a proactive system. I see little or no disadvantage with the Auto AWD when compared to the manual AWD. It is a proactive system. I have tested this out, and it is very difficult if not impossible to generate wheel spin.
Case #1: Raining Day. Flooring gas pedal from a traffic light. The AWD system automatically shift power to rear wheels to compensate for the weight transfer. In fact in this case, I think it is better than the
50/50 in the manual system, since the manual system waits to slip before transfer power!!!!
Case #2: Ice Storm. Again same as above flooring from a traffic light. No slippage what so ever. AWD system already compensates for weight transfer to rear wheels!!!
For what i have read, all AT subaru are now full-time AWD. Some models used to be part time AWD, but not anymore. In fact, the AT AWD is more advanced than the MT AWD. The automatic transmission pump pressure allows the design of this more advance AWD. It is the same system as the Audi A8. In 1st and 2nd gear the power distribution is 60/40. To reduce gas consumption, the distribution goes to 80/20 or 90/10 in 3rd and 4th gear.
For the MT AWD, when there is slippage, the center differential which is normally open, locks to 50/50. For the AT AWD, when there is slippage torque is sent to the axel with less slippage, hence torque on each axel is adjusted dynamically, which make the AT AWD better. Of course, it doesn't mean the the MT AWD doesn't do the job well. In fact it is a good system also, it is just less advanced and adaptative.
quote from "
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""Subaru deserves mention here because in the automatic version of theLegacy and Impreza (including the Outback variants), it uses acomputer controlled system much like those found in the Mercedes4Matic, automatic Audi A8/V8 and the earlier Porsches. Subaru has beenoffering this sophisticated system for a long time in a relativelyinexpensive car."
Funny, I was talking to a Subaru salesman and he claimed
50-50 for the non-VTD automatic. Due to my disagreement we went to the mechanic and he said 80-20.
It would be so nice if Subaru cleared all this up for those of us who like to understand things. Here are the different numbers I've come across, all from Subaru :
Off the Subaru Canada website
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-> "Click to view our AWD demo"
"The different Subaru AWD systems are matched to each vehicle's character. A 60% front 40% rear torque transfer on our basic system prioritizes safety while a 36% front 64% rear torque transfer on our STI helps maximize performance where sporty driving is the priority."
[--> no mention made of which transmission this might apply to]
Out of the Outback 2003/4 brochure
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"Manual transmission models split engine power 50/50 between front and rear wheels. Automatic transmission models use Active All-Wheel Drive, an electronically controlled multiplate transfer clutch. VDC model uses a Variable Torque Distribution System."
[--> no problem, except it violates the previous ]
Off the Subaru Global website
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[manual transmissions]
"Normally, the centre differential distributes the torque at a ratio of 50:50 to the front and rear wheels for extremely stable driving and maximum traction. Where the balance of traction is lost when front or rear wheels slip, viscous LSD automatically redistributes torque to maximise grip, so full traction and driveability are always available."
[automatic transmissions]
[--> no values for the MPT trnasmission]
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[--> ie WRX ]
"V-T-D AWD is an AWD system for automatic transmission vehicles that provides positive, sporty driving by making improvements in turning-in while maintaining the basic driving safety performance of Subaru AWD. The system uses complex planetary gear type centre differentials that distribute the basic torque at a ratio of 35 for the front wheels and 65 for the rear. Distributing more torque to the rear wheels reduces the tendency to understeer when accelerating while cornering to provide smoother, more confident handling."
[--> here are the gear reductions for the VTD WRX :
front : 4.111 rear : 4.111 ]
"Torque distribution is also optimally controlled to suit road conditions. This system provides both sporty driving and stability under any road conditions by automatically equalizing the front and rear wheel torque distribution to a maximum ratio of 50:50."
Out of the Impreza 2004 Service Manual (Mechanism and function)
Automatic Transmission : Transmission Control Module (TCM)
AWD transfer clutch control - Ordinary transfer control
Input signals :
Throttle position sensor Rear vehicle speed sensor Front vehicle speed sensor Inhibitor switch ATF temperature sensor FWD switch
[--> there aren't any 'torque sensors']
Automatic Transmission : AWD Transfer System
A: MPT MODELS
"This all-wheel-drive (AWD) transfer system uses an electronically controlled multi-plate type transfer clutch. The clutch is controlled by the TCM through the transfer hydraulic pressure control unit which consists of a duty-cycle-controlled solenoid valve and is located at the rear of the automatic transmission section together with the vehicle speed sensor.
The TCM has in its memory a set of duty ratio data, each defining at what ratio the transfer clutch should transmit the torque for a particular driving condition. Based on the driving condition information it receives from the corresponding sensors (vehicle speed, throttle opening, gear range, slip of wheels, etc.), the TCM selects an appropriate duty ratio from the memory and uses it to control the solenoid valve. The solenoid valve then regulates the pilot pressure of the transfer control valve which creates the pressure to the clutch from the line pressure. The clutch is engaged to a degree determined by the transfer clutch pressure thus created. Through this process, the torque from the engine is distributed to the rear wheels optimally according to driving conditions.
B: VTD MODELS [ie WRX]
The center differential performs the differential functions of absorbing the speed difference between the front and rear wheels and also distributes drive forces to the front and rear wheels at a predetermined ratio. In normal conditions (when there is almost no difference in the speed between the front and rear wheels), the drive force distribution ratio is 45.5 % to the front wheels and 54.5 % to the rear wheels. The hydraulic multi-plate clutch connected in parallel with the center differential between the carrier and 2nd sun gear functions as a differential action limiting device (LSD) and also as a device that controls torque distribution according to driving conditions."
From Subaru's Drive magazine
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"The VDC system first uses the transmission?s VTD AWD to help maintain control of the vehicle by transferring power between the front and rear wheels. Under normal driving conditions the VTD AWD system directs 45% of the engine power to the front wheels and 55% to the rear wheels. This slight rear-wheel bias produces a more neutral balance by reducing understeer and providing more of a performance driving feel."
For another non-Subaru description see :
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(this article mentions 90%-10%)
At least for the 2004 non-turbo Impreza (and Forester ?) the gear reductions are the same front and back 4.111 so any torque difference must come from the viscous coupling in the transfer clutch, which under normal conditions is mostly disengaged. The 90-10 figures I've seen look sensible for this condition.
With the transfer clutch fully engaged (locked) the distribution will be 50-50. An interesting thing here is that I don't beleive it's possible to spin a rear wheel without a front wheel also being in the spinning state.
Another thing, I don't see a transfer clutch as a replacement for a center differential. There's no balancing function. When the clutch is disengaged there is little going to the rear, then as the clutch closes, there is more and more going to the rear - but - there is never anything to handle the difference in wheel speeds front and back and vehicle handling must go out the window. (Just try driving a car with a locked center differential).
My personal vote is for 90-10 (if not 100-0) unless conditions dictate a shift to the rear, which can go to a max of 50-50.
It would seem that Subaru shies away from numbers when they talk about the MPT transmission...
Thanks for the confirmation, TransFixed. I should have read your post before writing my last one.
If I may pick a nit, in the above I'd replace the "where the traction is" with "where the traction isn't" since torque is being forcibly split front to back, but then the diffentials at the ends have to take care of things and unless they're limited slip, the torque will go to the wheel with the least traction (while not affecting what is happening at the other end of the car).
The VTD models are neat in that they can apply the brakes individually to prevent wheelspin and send the torque elsewhere.
Hi, I don't think you really don't know yourself. I have Honda, Subaru and Mitsu in my family manual and auto, all AWD. They do things it suppose to do all the time. It does it ALMOST real time. You're wrong saying it isn't doing anything most of the time. How old are you and how many years have you been driving? I am 63, drivivng for ~50 years. In those years one flat tire on the road, one broken steady bearing on my tow truck pulling trailer which stranded me until I got the new bearing. No other break downs. I maintain my vehcile very well and know how things work inside out. Tony
Funny, when I came here I learned that most americans are unable to drive stick, and Subaru had to take this fact into consideration as well, since they want to sell cars on the biggest market, so, from my point of view, they came up with an obvious solution: make a subaru a honda like, and apply as much marketing BS as possible, so consumers start spending their $$ on subarus. But since Subaru in a AWD business, they could not give up the idea completely, and market their ATs as very advanced to those who listen, but in fact a few people from this group reported that this (very advanced) tranny can be vary dangerous on slippery turns, when the back kicks in causing tremendous oversteering, but who cares, cars must be sold anyway.
On a funny note, I never lock my cars, they are equipped with a natural anti theft device - an MT. Never had a problem for years.
Hi, Vlad Maybe your car is not worth stealing. MT has nothing to do with it. Maybe you'are driving a rust bucket or wrecker, LOL. Don't be so sure. You think thieves steal only whole car? They steal tires, trims, stereo, etc. Even in America serious drivers drive manual and most older people know how to drive manual. In old days there was no automatic shift. Automatic is for just conevenience. Try to drive in a rush hour traffic in a city like N.Y. or LA. When you say, I never lock my car, it sounds STUPID to me. Just like guys who don't wear seat belt(I hope you do). Or you live in a small town where people know everyone each other. There are still places like that even nowadays. Tony P.S. What kind of dummy is going around slippery corner in high speed? Only people doing winter rally here. I live in cold Alberta.
Here in Philadelphia, most drivers forget how to drive in the snow from year to year (some years we don't get any to speak of), so the first snowfall always produces more accidents than subsequent snows. The second snow, everyone is driving real carefully after seeing the results of the first snow- if they even see any snow on the road, they slow to a crawl. After the third anow, drivers start to drive more reasonably- if you can see the road under the snow, then drive a bit faster. Unfortunately, that's also when the morons with the SUVs start driving too fast for conditions. They can go, but they can't stop. If they go fast enough (ka-boom), I refer to it as "natural selection".
You are right, maybe my cars do not worth stealing ;) But if a thief wants your stereo, he will take it anyway, the point here is what you end up with - a broken window, or in my case a slashed rug top, which is more expensive to replace then a stereo. You can call this STUPID, I call this WISE, up to you.
I drive stick in rush hour traffic, and think that it is safer then an AT, because drivers behind me tend to keep safe distance for obvious reasons. (hint - I rarely use brakes), but this is out of topic.
I do not know how those drivers went into a heavy oversteering condition, I only pointed that I saw such postings in this group. My previous post was about 'advanced AT' as some people think. The point here, regardless how advanced you auto tranny is, a simple stick is still far more advanced, and regardless how much marketing BS you absorbed the things will remain the same.
vlad
PS. Convenience is a marketing term I think, helps to steal your money
Hi, Okay, I respect your onw personal opinion. But the gap between MT and smart AT is narrowing. Some day in the future, most cars won't have MT. You don't even have to steer it, it'll be all automatic. With high power micro procesor and AI, why not? Just my personal opinion. Anyhow, I park my car inside garage at home/work. Tony
Unfortunately that can also be natural selection in reverse: When the hugh SUV goes KA-BOOM into a small car whose driver was driving with a high level of intelligence.....
Read the above posts carefully. You are dscribing the advanced VTD/VDC AT AWD in the more expensive models. The cheaper Subarus (including the FOrester and Impreza) don't have that.
Perhaps I did not make myself clear. Forget about the wheels and presence / no presence of a rear/front LSD for a moment. If you lock the center, or almost lock it, then torque will go to the axle with the most traction. No computer, differential, or other gizmo necessary.
Think of a toy car with locked center diff. Place the front wheels on fine sand paper, the back on wet ice. All wheels will turn at the same rate, the car will move forward. Where did the torque go? 99.9% magically went to the front. The back wheels did not do any work.
So when (almost) locking the center, a manufacturer can claim automatic torque distribution. Nifty, isn't it? Of course that 'almost' can get you, depending on the design, e.g., when trying to get up a wet boat ramp with a trailer, when almost all static and dynamic weight is on the back, but your car is a FWD vehicle with just a clutch pack to the back... (remember it needs to slip a little to compensate for different wheel spin during driving).
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