Lets talk about Competition Diffs

[zzyzx]

This is a tricky and difficult topic. Make no mistake about that. Worthy of a dissertation on the subject, no doubt. I'm hesitant to even comment because unless I spend the next two hours filling in the details, I know I'll have left something out.

I currently have a Quaife (torsen) in the front, CUSCO Tarmac 35/65 center (open), and CUSCO 1.5 way (clutch) in the rear.

I've had rear inside wheel lift, front inside wheel lift and bicycled through corners on the power before, depending on class/setup (GS, STS, DSP, XP). You'll note that's my 2.5 RS on two wheels in Pat's Pig from 2004 Nats (in DSP trim...).

I think one important thought to keep in mind that you can't generalize about setup... we're talking about autocrossing an Impreza. Depending on your class, there are setup situations you'll have to deal with that simply don't apply to other venues.

If you have any Qs, just ask and I'll try to address them.

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[angryfist]

steve,
you running the cusco RS or MZ?

btw. i tried getting a hold of you a few times earlier this season with no luck. shoot me an email sometime.

[solo-x]

Originally Posted by angryfist

nate, maybe you can answer this one...

so for a clutch type rear diff, do you think a higher or lower initial torque on the clutches would be better for autox? specifically the design difference btwn the Cusco RS and MZ type diffs.

I don't have "the" answer to that, but I can offer my thoughts. I assume you are referring to pre-load or break away torque in the diff, right? This would be the percentage of lockup applied before the ramps/ramp angles come into play. Assuming we are talking about the same thing, I high initial torque/preload would tend to increase understeer on corner entry. For Solo, I believe this to be undesirable. Therefore I'd lean towards a diff with a lower initial lockup. However, a higher initial torque would tend to speed the reaction of the diff., so you may find that you have to balance the compromise of corner entry balance with diff response time. At that point, it's likely an exercise in splitting hairs, with the more "damaging" penalty being the increased likelihood of corner entry push.

[bucket7788]

Originally Posted by Mechie3

Front right is severely compressed. The gap on the wheel is about 2 fingers or so (14" hub to fender). In that pic the wheel is probably a good inch up into the fender by that point. I only have street coils on.

The fact that the left side of your car is off of the ground is a major factor there. The effect is a major loss of dynamic camber and some caster as well.

I vote for stiffer springs.

[Storm]

^^^ I would go with a slightly higher preload....but I'm not Nate. It seems to be especially helpful in slaloms, where the car is in a constant state of directional change. The only time I see it to be less than optimal, is upon entry to slow speed turnarounds. That said, slow down enough to get the front end to bite, then add power through to the exit.

Jay Storm
hiding a bunch of bad habits with bigazz Hoo$iers!!!

[Mechie3]

My plan for next year is stiffer springs for the coils, hopefully diffs, and with any luck some 275 R-comps (need the stiffer springs for those anyways). I have increased preload a bit from when that pic was taken. I'll be keeping tabs on this thread to see what kind of diffs would suit me best. I'm interested to see how much stiffer springs lessen my need for diffs and how the overall characteristics of the car change.

Thanks again for helping me with the timing belt at the FLR prosolo btw Jay. !

[DS1]

You guys forgot the DCCD center diff also came in the 5mt variety

On our 5mt we run the STi helical up front / DCCD center / STi plated rear

A question for the guys having tire lift issues: What size sway bars are you using?

The difference between the Cusco RS and MZ:

RS:
Low power loss means better lap times for even the mild tuned of cars
Chattering is kept low
Improved durability as clutch plates are low pressured
The LSD efficiency is higher with smoother oil flow between plates.
The Benfits

According to CUSCO'S in house research, initial torque is 50 to 70% lower compared to the conventional cone plate type LSD enabling lower drive friction and better response.

The Type RS's Durability

Special precision springs, set in the pressure plate, assure steady coil movement
Clutch plates do not stick together like convetional cone plate type LSDs. Therefore less clutch plate wear or deterioration occurs.
The Benefits

MZ:

Featuring high tension chrome molybdenium steel housing and gears the Cusco LSD is up for the task of serious competition. With large oil windows for increased flow and Cusco's original groove cut technology for improved efficiency on the clutch plates, stability and longevity is promoted. ----- The 1.5-way Type MZ utilizes cone plates to develop the initial torque and features full engagement during acceleration and partially under deceleration.

Hope that helps

-Dylan @ DS1

[zzyzx]

Originally Posted by angryfist

steve,
you running the cusco RS or MZ?

RS.

That info posted above doesn't really help point out the differences, and I'm not sure what those differences are in either intended application or design.

[DS1]

The RS is sprung softer so the lock up is slower making it a more "streetable" lsd over the MZ.

[zzyzx]

So, the only difference is the springs, meaning somebody could swap out those and go from an RS to an MZ?

[DS1]

From what I took from the documentation I have, yes if you changed out the springs you can get the RS to lock up just like the MZ.

[angryfist]

Originally Posted by DS1 Motorsports

From what I took from the documentation I have, yes if you changed out the springs you can get the RS to lock up just like the MZ.

I read that same comparison you listed but it didn't really tell me that much. Reads more like marketing mumbo-jumbo to me...

The documentation I have makes it look like you cannot change from a RS to MZ very easily (if at all possible). Its not just replacing springs. The MZ uses a cone plate and not springs to cause a preload on the clutches.

[ArcticWolf]

Originally Posted by DS1 Motorsports

You guys forgot the DCCD center diff also came in the 5mt variety

On our 5mt we run the STi helical up front / DCCD center / STi plated rear

Is that for the older style 5MT only, or does it worth with both flavors (that I know of, anyway) of 5MT?

I seem to remember the 12kg center diff being for the older 5MT only, and unable to be used on a newer model one.

[grippgoat]

What's the difference between a Cusco 35/65 tarmac, and an 05 STI DCCD set to "full rear" (which I understand to mean fully open)? I'm guessing the cusco is probably lighter.

-Mike

[subydude]

There's also the JDM Ver7 6-speed DCCD that had an open setting of 45/55. IMHO that would be just about the right amount of torque to the rear without making it tail happy.

That + 1.5 clutch rear and 1.0 way in the front would be about perfect.

[Homemade WRX]

Originally Posted by subydude

There's also the JDM Ver7 6-speed DCCD that had an open setting of 45/55. IMHO that would be just about the right amount of torque to the rear without making it tail happy.

sorry to get a bit off topic but where did you hear that and do you have a tranny code for that?

[CatfaceType-R]

Originally Posted by Splash

Heh, I have always found it ironic that I can strand my AWD car simply by lifting a single wheel off the ground... <grin>

To contribute, your suspension will have a lot to do with your choices. If you ever get far enough that you lift a front wheel, the front helical will not like it one bit.

The stock STi clutch-type rear diff isn't strong enough to keep the car moving forward if you lift an inside rear and spin it. This makes the clutch ring re-ordering necessary, if not the pressure ring kit as well.

Suretracs act like helicals. Lift a wheel, it spins.

I'm not impressed with DCCD. Really, it's just a clutch. The fact that it tries to change things in mid corner for me doesn't help. The fact that the brake light deactivates it and leaves it fully open doesn't help either. In fact, I can't figure out why, when you do lift an inside rear wheel and it spins up wildly, why that doesn't trigger the DCCD to SEND MORE TO THE FRONT! When the car does that, it feels like a Mustang with too much rear bar.

Personally, I'd find a center that had some rear bias (but not much) and also didn't do anything automatically. Then I'd use a 1.0-way front clutch type and a 1.5-way rear clutch type diffs. I keep the front a bit on the loose side, wind the rear pretty tight.

interesting stuff, I agree especially with the last point; about the mustang'ish feeling...great comparison

[subydude]

Originally Posted by Homemade WRX

sorry to get a bit off topic but where did you hear that and do you have a tranny code for that?

Tranny code is: TY856WB1BA. It came in the 2001 JDM STi RA and then again in the 2002 JDM STi RA Spec-C.

This thread contains all the information you'll want to know about the 6-speed as it came from the factory and what type of diffs they have.
http://forums.nasioc.com/forums/show...5#post17412995

Personally I think that dccd would be about perfect. Very little difference between lock and open.

[Splash]

I dunno about the others with inside rear wheel lift problems, but I had a WL 24mm rear bar and various front bars from WL 24mm to WL 27mm to Strano 32mm hollow. I never tried the Niles 32mm solid because the Strano was already too big.

Anyway, on ST tires, the lift didn't stop unless I upped the front bar to 27mm on the inside hole and the rear set in the middle hole. I can also do it with the 27mm on the softest hole and the rear on the softest hole. I'd lie to go softer on the front bar to get my front grip back, but I think I'll have to go WAY softer on the rear bar to do it, which doesn't help rotation. In addition, larger rear bars also keep the inside rear up, no matter how much stroke the dampers have. You end up with killer rear springs instead of a big rear bar.

Where the DCCD on AUTO screws with me, is that I get 2 handling modes. 1 when I am on no or little boost, and another when it hits like a hammer. Just when I either get it balanced or have managed the mid-corner push, the boost comes in and it suddenly goes full-on dorifto-mode. I would have thought the DCCD would sense the rear slip and put more voltage to the clutch, but it either doesn't, or it's not working.

Either way, at this point, I'd rather have ONE setting and let me work around it. Right now, it like shooting at a moving target.

[zzyzx]

On the topic of wheel lift...

First, I'll define the problem of wheel lift as: one or more wheels off the ground long enough to affect the performance during an autox run. Either by loss of power, or otherwise.

Ok, now that we've defined the problem...

I adhere to the school of stiff springs and small bars. This wasn't an a priori position on how to make a Subaru handle. It's from real world experience.

Given the option, you're better off with more spring and less bar on our Subarus. In stock class, this isn't the case. The band-aid solution is a ridiculously large front bar.

For all other classes, you have the options you need to provide a more reasonable solution to the problem.

I can say that if wheel lift in an ST* class is a problem for you, a solution exists that you just haven't figured out yet. In the ST* classes if you're setup is working correctly, you could possibly get some wheel lift, but the type a quick photographer catches, not the type that loses races.

As grip levels increase, it gets harder to keep all four on the ground. Physics is against us. We race a nose heavy econobox unibody with a (relatively) short wheelbase. The best education on this is to watch FWD E Prepared cars race around on sticky concrete at Nationals. Or conversely, watch BMWs in SM or DSP. Different set of wheels, but same problem.

Bottom line, on R-comps, keeping all four on the ground so that it doesn't impact performance can be problematic.

There are two difference things to keep in mind here:

1) Body roll.
2) Jacking forces

Most people overlook #2. Infinitely stiff springs do not alter the laws of physics: lateral Gs are lateral Gs and that force doesn't care how stiff your springs are. That force acts against the CoG of your chassis and will lift the inside of your car.

When I ran on bias ply race slicks in XP, it was all too trivial to put my 2.5 RS up on two wheels. It was, honestly, not fun.

The important lesson here to remember: the higher your CoG is, the more likely the lateral forces will act on your CoG to lift the inside.

This is why the 275/35-15 has been somewhat of a God send for the DSP Imprezas. It not only reduced CoG for a given "geometric" ride height, but provided more than enough tire/fender clearance.

The alternative of course is to lower your ride height. But how low?

One thing to consider: if your CoG is coincident with your roll axis, then body roll and inside "lift" is minimized. This is a good thing. As your CoG and roll axis locations diverge, the "moment arm" created makes it easier for the lateral forces to "lift" your chassis. This is not a good thing. This is a technical topic and suffice to say there's a lot of info about it in various places.

If I had any rule of thumbs here, I'd say:

1) Get your CoG as low as possible.

2) Don't sacrifice your suspension geometry to the extent that it is, itself, contributing to the problem.

3) Try to do #1 and #2 at the same time.

Now back to differentials... Realize that any amount of problematic wheel lift, no matter what diffs you're using, will impact performance. Only a locked differential (which isn't really a differential...) doesn't care if a wheel is in the air. Unfortunately locked differentials, grippy concrete, grippy tires, and turns aren't particularly compatible.

[Storm]

I agree with that. Just running on fresh 13" race rubber dropped the CG enough that the car never took on much roll at all, let alone lift the inside wheels. The grip was there, but it just wouldn't roll. Now that we're on 275-15's, the car is slightly higher and I have noticed a little bit of lift at corner entry, but it settles down soon enough that forward power is not effected. Having the clutch type rear LSD in the back will still give me some forward thrust to keep the corner speeds up, but having the wheel on the ground fells much better!

Jay

[Scooby South]

Great Info guys...thanks..
So from what I am gathering....you want a Rear LSD thats most likely a 1.5 Clutch type...

the center is still in debate...but it looks like the Cusco 65/35 is the ticket

and for the front...the helical should be fine....but maybe a 1.0 Clutch type also...

so on Decel...the front is open..???? Accel its closed...so to speak...

Is there any drawbacks to handling with this type of setup...Is it going to push???? so then your into your spring rates to try and get sorted??? correct...

sorry just trying to understand..

Bill

[Homemade WRX]

Originally Posted by subydude

Tranny code is: TY856WB1BA. It came in the 2001 JDM STi RA and then again in the 2002 JDM STi RA Spec-C.

This thread contains all the information you'll want to know about the 6-speed as it came from the factory and what type of diffs they have.
http://forums.nasioc.com/forums/show...5#post17412995

Personally I think that dccd would be about perfect. Very little difference between lock and open.

Thanks

Originally Posted by Scooby South

so on Decel...the front is open..???? Accel its closed...so to speak...

for 1.0, essentially...and for a torsen its a bit different but they are very neutral without load on them (but stay "locked" under decel), hence when they loose load (pick up a wheel) they become like an open diff....or so is my understanding and experience running them. In a front diff on an awd car, they are smooth under turn in...on the rear diff of a formula car, it will lock both wheels when engine braking or using a diff mounted brake (diff distributed brake torque)

Originally Posted by zzyzx

On the topic of wheel lift...
Most people overlook #2. Infinitely stiff springs do not alter the laws of physics: lateral Gs are lateral Gs and that force doesn't care how stiff your springs are. That force acts against the CoG of your chassis and will lift the inside of your car

THANK YOU!! so many people don't understand that a stiff spring doesn't change the force acting on the car. It does however require more force to cause the same degree of pitch by reducing weight transfer to the outside wheel via body roll/change in CG.

[Homemade WRX]

oops...ignore

[speedyHAM]

I had a plan pretty much set in my head for my tranny build over the winter before this thread started. I questioned it a little bit due to the wisdom shared here- but now I am back where I started and more confident about my plan.

I am going with a torsen up front, a Cusco 35/65 center, and a 2 way clutch rear. Now I just have to figure out which torsen and which clutch.

Thanks everyone.