Viscous vs Electronic Transfer Clutch?

[blitz]

I'm gonna ask the question that I'm sure many Sube owners have wondered about: Why two AWD systems?

 

Personally I like the viscous clutch system better than the electronic transfer clutch. The electronic clutch system doesn't give a true AWD handling characteristic and also has that irritating (dare I say dangerous) delayed and sudden rear-engagement. Aside from blatent abuse or improper towing, the viscous unit doesn't seen to have the reliability issues of the electronic clutch (binding, et. al).

 

Supposedly the electronic system gives better fuel mileage, but I'm not seeing it. If it were true, why not use it across the entire Subaru line regardless of tranny type? CAFE point are precious. Which brings me to my second question: Why didn't Subaru do it the other way around (viscous on A/T)?

[nipper]

Actually the electronic one is far more sophistacated as it varies the split from 10/90 to 50/50, as oppsed to 50/50. The computer is always montiring things, and reacts very quickly on the move.

There is a delay in the viscous too. I have a freind that has the manual with the viscous, and he really prefers the computer control. The reason there seems to be a delay from a standing stop is that first off you notice the computer determing that you need to go from 10 to 50%, Thers is a delay in the visous coupling as the plates inside the coupling start to shear the fluid. The viscous coupling does not have clutches. Yes it is a simpler system, and far less sophisticated. Once the fluid shears it thickens and you have torque transfer. The thickining of the fluid actually takes as long if not longer then the computer controled one, but since it is more fluid in its engagement it is not noticble. The computer engages clutches, so its seems more jerky.

As far as torque bind, in any car the most over looked fluids are the trranny and differnetial fluids. As long as you change your tranny fluid at regular intervals, you wont get torque bind. No one exects engine oil to last 100K, but yet so many will let tranny fluid go that far or longer without changing.

 

Also for traction on the fly while manuvering on poor surfaces, you dont always want 50/50 to keep the car true, sometimes you only need 30/70.

Also if you look at any other car out there with AWD they all have a delay for the computer to figure out whats going on from a dead stop, some are worse then others.

Motortrend? or Car and Driver (ill try to find it) just did a test on in ice rink of mid-price full size cars with AWD, and some couldnt get out of their own way on the ice, where others where very impressive. Surprisingly i think the mercedes sucked and the Audi kicked but. They all used diffent drive systems... viscous, computer and hybrid.

You have visocous coupling in manual tranny cars because its a cheap system. You have a computer in the automatics because its no longer a base model.

 

 

nipper

[All_talk]

Just to add a few points… my experience is based on my ’91 Legacy SS 5MT and ’92 Legacy SS 4EAT (now sold).

 

The 4EAT reacts faster and in most cases more positively, the center VLSD of the 5MT (and it is a true VLSD diff, not just a viscous coupling) can require quite a bit of wheel spin to work in some circumstances. That said, I feel the manual system is very smooth and balanced and gives that car a very predictable feel. Not to say the 4EAT system feels unpredictable, the handling quite nice, but it doesn’t provide the same feedback as the 5MT, its more of a “drive it and forget it” kinda thing.

 

No question the active computer controlled system of the 4EAT is more a adaptable, having the ECU gives complete control over not only the AWD but the entire drive system, I’m sure Subaru considers it the superior setup and for the majority of drivers I’d agree. And they would likely use it across the model line except for the fact that the 5MT lacks the hydraulic system to power it.

 

Gary

[blitz]

That said, I feel the manual system is very smooth and balanced and gives that car a very predictable feel.

Yeah, that was my point about the handling difference.

 

The manual trans is applying driving torque to both front and rear axles at all times via the center differential (with any wheelspin overload being moderated by the viscous clutch), whereas the 4EAT is front wheel drive and only clutches-in the rear after front wheelspin occurs.

 

The "10%" static setting on the rear is just a preload to take up driveline slack (makes the rear engagement event less obtrusive).

[audley59]

hi guys,

i am a newby but i have been driving a 96 impreza for the last four years. i recently decided that i wanted to go faster so i made an engine swap, a ej18 to a ej20t from a 98 forrester. i also changed the ecu. my problem is that the gears no longer change smoothly and only when i drive fast also when i do an engine wash it run through the gear smoothly, but hardly any power. i also changed the clutch in my transfer box but there is still that sound and viberation when going around corners. can anyone tell me whats happening.

[nipper]

Yeah, that was my point about the handling difference.

 

The manual trans is applying driving torque to both front and rear axles at all times via the center differential (with any wheelspin overload being moderated by the viscous clutch), whereas the 4EAT is front wheel drive and only clutches-in the rear after front wheelspin occurs.

 

The "10%" static setting on the rear is just a preload to take up driveline slack (makes the rear engagement event less obtrusive).

 

The visous dose NOT do it at all times, the electronic one does. In order for the viscous to apply 50/50 there has to be speed differential between the front and rear axles, otherwise it is biased towrads to the front. The viscous coupling is NOT a differnential.

 

A center differnetial is a mechanical unit that allows for the front and rear of the car to be driven at the same time, allowing for the front wheels to travle faster then the rear (as they do normally) without torque bind. In this system it is still possible to have 100% of the torque go ot the one wheel with least traction if it does not have a lock up mechanisim.

 

2wd/4wd transfer case is a direct gear connnection between front and rea wheels, but will not allow for the binding that develops from the axle speed.

 

Visous coulping is a series of plates that are in a sealed unit filed with silicon fluid. As long as the front and read axles are within a % of each other, there will be only fwd. Once the speed differnce is great enough, the plates start shearing the fluid creating friction. The silicon fluid is one of those rare fluids that get thicher with heat, and create a mechanical connection between the front and rear axles givein you 50/50 split.

http://auto.howstuffworks.com/differential9.htm

 

The electronic system has a duty soleniod and a set of clutch plates like a auto transmission, and needs pump pressure to make it work. The minimal setting is 10/90, ans is adjusted by the amount of time the duty cycle opens and closes up untill you get a 50/50 split.

 

The electronic one is always on, the viscous is not, but always at standby.

 

nipper

[All_talk]

The viscous coupling is NOT a differnential.

 

Oh yes it is you stinky poopy face :-p

 

 

OK, grade school playground insults aside... you have a misunderstaning about how the Subaru EJ 5MT AWD system works (dont feel bad, lots of Subie fans do). The center diff certainly IS a true viscous coupled bevel gear type differential. These scans from the FSM will shed some light...

 

basic description

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWDMT_description.jpg

 

innner/outer pinion shaft

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_AWD_drive_pinion_assy.jpg

 

the center diff

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff1.jpg

 

how the diff transfers power

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff2.jpg

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff3.jpg

 

Thanks to Josh for the scans

 

Gary

[ferret]

I also beg to differ...I have drive both extensivly and agree whole hardly with AllTalk.

 

 

Just go to www.Subaru.com and click on the AWD flash in the lower left. Then click on 'Whats the Difference'.

 

The Manual tranny is Continuous, The 4EAT auto is 'Active'. It also explains the other VTC...VTD and DCCD used on more expensive models.

 

 

My wife prefers the Automatic tranny (nothing to do...but an occasional 'clunk' as the duty solenoid kicks in). I prefer the Manual so I have control and some fun in the snow.

[nipper]

Oh yes it is you stinky poopy face :-p

 

 

OK, grade school playground insults aside... you have a misunderstaning about how the Subaru EJ 5MT AWD system works (dont feel bad, lots of Subie fans do). The center diff certainly IS a true viscous coupled bevel gear type differential. These scans from the FSM will shed some light...

 

basic description

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWDMT_description.jpg

 

innner/outer pinion shaft

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_AWD_drive_pinion_assy.jpg

 

the center diff

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff1.jpg

 

how the diff transfers power

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff2.jpg

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/MT_centerdiff3.jpg

 

Thanks to Josh for the scans

 

Gary

 

good scans ..... its a hybrid trying to do the best of both worlds, but still has a lag as the vsicous couplings "hook up". And i said visocous coupling .... in its pure form so thhhrrrrrppppppttttttttt :-p

Just means it react faster then a normla couling but i bet if you could get one of these buggers an a 4wd wheel dyno yould see the electronic version reacts faster ..... if there is such a thing .... always wondered about that

[Setright]

Yep! I think Nipper, you might have been swayed by all the "part time AWD" vehicles. Welcome to the world of Subaru, we do things differently here

[blitz]

OK, here's what I've gathered: The "premium" (more expensive) system is the one that doesn't give an AWD handling charateristic on the road and has a lower track record of reliability.

 

Do have it right?

[ktdenali]

Do either of you know when Subaru swtiched from viscous coupler to the electronic system?

I have a 96 legacy, in which the VC is shot. Quit at 230000 miles. Just bought an Outback 2000 and was wondering if i can expect similar problems with it, or does it have a different sysyterm in the AWD?

 

I'm gonna ask the question that I'm sure many Sube owners have wondered about: Why two AWD systems?

 

Personally I like the viscous clutch system better than the electronic transfer clutch. The electronic clutch system doesn't give a true AWD handling characteristic and also has that irritating (dare I say dangerous) delayed and sudden rear-engagement. Aside from blatent abuse or improper towing, the viscous unit doesn't seen to have the reliability issues of the electronic clutch (binding, et. al).

 

Supposedly the electronic system gives better fuel mileage, but I'm not seeing it. If it were true, why not use it across the entire Subaru line regardless of tranny type? CAFE point are precious. Which brings me to my second question: Why didn't Subaru do it the other way around (viscous on A/T)?

[GeneralDisorder]

Do either of you know when Subaru swtiched from viscous coupler to the electronic system?

I have a 96 legacy, in which the VC is shot. Quit at 230000 miles. Just bought an Outback 2000 and was wondering if i can expect similar problems with it, or does it have a different sysyterm in the AWD?

 

Are you talking about Automatic or manual. If you are talking 5 speed they haven't switched - all the 5 speed's are Center Diff w/VC. There is absolutely zero electronics in a Subaru 5MT.

 

If you are talking automatic, then they switched to electronic control in 1988 with the advent of the 4EAT transmission (which is still being used in upgraded form). Prior to that there was a 3AT (not electronic) 3 speed automatic that did work off a VC center coupler (no diff) for the rear output.

 

You can expect exactly what you put into the tranny out of it. If you regularely flush the fluid and change it and make sure to not overheat it, or run it with wildy dissimilar tire wear/inlfation then they will last the life of the car. If you don't heed the proper maintenance then they will have transfer clutch and duty-c solenoid failure.

 

GD

Edited December 2, 2009 by GeneralDisorder

[ktdenali]

I was talking 5 speed. I do not like driving anything automatic (glitch of being from Europe - also wish they had Subaru diesels here too)

 

What you say makes complete sense. That is why the automatics have the fuse on top of the engine to turn off AWD, which some call "fix" the viscous problem, by just driving 2WD.

[bulwnkl]

Whoa! Can you guys see what the FSM says? 5MT Subarus are RWD!!

 

The viscous unit tightens up in order to send power forward! That's awesome!!

 

Personally, I really like the planetary center power divider system in my A/T Baja turbo.

[ausubaru92]

Whoa! Can you guys see what the FSM says? 5MT Subarus are RWD!!

 

The viscous unit tightens up in order to send power forward!

.

 

Um.... No

 

The 5MT has a centre diff that sends power evenly between the front and rear wheels (the exact same way the rear diff divides power between the left and right wheels)

Buy it also has a viscous coupling, which tightens when wheelspin occurs and limits the difference in rotating speed between the front and rear axles, effectively acting as a diff lock

[bulwnkl]

Yes, you're right. I scanned the pages too quickly.

 

To reiterate for clarity: The 5MTs utilize a mechanical center differential with a viscous coupler. IOW, they have a center VLSD.

Edited February 4, 2010 by bulwnkl

[Gsir08]

I haven't driven to many Auto subarus, but I love the viscous setup that was in my 96 Legacy 5 speed. Ive been driving it long enough in the snow to be able to tell exactly how much throttle it needs to get the rears engaged and how much is too much to get the rears to spin.

[ausubaru92]

I have a 2001 outback auto and i rekon the awd system is great.

I cant even notice the lag when it locks up.

 

This is a video, i made of a duty cycle meter and hooked it upto the duty solenoid so it displays the amount of MPT (multi plate transfer) lockup

In the video, i drive up to an intersection, turn left and accelerate to 100km.h

[nipper]

That is very interesting, what i've been wanting to see for ages. I would love to have seen how 1;2;3;R were affected.

 

 

nipper

[johnceggleston]

That is very interesting, what i've been wanting to see for ages. I would love to have seen how 1;2;3;R were affected.

 

 

nipper

 

+1,

 

so it's counting cycles, is that per minute or second or what??

 

this is a guess,

 

but it looks like 30 is the default. and maybe it jumps up as the trans shifts hitting the 70s, and higher, as the trans reaches high speed in 4th bleeding off most of the trans fluid pressure.

 

does this fit with the way the duty c works?

[idosubaru]

having driven dozens of different subaru's as daily drivers i've never really noticed much difference. it's negligible for me since snow tires or studs are the real solution for those of us that have serious conditions (meaning economically destitute states that can't afford to/don't maintain snow packed mountain roads :lol:) to cover.

[nipper]

+1,

 

so it's counting cycles, is that per minute or second or what??

 

this is a guess,

 

but it looks like 30 is the default. and maybe it jumps up as the trans shifts hitting the 70s, and higher, as the trans reaches high speed in 4th bleeding off most of the trans fluid pressure.

 

does this fit with the way the duty c works?

 

Pretty much it does. The more the cycles the more pressure applied to the clutch pack. Would be nice to see it work on something slick too.

 

nipper

[ausubaru92]

That run was tested on bitumen.

 

I found no difference between 1, 2, 3 and D

 

That display was measuring duty cycle. So it is the ratio of on and off periods.

 

0% duty cycle would mean zero pressure to the MPT and thus a torque split of 100% front and 0% rear.

 

50% duty cycle means there half pressure to the MPT and a torque split would be 75% front and 25% rear.

 

100% duty cycle will give full pressure to the MPT and a torque split of 50 50

 

The TCU MPT output is mostly related to throttle position, but sudden changes in wheel speed (loss of traction) quickly sees a rise in MPT output.

Yes the lowest default is 30% (which gives a front/rear torque split of 85/15)

Full throttle gives around 70% ( front/rear torque split of 65/35)and if there is wheelspin detected, it jumps as high as 97% (almost 50 50 split)

 

I took it for a drive on gravel, but didnt record it.

I could see the TCU applying more pressure because of wheelspin, even though i could not feel the wheels spinning. The TCU is very quick and quite clever. Subaru obviously put a lot of time into programming it.

[AWDfreak]

I've always loathed automatic transmissions, but I am quite interested to see a test between auto and manual Subarus on a video to see the difference. And as much as I hate automatics, I would actually like to drive an AWD auto in the mud patch I've driven my 5MT Outback in to feel the difference.

 

Is there any reason why the AWD manual Subarus are always mechanical AWD?

 

And how about the STI? I know that's a huge can of worms I'm opening, but what is it more related to; the manual (viscous center diff) or the automatic (electronic transfer clutch)?