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Old 02-20-2001, 02:25 PM   #1
Reciprocity
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Question AWD systems.....manual vs. automatic...what REALLY is the difference?

This has come up several times...the whole automatic vs. manual debate.

Of course I know the difference between an automatic tranny vs. a manual tranny.

But what I really want to know is the REAL difference between the AWD systems.

The manual is mechanical (viscous coupling? plates in a silicon liquid?), the automatic is electronic (VTD? and electronic clutch?).

The manual puts 50% of the power to the front, 50% power to the rear, unless the tires slip...then it adjusts the power output to the wheels with the most traction...all done mechanicall... Is that correct?

The automatic splits the power 45% to the front, 50% to the rear...again unless slip is "detected", then it adjusts the power output to the wheels with the most grip... right?

Questions:
1. How much power can be directed to each set of wheels (for both auto and manual) 90/10? 100/0? 50/50?
2. How much faster does the automatic adjust power vs. the manual? Is it full seconds faster? Fractions of a second faster?
3. Can the manual even change the power output, or is the car 50/50 all the time?


In essence...what is the difference???

I really want to get some real world info here, not speculation...I love to hear stories about what the cars have done, what you made them do...but I really want to know the technical differences between the two.

Thanks in advance for the info!

D. Neil Crawford
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Old 02-20-2001, 02:34 PM   #2
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i'm not going to get into a lot of specifics here as the technical differences between auto/manual/other AWD systems has been beat to death. try searching in this and the FAQ forum, lots of useful stuff.

just a few quick points i noticed going over your post.
1) the auto torque split is 90/10. (note, this is RS were talking here. the manual for the WRX is 45/55 and i don't know about the auto WRX and the STi's get crazier)
2) yes, the 4EAT can vary torque almost instantaniously whereas the viscous coupling takes a bit more time.
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Old 02-20-2001, 03:07 PM   #3
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Actually, I did start this one with two thoughts in mind...

1. To learn for myself.
2. To have a thread that is just about the technical differences between the two...all other debates aside.

From what I understand, once the viscous fluid heats up (in the manual diff) the AWD system will react faster. But just how much faster? I still would like to find out how much faster the VC is vs. the VTD...

And from the other thread...

The automatic uses electronic sensors to continously adjust the torque split between front and rear, even before slip occurs. The manual will only adjust when slip happens. Correct?

D. Neil Crawford

[This message has been edited by dcrawford (edited February 20, 2001).]
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Old 02-20-2001, 05:34 PM   #4
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I'm not sure how the new WRX autos transfer power but here's how it works on the older cars...

AT starts from a split of 90/10. Bassically the front wheels are driven all the time, just like a FWD car. An electronic clutch which is slightly engaged all the time (hence the 10%) drives the rear wheels. When the clutch is fully locked the rear wheels are driven at the same speed as the front, making the power delivery a max of 50/50.

As for engagement, it does engage when wheel spin is detected, and works very quickly. It also engages at other times, during normal driving to prevent wheelspin before it occurs. A computer controls that function and makes a determination on how much the clutch should engage based on throttle position, gear selection, vehical speed, steering input, ect...

The manual uses three differentials. A differential tends to send power to the path
of least resistance. In normal driving this means 50/50, since both sides are going the same distance. When turning, slighty more power is sent to the side which has to travel farther, and less to the side which travels less.

Unfortunatly, the fact that a differential splits power in this way also means that one side can receive all the power, such as when on a slick surface or with lots of power. So a differential that splits power around 50/50 can go to 100/0 or 0/100.

The viscous unit is there to prevent that extreme. When large speed differences occur, the unit heats up, and tries to lock the two sides together. There is a delay while the unit kicks in, and it can't really bring the split back to 50/50, since small speed differences are allowed for the car to be able to turn. If the rears spin faster than the fronts, you can get power oversteer, which is quite easy to do in snow.
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Old 02-20-2001, 06:21 PM   #5
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Quote:
just a few quick points i noticed going over your post.
1) the auto torque split is 90/10. (note, this is RS were talking here. the manual for the WRX is 45/55 and i don't know about the auto WRX and the STi's get crazier)
On auto RS and other Subarus (exclude OB VDC), the power split is 90/10. On all manual Subarus (WRX included), the power split is 50/50. The WRX auto and OB VDC model gets a new AWD system called VTD which splits power 45/55.

For more info on VC diff and other types of AWD systems check this out: http://www.fortunecity.com/silversto...tion_4wd_2.htm

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Old 02-20-2001, 08:27 PM   #6
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Q: What is the AWD difference between the AT and MT Impreza's?

A: Lets talk about MT cars first. If you think about the way a viscous limited slip works, it's obvious that "50/50" is a huge oversimplification. Put the car up on a lift and run the engine (not recommended). Now put a brake on the rear driveshaft (i.e., both rear wheels), and measure the torque there. Initially, there will be very little torque as the rear end slows down due to the braking force. 50% of the engine's torque is obviously not going there right now. Only after the rear shaft has slowed down, and the fluid heated up causing viscous drag, does torque begin to be delivered there. In other words, some amount of slip must occur before the torque "transfer" occurs.

Now let's take AT cars with the MPT (multi-plate transfer clutch) system. This is a very sophisticated system similar to the one used on the WRC (but built for MUCH lighter duty use of course). TCU, or Transmission Control Unit, does indeed "anticipate" slip by applying clutch lock-up depending on throttle, speed, and gear position. So when you accelerate from a stop at WOT, the TCU always sends torque to the rear, regardless of wheel slip condition. Note that the Haldex center diff used on VW's is mechanically identical to our MPT, however by not being connected to the TCU it is lacking this very powerful feature -- like the viscous coupling, the Haldex must sense slip before lock-up occurs.

So the "90/10" myth is exactly that. With the MPT clutch fully disengaged (low throttle or braking at highway speeds), maybe this is somewhat accurate, but at every other situation, it is not.

Note that you can force fully locked 4WD ("50/50") on the autotrans by putting the selector in "1".

From "I" Club FAQ
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Old 02-20-2001, 09:20 PM   #7
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In the SVX 50/50 can be forced by shifting into 1 or 2.
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Old 02-20-2001, 09:32 PM   #8
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I absolutely hate that description of the MT system, even if it is from the i-club faq.

It describes a system that is more like a simple viscous coupling rather than a viscous locking differential, and uses as an example a situation that would never occur. It's flat out wrong.

The big difference in the two cars is the fact that the manual has a center diff, and the AT does not, and that has a big effect on the handling of the car. The MT is not as FWD biased as the AT is, meaning that it should be easier to get rid of power understeer and other FWD characteristics.

Read the above linked artical, especially the part in the area of viscous couplings and how they are actually a part time 4wd system and the effect that it has on handling vs a true 4wd system. The AT is that same kind of part time 4WD system but with a electric clutch system instead of a viscous coupling. (And some other important differences with make it AWD and not part-time 4wd)
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Old 02-20-2001, 10:45 PM   #9
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Alright! This is EXACTLY what I wanted to see!

I am awfully tired right now and am printing this all out to read as I lie in bed...so I will talk about it tomorrow...but I just wanted to say thanks for posting this up folks, it is just what I wanted to read.

Damn I love this board!

D. Neil Crawford
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Old 02-21-2001, 06:03 AM   #10
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This is what this board is up for: mature discussion not religious beliefs.

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Old 02-21-2001, 06:08 AM   #11
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Quote:
The big difference in the two cars is the fact that the manual has a center diff, and the AT does not, and that has a big effect on the handling of the car. The MT is not as FWD biased as the AT is, meaning that it should be easier to get rid of power understeer and other FWD characteristics.
That's true. However, in WRX, the VTD system is more rear biased than MT. Also, VTD system uses multiplate clutch to control a center differential to distribute the power. A far better system than current generation of AT AWD system, IMHO.

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Old 02-21-2001, 10:33 AM   #12
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Question

A couple things here...

I just want to clarify this...the automatic can split a maximum of 50 front and 50 rear. The manual can split 100/0 or 0/100. The auto will adjust the split prior to wheelspin...however, with both cars doing a drag launch...a 1/4 mile run...the manual will start at 50/50, the auto will start at 90/10, but can adjust to 50/50 if needed, but no more. (this is on the pre-WRX Imprezas)

So the big advantage of the VTD system is the speed at which it operates, and the ability to "anticipate" wheel slip and adjust accordingly.

Does anyone know the weight difference between the two? Not the weight difference between the transmitions, but of the AWD systems themselves?

Also, in theory, you could adjust the split in the automatic yourself, right? Hook up a dial to it and adjust the split on your own? (oversimplified yes...but possible I think!)

I am still a bit fuzzy on the "Viscous Coupling Differental Lock" (remember, I am an accountant! ). From what I understand from the article...it is a vicous coupling (which is a device I do understand) that has a locking ability? (not sure where the "lock" part comes in) In essence...from what I understand...it will split the power 50/50 (regular center diff.) but will adjust the power (a la viscous coupling) as wheel slip occurs (up to 100/0, 0/100). Correct?

D. Neil Crawford
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Old 02-21-2001, 10:36 AM   #13
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Oh yeah...Foxbat...I totally agree. Save the debates for other threads, I am happy this is not an arguement. I want this thread to be technical information, not a fight. Keep it up!

D. Neil Crawford
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Old 02-21-2001, 10:55 AM   #14
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Quote:
I just want to clarify this...the automatic can split a maximum of 50 front and 50 rear. The manual can split 100/0 or 0/100. The auto will adjust the split prior to wheelspin...however, with both cars doing a drag launch...a 1/4 mile run...the manual will start at 50/50, the auto will start at 90/10, but can adjust to 50/50 if needed, but no more. (this is on the pre-WRX Imprezas)

So the big advantage of the VTD system is the speed at which it operates, and the ability to "anticipate" wheel slip and adjust accordingly.
What you described here is non-WRX AT AWD system not VTD system. VTD system's initial power split is already 45/55 and should be able to do 0/100 or 100/0 if allowed because it has a center diff as manual does.

Quote:
In theory, you could adjust the split in the automatic yourself, right? Hook up a dial to it and adjust the split on your own? (oversimplified yes...but possible I think!)
I think STi version has a button to do just that.

Quote:
I am still a bit fuzzy on the "Viscous Coupling Differental Lock" (remember, I am an accountant! ). From what I understand from the article...it is a vicous coupling (which is a device I do understand) that has a locking ability? (not sure where the "lock" part comes in) In essence...from what I understand...it will split the power 50/50 (regular center diff.) but will adjust the power (a la viscous coupling) as wheel slip occurs (up to 100/0, 0/100). Correct?
Think of this AWD system as a LSD(this is a LSD). Without a VC, the open center diff will operate from 0/100 to 100/0 freely. The effects of VC is to put a limit force on center diff to bring the power split back to 50/50 whenever it deviates.

Foxbat
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Old 02-21-2001, 11:07 AM   #15
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Again..this is fantastic, learning a lot here...

Foxbat...actually I was trying to describe the non-WRX system (I noticed I said VTD, I should have said Impreza RS system...which is called what by the way?)...anyway, can the AWD system in the automatic 2.5RS (non-WRX, prior to MY02) adjust the split beyond 50/50?

D. Neil Crawford
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Old 02-21-2001, 11:09 AM   #16
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Quote:
Foxbat...actually I was trying to describe the non-WRX system (I noticed I said VTD, I should have said Impreza RS system...which is called what by the way?)...anyway, can the AWD system in the automatic 2.5RS (non-WRX, prior to MY02) adjust the split beyond 50/50?
Subaru calls it Active AWD. From what I understand, this system can not adjust the split beyond 50/50 for the rear.

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Old 02-21-2001, 11:23 AM   #17
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Thanks for the info! I think that takes care of all of my questions (except the weight...but that isn't terribly important to know).

Alex, save this thread somewhere!

D. Neil Crawford
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Old 02-22-2001, 12:17 AM   #18
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This is going good...

Just want to clarify a couple things...

When the power split on the AT system goes to 50/50 by fully locking the clutch, what that means is that the front wheels and the rear wheels turn at the same speed, all the time.
That works great for when you get stuck or loose traction, but is not good for turning. A system locked at 50/50 would scrub speed and be very hard to turn tight corners.

On a "drag launch", the automatic would probably be activating the clutch to prevent wheelspin based on thottle position, and the computer knows that the front may loose traction as wieght shifts to the rear so it activates even before slip occurs. It's predictive and actually a very impresive system.

The manual at 50/50 is quite different. The differential allows for different front to back speeds while cornering, which is why it can spit power that way all the time. The limited slip function is just there to keep the system working when when one or more tires looses traction.


A viscous coupling is a very simple way to make a car AWD. You take a fWD car and extend a driveshaft rearward, connected to the rear wheels with a viscous coupling. (Kinda of like the impreza auto RS with a viscous coupling replacing the electric clutch)

The front wheels are driven, and undernormal circumstances the rear wheels just rotate freely, and follow the fronts. But when the fronts spin faster than the rears due to tire slip, the viscous reacts and sends power rear. (up to 50/50 if it fully locked, but it doesn't fully lock)

This is very simple, cheap, but is in fact used for some high end cars the opposite way. The Lambo diablo and some porshes use this I think, rear wheels driven with the fronts coupled via a viscous coupling.

The viscous coupling diff lock is indeed just another type of LSD, which means is just there to stop all the power from going to the spinning side.

The new VDT on the new WRX autos should help them get rid of the FWD caracteritics that flaw the current auto system.

And finally, weight...

Hmmm... A simple viscous coupling would be lightest, But a transfer clutch like on the current auto RS, would be about the same as the manuals LSD. Of course the auto tranny wieghs more than the manual I think, so that probably is the biggest difference. I think the VDT on the new auto WRX would be the heaviest.
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Old 02-22-2001, 12:30 AM   #19
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According to WRX brochure, the weight difference between AT and MT is only 55 lbs.

Foxbat
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Old 02-22-2001, 07:25 AM   #20
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Ever since I started reading about Subes on the internet in '97, I've always read that you can lock the AT's torque split in 1st gear.
Could someone point me to some factual information?
Dennis
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