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Old 05-02-2000, 07:00 PM   #1
Joel Gat, 1.8L
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Arrow Center diff options

Hello,

I'm not sure the tech forum is an appropriate place, but the other forums don't seem any better.

I'm still debating replacing a blown center diff or just replacing the entire tranny. What options do I have out there, and at what prices, for replacing the center diff.

I know Axle motorsports has the STi heavy duty viscous diff at about $550 plus shipping, new. KAAZ makes a center diff, but they want $1360 for it. Any other thoughts? I'm assuming that the diffs are exchangeable with the non-turbo tranny, and I'm willing to try putting my money where my assumption is...

Lemme know if you know of anything...

Joel
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Old 05-02-2000, 07:49 PM   #2
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Bugetwise, maybe its better to get a used trans or something. I just think why get the high performance ones if you do not have the power to use them (no offence) I had a Sti center in my RS before I did the motor swap and it was kind of useless. The car had alot of traction but no power to pust it.
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Old 05-02-2000, 08:04 PM   #3
Joel Gat, 1.8L
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Hello,

Ver. III, I'm thinking along the same lines as you. However, the STi center diff is $550. I have a '99 tranny (that's the tranny that I blew the center diff on) and a new junk tranny runs about $750. I know time is money, but even a lawyer can get a lot done on the car in the time it takes to make $200

It's actually not looking like a lot of work since the back portion of the tranny comes off - I'd need to disconnect the driveshaft, pull off the shift linkage, pull out the 6-10 bolts that hold the tranny's butt on, and pull out the center diff... Seems like a single-day job (accounting for difficulties which always arise). Okay, so that's more than the $200 difference, but hey! Installing a tranny would probably take just as long, if not longer!

JYD - still undecided.
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Old 05-02-2000, 08:10 PM   #4
Ver.III
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Well, when I had my 99RS tranny apart. The center diff was a different style to the one I got from Japan. There are different ones for different years. I ended up using a 94 Legacy style tranny because I then found out that the motor I was getting did not fit the 99 RS tranny.Sh#t!! But it all worked out in the end.
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Old 05-02-2000, 08:17 PM   #5
Joel Gat, 1.8L
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Hello,

Hmm, the '99 tranny should fit any motor... The only problem is the 4 bolts versus 7 bolts, but what the hey! I have three bolt holes unused (99 tranny on '95 engine) and I had to use a drill and a sunken bolt to hold the starter motor on. But aside from that, they were compatible.

As for the center diff differences, do you know what year center diff you got? I assume that the center diffs are like the trannies themselves, so a pre-99 tranny needs a pre-99 center and a post-99 needs a post-99. If that's the case, the STi model to use would be the v.5 center diff or the equivilent. But do you know of any other differences?

JYD - interest piqued
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Old 05-02-2000, 08:27 PM   #6
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The center diff I got was a 20kg Sti. As far as I know, all of the Sti motors use the 4 bolt trans with the pull style clutch. At least up to a Ver5. The NA Legacy Turbo uses the same style. I bought a used trans for $150 and swapped the guts with ratios from a 96 Sti RA (close ratio). As for the center diff. But if i can remember,the base model versions (Jap spec) use different centers which is simular to the RS. I have to look at the Jap parts book I have. But I think its still 60/40 split.
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Old 05-02-2000, 08:33 PM   #7
Joel Gat, 1.8L
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Hello,

1999 was the year Subaru moved to the new style trannys. Although I have not held a v5 tranny in my hand (so I could be wrong), Brett at MRT says the '99 v5 trannies are the newer style. The 4-bolt pattern that you found is the exact same 4-bolt pattern that 1993-1998 US imprezas, from the 1.8 to the 2.5 all have. It's also the same pattern as the 2.2 turbo legacy engine and the EJ20s upto 1999.

It's the internals that I don't know much about. Did you swap the internals or have someone do it for you? I must admit, when I took apart my FWD tranny (exact same as AWD, but missing some rearward parts), I was daunted by the sheer complexity of it all. If you did it, describe the process. How much time, how much experience do you have, how many bloody knuckles, etc...

Thanks!

Joel
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Old 05-02-2000, 08:47 PM   #8
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Well, the Ver5 that I seen has the exact trans that I had,4-bolt pull style. The only way I could tell it was a Ver5 was it has the same intake identical to the 99RS with the coil pack off to one side of the top of the intake. Of course the other things are red intake and silver intercooler with the Sti badge on it.
I did the tranny myself. Yes it is scary initally looking at it when you split the casing but it was not as hard as I thought after looking how the shifting works and etc.... Most of the differences between trannys were different ratios of course but different style shift hubs and bigger syncros but that was about it. The basic layout is still the same. The key thing to remember was making sure that I knew where everything went when re-assembling.
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Old 05-03-2000, 03:26 AM   #9
patr
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Hey guys

(how come its always the same people talking tranny ?! :-)

ANyhow, the guts of all of them are interchangeable, I believe, with the exception of the shifter fork on the WRX/turbo Legacy vs. the RS, and the tranny housings on the 99+ versus the 93-98 tranny housings.

In other words, you can take the guts of anything and pretty much put it anywhere as long as the ratios match and the clutch type matches, and the housing of the 99+ is different, but can be adapted (Joel sez it is easy, and I have done this both ways myself but it wasn't "clean"). Inside the trannies actually I thought was less complex, you can just pull gearsets out of almost any box and just shove them in !!

On the center diff options, I have seen an RS viscous, STi viscous, STi adjustable center (dunno if this is an electronic clutch pack or viscous or what), CUSCO mechanical, KAZZ mechanical.

I wouldn't mind hearing what else there is as mine will eventually be toast so I wanna know what is out there.
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Old 05-03-2000, 07:16 AM   #10
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Joel's right, center diffs are different pre-99 and 99-on. Front diffs are the same for all models, though you'll need to swap over to different style axle stubs if you go with something like a KAAZ. STi makes different viscous center diffs or a locked center diff (which might be the inexpensive one you're looking at Joel??). KAAZ makes mechanical centers but they are more expensive than their front or rears. Cusco also has many opinions including their new Tarmac gear which has a 35F:65R split.

There is a good write up in the Feb/March issue of Fast Fours that involves installing front and rear KAAZ diffs on a MY99 WRX you might be interested in. Great picture of a the transmission internals as well.

Or if you're interested Joel, send me your tranny and I'll convert you to full time FWD or full time RWD so you don't have to worry about those pesky center diffs anymore. :-) I'm sure that's just what you'll want after going through the trouble of getting the AWD drivetrain. How did you blow the center anyway???? Did you overspeed it by trying to run it on a 2WD dyno??

Trey
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Old 05-03-2000, 07:43 AM   #11
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Sorry no answers, but I do have a few, slightly off topic, questions.

Does anyone know if there are significant efficiency differences between the different center diff options mentioned above?

Most of us have seen the numbers in SCC where only about 96hp of the MY99 RS's 165 peak hp is getting to the ground. That's a loss of almost 70hp. Not too efficient. I recall reading once that the Audi quattro system is very efficient, something in the ballpark of only a 10% loss (don't quote me on that, it's been sometime since I recall reading about it and I have never seen any AWD Dyno numbers to back it up).

Now the quattro system (in most Audii) uses a Torsen center differential. Torsen differential are suppose to be very efficient. I wonder how the different types of center differentials stack up in this regard? What about the Impreza's with automatics, how efficient is their electronically controlled clutch pack center diffs?

Image if we could get even half of that loss back through drivetrain efficiency improvements. That would be about 35hp. Thats a lot more than a lot of other mods.

Remember, I'm just speculating here. I just wanted to stir up some discussion in this regard. Most topics related to power are on how to get more power out of the engine. I just think a little more board space could be used to weigh the possibilities of getting more of the power thats already there to the ground.
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Old 05-03-2000, 09:14 AM   #12
Joel Gat, 1.8L
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Cool

Hello,

First, about the drivetrain loss tangent. SCC's dyno figures do not represent drivetrain losses alone. They also represent AWD Dyno losses. The AWD dyno is known to give low hp figures. It's really only useful to compare to other runs on the same type of dyno - not for comparing to 2WD dynos.

Trey, nice to see that your shop is up and running now. You should have made some more visible announcement, eh? With the grand opening a week before the June 10-11 Buttonwillow event, I'm not going to be able to convince the fiancee to let me cruise out there

Trey, what can you tell me about the Cusco "many options"? The STi center diff that I mentioned, sold by Axle Motorsports is the 20kg/100rpm normal hd STi center diff at about US$550.

As for how I burned my center diff... I'm embarrassed to say that the '99RS center diff is very very very durable. It ran for about 29 minutes at 130 mph with a 4.11 front diff and a 4.44 rear diff. Minute 30 caused the viscous fluid to completely overheat, lock up, and give me the most exciting wheel-hopping-driveline-bouncing-wheels-slipping-car-screaming-(seriously, my ears hurt for hours) terror I've ever experienced - and at 130 mph, nonetheless.

How'd I make that mistake? By being a cheap bastard and using junkyard parts. I havent completed dissassembly of the rear diff, but it looks like the rear diff had some major damage to it internally and spinning the axles was showing me 4.25:1 turns. Since the junkyard guys said t came off the same car as the tranny (4.11), I figured something wierd was happening and didn't think anything of it. Oops. The wierd was not a 4.11 acting strang, but a 4.44 acting very strang.

Whee! So now, what are my options? What are the various cusco diffs? What are the prices of the various options? Anyone? Advantages, disadvantages, personal experiences?

Joel - losing interest in being the JYD
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Old 05-03-2000, 10:26 AM   #13
Trey
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Regarding AWD Dyno results, each dyno will give you various results. The purpose behind dynos is to see if changes IMPROVE/HURT your results though many people try to equate the power the car makes on a dyno to the power it's actually making on the road. If you take HP@wheels numbers from a Dyno Dynamics dyno for instance (popular in Oz), you'd assume the Subaru loses nearly 40% of it's power in the drivetrain. If that were true, we'd have a very hot drivetrain on our hands as the power (heat) has to go somewhere. What's actually happening is that the Dyno Dynamics dynos just read lower (about 15% lower than Dynojet for example). The Clayton dynos (which UPRD has) read even lower than the DD units. They are an emissions dyno and is far from being the most accurate thing to determine actual power to the ground. It is a measuring tool for testing emissions and driving problems, not giving you bragging right numbers that the Dynojet dyno is known for. The Subaru AWD system is actually very efficent compared to other designs currently used. This is a whole other topic by my estimates are 15-20% drivetrain loses with the Subaru AWD system.

As for Cusco center diffs, they have the MZ 1-way unit which is designed for rally and dirt use. It's a mechanical diff which will engage under acceleration and be a bit more abrupt compared to the viscous. They also have their 35:65 tarmac gear which is nice but it is a locked diff so day to day street driving might get rough. That's basically it actually, I thought they had more. They make 3-4 different front and rear diffs as well. As for prices, Cusco is usually more than KAAZ. I can give you exact prices if you contact me directly.

I've used KAAZ and are extremely happy with their units. They are the #1 LSD manufacture in Japan and the quality is top notch. You have to use their oil though which is expensive. If you use regular gear oil (w/ additive), you get ALOT of noise. Even with the oil they are more noisy than the Viscous but they work real well and are 100% rebuildable.

And we'll have more Subaru meets in the future so I'm sure there'll be one we can plan that you'll be able to make it out to.

Cheers,
Trey@CobbTuning.com
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Old 05-03-2000, 10:59 AM   #14
patr
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I am using the KAZZ rear and looking into the options for the front/centre. The nice thing about the KAZZ mechanicals is that you can play with the lockup, by changing the clutch plates. I've been using regular oil with the additive too and it doesn't actually seem noisy. Just really grabby on tight turns on the street (it is set to 100%). I hear that the hot setup is 100% rear and 50-60% front with a 'standard' center (whatever that means).

Where are you getting your info on the Cusco diffs ? Are they adjustable mech. with clutch packs too ?

Does anyone know how the RS center diff differs (hee hee) from the WRX, WRX STi, and RA center diffs ?
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Old 05-03-2000, 11:18 AM   #15
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Couple things I want to throw out there. 1. I have a stock '98 2.5RS tranny @15k miles works great, and is super clean - make offer. I'm not posting this in the classifieds yet, due to the nature of the conversation, and the members involved. Racers need gearboxes, plain and simple. Right now I need money more than gearboxes.

2. Mike Shields mentioned that Kaaz has, or will soon have available a complete hardened gear set (incls all the bells & whistles you need for it to work), that will accompany their fine diffs nicely. I think it will be around $1800, but if you wanna play, you're gonna pay. I plan on rebuilding my '96 WRX transmission with this gear set as well as changing the diff. I figure since it's coming apart, might as well put the good stuff in right? Take it easy my Subaru brethren! Later from L.A. - Arik
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Old 05-03-2000, 11:46 AM   #16
Joel Gat, 1.8L
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Hello,

Pat, by 'standard' setup, I believe the reference is to using a viscous center. I've heard many people say that 3 mechanical lsds makes a ****ty ride in all but the most powerful race-only cars. By keeping the center viscous, you can at least make turns and stuff at slow speed without serious wheel hop and binding.

So we're at, for center diffs:

1. stock unit (what viscous rating?)
2. STi HD unit (20kg/100rpm)
3. STi non-R/RA unit (12kg/100rpm)
4. KAAZ 1.5-way mechanical
5. Cusco 1-way
6. Cusco 35:65 (2-way, I'd guess)

BTW, my understanding of LSDs is that 1-way means the mechanical LSD engages upon acceleration only (open diff otherwise), 1.5-way means LSD for accel. and some limitation on slipping off accel., 2-way is always limiting slip. The Subaru viscous units are basically open diffs that have a viscous coupling that heats up and locks whenever the open diff is spinning at different speeds. The rating is how much lockup you get per 100 rpm of speed difference from front to rear.

Anything else? Any other options? Am I right, above?

Joel
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Old 05-03-2000, 11:54 PM   #17
patr
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hey Arik, email me about the box. I sent you (an unrelated) msg earlier today.

As far as the centers go, I like the sound of the RA viscous. Has anyone heard of these blowing themselves when driven hard ? this isn't the ricey adjustable one, or is it ?

-Pat
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Old 05-04-2000, 12:36 AM   #18
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Ok anyone know if these will work in an early Legacy tranny? I ask because from the Chiltons 85-92 manual the Old fulltime 4wd uses the same case as the early legacy AWD. Therefore I could replace the difflock with a center LSD of some kind.
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Old 05-04-2000, 07:43 PM   #19
Ver.III
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Matt,
I have a gearset ratio from a 96 Wrx Sti RA. The actual gearset is from Possum Borne Motorsports in New Zealand. It is a hardened set and alot of Grp N Imprezas use them. It is very expensive but I bought it used. I am running a Sti 20kg center diff and a Sti rear LSD. For the clutch, I'm using a Helix race 4-puck race. This is also used alot in Grp N. As for the prices....don't ask!!!!

Patr,
The Sti center diffs have a 12kg or a 20kg like the one I have. The RA is the adjustable.
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Old 05-05-2000, 12:48 AM   #20
MattC
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Ver.III,
Where did you get your gearset from, and how much did it cost. Also, what clutch are you using, how much did it cost, and where did you get it from?

TIA,

MattC
Minnam@9psi
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Old 05-05-2000, 02:48 PM   #21
MattC
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Is there a number I can call to reach Possume Borne, or a website. I am not just fiddling with prices, I need a new tranny. Some how my first gear started making noises. It is getting worse and worse. Of course this has happened a week after I put the turbo in So, I have decided to replace them with parts that can withstand the abuse I am going to put on them. If I replace the gearset do I have to use an STi type clutch(pull)? I hope not, because I just bought an ACT.


Thanks,

MattC
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Old 05-05-2000, 03:47 PM   #22
Joel Gat, 1.8L
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Hello,

With a turbo tranny case, you have no choice. You have to use a turbo clutch - the fork is all wrong otherwise. I dunno what happens if you swap a gearset, though...

JYD - looking for a new clutch
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Old 05-05-2000, 06:05 PM   #23
Ver.III
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Matt,
I think without having to change clutches.....etc. Maybe getting your present tranny rebuilt would be cheaper. Or alternatively, how about getting the guts out of a 94 Legacy Turbo?.............the website is www.possumbourne.co.nz . Like I said, its expensive. The gears they have are close ratio sets and I am not sure if they have regualr "stock" gearsets.
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Old 05-06-2000, 04:40 AM   #24
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Umm, Joel, this isn't really relavant to the topic, but how did you get a 1.8L Impreza to do 130 mph, and where were you able to sustain this for 30 minutes? Or am I reading your post incorrectly?
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Old 05-08-2000, 03:06 PM   #25
Joel Gat, 1.8L
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Hello,

The 1.8 is just as fast as the 2.5 - given I guess better aerodynamics or something, because last time I checked, my car tops out at an indicated 13x mph (don't remember exactly what it was), which corresponded to 130 mph according to the tach on the 3 or 4 RSs with me. We were all within 1 or 2 mph of 130. The eclipse with us indicated 130ish, too (though he was not topped out - in fact, he used his cruise control to laugh at us as he pulled away with the accel feature

As for where I could sustain such a speed... Umm, at a closed circuit, safe track. Yeah. That's the ticket. Uh huh.

Joel
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