I need a little help understanding the LSD difference between the '07 WRX, and '07 2.5i.

So far what I understand is that the 2.5i Manual uses a Viscous-type Locking Center Differential (Center LSD). I know that the viscous uses liquid to distribute torque. So having this in the center distributes the torque between the front and rear axles depending upon which has less traction, and which has more. (Distributing from the one with less, to the one with more). So the 50/50 split could easily turn into a 60/40 front biased drive if the rear wheels are slipping more.
Since this is a locking differential, even if one wheel is up in the air, all the remaining will still turn. This also allows all wheels to turn at the same rate when needed. This allows the wheels with traction to still keep high amounts of useful torque, as opposed to an open differential, which if one wheel is up in the air, the wheel on the ground will basically be drained of any useful torque. (Correct me if I'm wrong in any part of this).

Alright, now my understanding is that the WRX models make use of a Viscous-type Limited-Slip Rear Differential (Rear LSD). So If say the Right-Rear wheel is slipping, the Left-Rear wheel will still retain torque as long as it has traction. (Basically a torque transfer from wheel to wheel in the rear, instead of axel to axel).

So what exactly is the difference between the WRX's differential set-up, and the 2.5i's differential set-up? Any Pros or Cons for either?

I've only been looking into AWD systems for a day, all these different kinds of set-ups and styles of differentials are fairly confusing. If anything I said is incorrect, feel free to correct me, or add anything I may have omitted.

 

I wanna say they both have the LCD center diff, but the 2.5i has an open diff front and rear. While, the WRX has an open front diff, and LSD rear diff. Meaning, they both transfer torque front and rear, but on the 2.5i if one wheel (say the left rear) is in the air the other (right rear) gets no torque, at which point torque would be increased to the front...

I think...

 

Hmmmm.. yeah that makes sense. I just wonder if there's any pros or cons to either set-up, and under which conditions one might be better than the other.

Thanks for the explanation tho. Definitely helps out.

 

i don't wanna hijack this thread but i've been wondering this...

so does the STi have the same setup as the WRX...and also is there a way to put the WRX rear diff on a 2.5i without massive mods?

 

Hey I was headed towards that line of questioning, so feel free to ask away zavier. Maybe for once and for all we can get everything laid down about the difference in the set-ups, the pros and cons of each, in various situations and conditions, and the ability to do mods and swaps to these areas of the vehicles, and if it is done, what else needs to be modded or swapped along with it.

I know some individual threads mention bits and pieces of these subject areas, but it would be nice to just have it all layed out here.

Not to mention that everyone's current understanding and experience up to this point in time would be extremely helpful.

 

The STi differentials are very, very differnt than the WRX, but you can use a WRX rear diff unit in an RS or 2.5i as long as you swap the ring and pinion.

 

so the front diff on a wrx is the same as the 2.5i as stated is true then?

 

Yes, both the WRX and RS/2.5i have open front differentials.

 

Ok I think you guys need to simplify the AWD thing.

The RS has open diffs front/rear, meaning only one wheel spins on each axle at any given moment. So the RS is really two wheel drive.

The WRX has an open front diff and a limited slip rear diff, meaning that under heavy throttle the rear diff locks both rear wheels while the front diff only spins one wheel at any given moment.
So the WRX is really three wheel drive.


The STI has front and rear locking diffs, meaning that ALL FOUR WHEELS WILL BE SPINNING NO MATTER IF ONE IS TOUCHING THE GROUND AND THE OTHER 3 ARE IN THE AIR.
The STI is actual AWD/4WD.

All this being said, the usefullness of the vehicle you choose depends on what you use it for.

If you just want an inexpensive AWD thats capable in most everyday conditions pick the RS

If you want a little more than the above and want HP and go offroad where most other AWDs couldnt go(Rav4, CRV to name a few) get the WRX.
Just as an example: I had an S10 4x4 Blazer w/ just a rear lsd and a friend with Rav4 Fulltime 4x4. I took him to an old hunting trail with some small loose rocky hills. Most of the time I dont need 4WD to climb any of these hills when its dry but when its wet I can never make it to the top unless in 4WD. Well, the RAV4 couldnt climb the hill when it was dry. I wish I had a video of it. I watched as only one wheel in the front spun like mad and only one rear wheel dug itself in. Even when he got a running start. If he only had a rear lsd he could have climbed the hill. If it was muddy he wouldnt stand a chance.

The WRX STI is TRUE AWD, meaning all 4 wheels spin no matter if 3 wheels are in the air and one is touching the ground all 4 wheels will be spinning.
While the STI does have front and rear LSD's during normal driving conditions they stay unlocked till the torque builds up enough to lock both diff's.

Please correct me if I'm wrong. I tried to keep it as simple as I could.

 

You're thinking in terms of 4x4s instead of AWD. Basically, everything you said about the Subaru diffs is wrong...

 

an open diff means the wheel that slips gets all the torque and the other tire stops moving (locks)...and lsd means the torque is split to make sure both tires are spinning...

if you have open front/read and an lsd center then if one of the front tires slips then the lsd splits the front/rear power to make sure you keep going forward...where in 4x4 if you lose front traction you lose all traction because of the locking case marrying the front and rear axles...

if you have LSD all around that just means you will always have all 4 tires going no matter what...unless one is in the air of course...cause then in that situation...torsen style lsd's just become open diffs lol

i think this is how it actually works

 

You are closer but still not right. I would try to break it down for both of you, but honestly there are already better writeups than I could give you posted in many places.

 

/sigh...i say don't worry about how they work if they are all mechanic and require no user input haha...simple way not to confuse myself :P

 

Well ****, I guess I wasnt even close. I guess I'm just spinning my wheels then..lol

 

Heh,

Basically, there is only a difference from one type of diff to another when a wheel loses traction relative to the other one on that axle. In the case of an open diff, almost all of the torque transfers to the slipping wheel and very little is left for the wheel with traction, wehter it's front or rear. When both wheels at either end of the car lose a lot of traction relative to the opposite end, then the center diff comes into play. (Also, it matters when the vehicle is turning, but that's a different circumstance.) If your front wheels are almost without traction, the viscous center diff will ensure that much of the torque still gets to the rear wheels to keep you moving and vice-versa. Open diffs at the axles only suck when one side has MUCH less traction than the other. On surfaces liek slush and ice, it's not such a huge deal because traction at either side is relatively equal, so you get some wheelspin at both wheels instead of one stationary wheel and one madly spinning wheel, neither of which puts torque to the ground very well. A viscous LSD at the axle makes this even less of a problem, because then even with one wheel spinning, you get good torque to the other side. You can get a Subaru moving with only one wheel having traction, if that wheel is on an axle with an LSD. The big, fat huge benefit of this system over true 4wd, which is a locked transfer case (not center diff; there is no differential action) is that all 4 wheels can spin at different speeds, which they all naturally should be doing when you are turning. 4x4s are at a major disadvantage when cornering because of this, and will want to severely undeersteer on loose or slick surfaces with any throttle applied.

That's more or less it. All of the differences in types of LSDs are to get various different characteristics on dry, grippy surfaces mostly.