AWD Diff's?

[Mylesofsmyles]

New to Subaru's and AWD but always eager to learn.

Thinking about a 1st Gen Legacy Wagon as a DD/winter vehicle and want to know more about Subaru's AWD system(s)/

Is the AWD system in the 1st Gen Legacy Symmetrical AWD? If not, what's the difference?

Wassup on the Diff's? I hear the center diff is a Viscus LSD, but what about the front and rear axles?

On some of the later STI and WRX cars, Impreza's, there were some electronic controls for the AWD system, no? What do they do and can they be retrofit to the 1st Gen Legacy?

If this is too much of a NOOB post, I'll take referrals to threads, posts or articles that I should read to get my info.

Thanks!

[Legacy777]

Welcome to the BBS.

The symetrical AWD thing is just a marketing term. All of Subaru's AWD setup are that way. The MT's AWD system is pretty much identical to all the newer 5mt AWD systems, and uses a center diff with a viscous coupling. The AT's AWD system uses a multi-plate system to transfer power to the rear wheels. It's default power split is 90/10, while the MT's is 50/50.

As for the front and rear diff's, they are all open diffs, with the exception of the 91 turbo legacy, which came with a vlsd in the rear.

As for the electronic controls on the newer models, no they can't be swapped over. Some people have modded the AT's with a switch that intercepts the duty c solenoid, which transfers power to the rear wheels to get 50/50 split. This mod should only be engaged on slippery surfaces like snow and gravel.

Here's some more info on the AT's AWD system:

http://main.experiencetherave.com/subar ... ystem1.jpg
http://main.experiencetherave.com/subar ... ystem2.jpg
http://main.experiencetherave.com/subar ... ystem3.jpg

Info on the MT's

http://main.experiencetherave.com/subar ... iption.jpg

[Mylesofsmyles]

wow, so automatic Legacy's are 90% FWD?

i think i saw someone's build on here where a 6mt was swapped into a 1st gen Legacy...can only assume a 6mt from a recent WRX or STI. figured the electronics in the 4wd system would have transfered too.

so manual transmission subies have a 50/50 split...nice, that's what i'm looking for.

so stock, they've got open diffs front and rear, but select models had an LSD rear...stupid question but what's the trend for rally sports vehicles. are many using open diffs front and rear, or doing a split combo like an open in the front and lsd in the back, or do people ever go the route of an lsd front and rear?

[Mylesofsmyles]

K, so got wise and read specs for the 2010 Subaru WRX STI...says under DCCD...

"Exclusive to WRX STI, Driver Controlled Center Differential (DCCD) All-Wheel Drive uses an electronically managed multi-plate transfer clutch and a mechanical limited-slip differential in conjunction with a planetary-gear-type center differential to control power distribution between the front and rear wheels. Featuring manual and three automatic modes, DCCD splits power 41% front and 59% rear. Sensors monitor parameters such as wheel slippage, steering angle, throttle position and braking to help determine torque distribution and direct it to the wheels with optimum traction. DCCD also features a limited-slip helical front and Torsen® rear differential."


k, so i guess having helical gear diffs in both the front and rear could be fun?

get lost in the explanation of the center diff, just wondering....sounds like it's electronically managed instead of being speed sensitive vs. torque sensitive...right?

what do other performance 4wd systems use and why? as in, if it's a mechanical system, are speed sensitive or torque sensitive diffs used, and why?

[mike-tracy]

...

I admit I'm not an expert, but a given awd system would be designed around the anticipated usage. Having front and rear LSD would be beneficial to motorsports, with different types of front LSD's depending on the surface. On a street car having certain types of LSD would cause you to spin out of control on a turn if you lifted off throttle.

Also, only the STI and certain 80s turbo Subarus have real LSD. The others have VLSD, which is better than an open diff really isn't very effective (also depending on usage). The clutch types can be rebuilt when they are worn, while the VLSD types are not serviceable.

[Mylesofsmyles]

...

I admit I'm not an expert, but a given awd system would be designed around the anticipated usage. Having front and rear LSD would be beneficial to motorsports, with different types of front LSD's depending on the surface. On a street car having certain types of LSD would cause you to spin out of control on a turn if you lifted off throttle.

Also, only the STI and certain 80s turbo Subarus have real LSD. The others have VLSD, which is better than an open diff really isn't very effective (also depending on usage). The clutch types can be rebuilt when they are worn, while the VLSD types are not serviceable.

sure, i come from a world of RWD cars, where the trend is to opt for a helical type LSD unit for more tire spinning fun. going for a similar type path for a 4wd car.

i guess i'm just surprised that the center diffs in these cars are speed sensitive and not torque sensative...might make sense though...never really thought about it till now. fwiw it appears as though VW?Audi does use a torque sensitive center diff in their quattro system.

[Legacy777]

Some more info probably worth reading.

http://en.wikipedia.org/wiki/Limited_slip_differential

[Mylesofsmyles]

Some more info probably worth reading.

http://en.wikipedia.org/wiki/Limited_slip_differential

yep, i've been through that article at least a dozen times, when i was trying to determine the difference between a Viscous and Torsen LSD available in Mazda Miata's.

I suppose a viscous lsd will engage at low speeds better, say for traction off the line, on a wet, snowy or gravely day, lolz. where a helical or clutch LSD will make better friction at higher speeds when torque output is greater?

maybe someone can help me through this.

so on an AWD subie, with a helical in the front, Torsen in the back and Viscous in the middle...it still allows more give in the system at the viscous center diff, so wheels in the front and rear can travel at the same speed, but the helical in the front and torsen in the back make for a more slideways car when your putting out the torques.

maybe, possibly, just really trying to vividly grasp this concept. i've never had awd before, and until recently, the fact that all four wheels some how and sometimes get power is about all i've known.