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Old 08-19-2009, 06:41 PM   #1
subydude
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Default Differentials for an SM/SP car

I have a rather unusual setup on my SM car. It's a non-dccd (read: viscous center) 6-speed with a suretrac front diff and a regular R180 in the back. Recently I've been dealing with more and more wheel lift and general understeer mid-corner/corner exit. Specifically power on understeer. I've got the suspension side down for what to fix, but I was thinking about starting on the differentials as well to get the whole package together.

From my own research:

I've read some conflicting information on what a rear diff will do. Some say if you have a 1.5 clutch pack it will actually hurt mid-corner and corner entry understeer, but help on corner exit. Others say it helps pretty much all the way around compared to the stock diff in terms of understeer in a corner. What are peoples experiences?

Then there's amount of lock. Between the Cusco and the Carbonetics rear diffs that I was looking at it seems the Cusco is changeable between 60%/80%/100% while the Carbonetics is set permanently at 100%. I would think the Cusco would be "better" since it's configurable, but I'm not sure I understand what the percentage of lock up means. Is it the amount of power it transfers between wheels, or does it mean the wheels are completely locked together? Any clarification here would be helpful since the Carbonetics can be had for several hundred cheaper and looks like it will last a lot longer than the Cusco.

For the center diff, there really isn't that many options. I can either keep my stock 50/50 viscous split, go to the Cusco Tarmac 35/65 split, or find a different trans to get DCCD. Some people seem to really like the Tarmac split, while others think it's two RWD biased, especially when paired with a 1.5 way clutch rear diff. Anyone care to comment on which they think would be best?

What could be considered the best front/center/rear setup when dealing with autox in terms of SM/SP?
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Old 08-19-2009, 08:26 PM   #2
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get the 1.5way. the inside rear is, or nearly is, off the ground with lateral acceleration so the corner entry/mid corner understeer "issues" with a 1.5way are hardly noticeable.

the way you adjust "lock up percentage" is by rearranging the clutch plates inside the diff. if all the the plates alternate between driven and driving plates then you get 100%, if you stack a couple of the driven plates together you lose available friction area thus reducing the lock up percentage. Both the cusco and the carbonetic are the same in this regard.

once you get a good rear diff in the reliance on the center diff for power application will diminish. you'll find that putting 50% of your power out through an axle that has <50% of the weight on it will be more then tail happy enough. You'll be fine with the stock 50/50 viscous diff.

there is still the possibility that you could get front inside wheel spin. At that point you would have to decide whether you put in a clutch type center or clutch type front diff.
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Old 08-19-2009, 08:38 PM   #3
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I concur with what griceiv says but I would recommend a full locking rear diff.

I had similar issues with my Evo lifting the inside rear wheel under hard cornering. I have tried everything, stiff swaybars, lose swaybar, changing springs rates on all corners, but nothing prevented the rear wheel lifting until I installed a TRE maxlock diff in the rear.

Once I had that installed, I now have a new problem, too much oversteer with the tendency to spin all 4 wheels exiting corners... It's so much fun my face hurts....

the great part is that I am not lifting the inside rear at all.
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Old 08-19-2009, 09:01 PM   #4
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I concur with what griceiv says but I would recommend a full locking rear diff.
agreed. the only reason to run less the 100% lockup on a 1.5way diff (in our applications) is to reduce the clunking and chatter, AKA daily driver duty.

Or you just get tired of people running up to you in grid to tell you your car sounds broken when you turn. I mean they hear it clunking from 20 feet away, do they think that the driver can't hear it?
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Old 08-19-2009, 10:00 PM   #5
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yeah, there is that... parking lots can be a beotch but it's all good when you have that crap eating grin as you execute that perfect Scandinavian flick around the apex cone..

got an excellent action shot recently...

This photo was taken on my first run of the day, 2nd corner, cold tires...

Follow the skid marks (all 4 of them)


click image for larger pic

Last edited by ChrisW; 08-19-2009 at 10:05 PM.
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Old 08-19-2009, 11:57 PM   #6
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Very good information! Thanks for the explaination about lock griceiv, that really makes sense.

I think overall it's worth it to go for the 100% lock carbonetics and see how it does with my center diff. Plus, I'm already daily driving it without a backseat and tirerack stickers all over it so I don't think it's really going matter if it makes a little clunking noise when turning

But by all means, keep the suggestions flowing in. I'm sure other people have questions about diffs as well! It'd be great to get a compilation going of what differentials people have and how they affected grip/handling in terms of autox.
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Old 08-20-2009, 03:34 PM   #7
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Good info.

I didn't know if you had seen or read through any of this one as well.
http://forums.nasioc.com/forums/show...=differentials

Seems like you have your mind made up however.
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Old 08-20-2009, 06:16 PM   #8
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I did read through it last year when it was being updated, but I hadn't looked at it recently. It's conclusion was basically what I had kinda thought anyway. Clutch rear, helical front with something in the middle. No real decision on what was in the middle though. I do think with the cusco that the car would be to tail happy with the power an SM car should make, so I voted to stick with my viscous 50/50 for now.

Greg, do you have a different rear diff? thought you did, but I wasn't sure.
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Old 05-23-2012, 01:40 PM   #9
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Almost 3 year bump. I did the carbonetic rear diff and it was just about perfect...for a while.

I do have a new question though regarding front diffs. Since my initial post I have added more grip and more tuning to the suspension and while it's getting faster, I was still working around the viscous center diff. This past winter I found an 07 STi trans to put in and now I'm wondering if I should drop it in the way it is, or look into upgrading the front diff while it's out. For a quick reference the 07 trans nets me a helical front (instead of suretrac front), the DCCD center (instead of a viscous center), and a slight gearing change.

My thoughts are the DCCD with it's aftermarket controller might be exactly what the doctor ordered and I don't need to replace the front helical. However, I didn't know if anyone had actually done the front and can say one way or another. I don't know if the Evo guys swap or not either, so Griceiv, if you want to comment please do

The two proposed setups would be:

1. Leave it alone. Helical front, DCCD middle, 1.5 way clutch pack rear.

2. Mess with it. 1 way clutch front, DCCD middle, 1.5 way clutch pack rear.

The carbonetic diff really eats the fluid up, and I don't want to be doing trans fluid changes like I did rear diff changes so I'd probably knock that one off the list which really leaves the Cusco. How much servicing and fluid changes does that require? Can I leave my standard every other year trans change schedule in tact or is there other "care and feeding" I should know about in terms of a clutch pack front diff?
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Old 05-23-2012, 02:21 PM   #10
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Quote:
Originally Posted by subydude View Post
so Griceiv, if you want to comment please do

The two proposed setups would be:

1. Leave it alone. Helical front, DCCD middle, 1.5 way clutch pack rear.

2. Mess with it. 1 way clutch front, DCCD middle, 1.5 way clutch pack rear.
Well i guess since you asked...

A helical diff can still perform similar to an open diff (read: crappy) if you have enough load transfer on the front axle. Which would ask a lot of your center diff but assuming the dccd can keep it locked would result in lots (all?) of your power going to the rear axle.

On the other hand if you put a clutch type diff in the front and you're not getting a ton of load transfer on the front axle then that could cause more corner exit understeer than a helical would depending on how aggressive the clutch diff is setup.
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Old 05-23-2012, 02:33 PM   #11
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Originally Posted by griceiv View Post
Well i guess since you asked...

A helical diff can still perform similar to an open diff (read: crappy) if you have enough load transfer on the front axle. Which would ask a lot of your center diff but assuming the dccd can keep it locked would result in lots (all?) of your power going to the rear axle.

On the other hand if you put a clutch type diff in the front and you're not getting a ton of load transfer on the front axle then that could cause more corner exit understeer than a helical would depending on how aggressive the clutch diff is setup.
I was specifically asking if the Evo guys liked to swap front diffs, and what the difference was in care and feeding, but hey, discussion is good

To your first paragraph, the center diff when locked could only send up to 59% of the power rearward. It should start at 50% and go up to 59% as the diff sensed less and less front traction. Still would happen very quickly. I've read the thoughts in other threads, thought about it myself, and concur that when the front unloads the rear gets a shock of power, during which the front comes down to earth, then as the rear get's it's full compliment of power it loads up, transfers weight to the rear, and unloads the front which starts the process over again. All assuming that the car is able to generate the required lateral acceleration.

To your second paragraph, that's true about the diff lock. So in steady state where the car is unloading the front, the 1 way diff would help keep things rolling forward, but when you go to unwind under power the front would wash out some as the diff tried to keep the front locked together. You'd have to find the proper balance of lock vs rear bias from the DCCD.

To me it sounds like the answer is a clutch pack front since it "solves" the front axle unloading issue, and the understeer on corner exit is solved through other means. All assuming that the car can produce the required lateral acceleration to lift the inside tires, which any decent SM/SP car should be able to do

Should I expect your answer to be just "Yes." For reference, I'm just joking here since I know not all secrets can be shared
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Old 05-23-2012, 02:40 PM   #12
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sounds like you have a good handle on what's going on.
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Old 05-23-2012, 05:57 PM   #13
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Since we're talking diffs...

I've got a 09 WRX that I run in SM with an 05 STI trans (w/DCCDPro) and an open rear R160.

I'm looking into rear diff upgrades, and will get a 1.5 way clutch type, but I'm still figuring out which one. As mentioned above, the Carbonetic requires an 1800mi oil change interval, which is a mark against it, and the Cusco is the priciest option. The one I'm most interested in now is the new Kaaz SuperQ, which uses treated metallic plates, with a longer oil change interval. Its supposed to be quieter and more dd friendly with no break in period, as well.

Any thoughts on diff brands or will they function mostly equivalently?

Last edited by dur4ce; 05-24-2012 at 05:40 PM.
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Old 05-23-2012, 06:03 PM   #14
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Carbonetics or Cusco. The Carbonetics is a 3k mile change and you can feel the fluid going as it get's older. Fresh fluid and it locks a LOT. Old fluid with 5-6 autox events and a few thousand street miles and it feels...good, but not like it used to. The upside to this is if you have an event coming up that you don't want the car to be super tail happy for, don't change the fluid. If you have a big event that you know has tight corners, change the fluid. Just another tuning tool.

I've only driven a Cusco equipped car once and it felt a lot like my Carbonetic diff for that event so not a good sampling. I think if I had it to do over again I would probably have gone with a Cusco, but I found a deal on the Carbonetic from a shop that was going out of business that I couldn't pass up.
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Old 05-23-2012, 06:24 PM   #15
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Any thoughts on diff brands or will they function mostly equivalently?
well they all have slight design differences, but any of them will be orders of magnitude better than an open/viscous rear diff. The main differences are in how the diffs provide preload springing and the friction materials.

I'm not going to run through all the details as they do a pretty good job of explaining them on their own websites but a Cusco RS would be my first choice.
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Old 05-23-2012, 07:12 PM   #16
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^^yea, I have a conception of the design differences between the various brands, but I was hoping for some first hand experiences like subydude provided.

Neither of you mentioned anything about the Kaaz. Is that just because you haven't had any experience with their diffs? Or have you had negative experiences? I saw that pignoseSTI has the Kaaz in his car and seemed happy with it.
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Old 05-23-2012, 07:31 PM   #17
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<--- Love my Carbonetic and they can be found for ~$4-500 cheaper than the cusco. I'm using a redline cocktail in mine and I'd have to do 28 fluid changes to equal $400. I only have about 600 miles on mine with 2 races. I'm planning to drive to lincoln and back next weekend then change the oil again. I changed it twice already while breaking it in.

The carbon discs are suppose to last longer as well in the carbonetic. Who know's if that's really true though. Mine is pretty loud especially after highways miles (to and from events). It completely transformed the lines I take on course and it'll probably take me most of the year to re-learn how to drive the car (plus I took all last year off).

subydude, what oil are you using in yours?


ps
I'm also running a Modena front diff that also totally transformed the car. Love it.
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Old 05-23-2012, 07:55 PM   #18
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Yall are sayin that the carbonetics is cheaper. But Im finding all of them around $1000.
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Old 05-23-2012, 08:03 PM   #19
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Quote:
Originally Posted by dur4ce View Post
^^yea, I have a conception of the design differences between the various brands, but I was hoping for some first hand experiences like subydude provided.

Neither of you mentioned anything about the Kaaz. Is that just because you haven't had any experience with their diffs? Or have you had negative experiences? I saw that pignoseSTI has the Kaaz in his car and seemed happy with it.
No experience = no comment. You should PM pignoseSTI and see if he can join the discussion. That way the good info can be in one spot

Quote:
Originally Posted by xcdhridr View Post
<--- Love my Carbonetic and they can be found for ~$4-500 cheaper than the cusco. I'm using a redline cocktail in mine and I'd have to do 28 fluid changes to equal $400. I only have about 600 miles on mine with 2 races. I'm planning to drive to lincoln and back next weekend then change the oil again. I changed it twice already while breaking it in.

The carbon discs are suppose to last longer as well in the carbonetic. Who know's if that's really true though. Mine is pretty loud especially after highways miles (to and from events). It completely transformed the lines I take on course and it'll probably take me most of the year to re-learn how to drive the car (plus I took all last year off).

subydude, what oil are you using in yours?


ps
I'm also running a Modena front diff that also totally transformed the car. Love it.
I bought the diff oil that Carbonetics sells. Bought something like 6 gallons in bulk. I've used 3 gallons in 2ish years, so the other 3 should last about that long or a little longer now that I'm not using the car for track days any more.

I haven't heard of a modena front diff. Details?
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Old 05-23-2012, 09:00 PM   #20
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Yall are sayin that the carbonetics is cheaper. But Im finding all of them around $1000.

Just found it for under $900 shipped (after coupon).

Here is the site and info for the Modena...

http://www.flatironstuning.com/p-197...02-07-wrx.aspx

I'm fortunate enough to be local to the US distributor and his knowledge about gears and differentials helped me decide what diff I wanted.
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Old 05-23-2012, 09:54 PM   #21
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I really liked my cusco rs 1.5 way as it totally transformed the car from stock, only downside was the sucker blew up. Broke a spider gear. Went with a Kaaz because they are in Socal and have all parts readily available to rebuild when the time comes. Support is great with the Kaaz as well. Put the super Q in and it feels just like my Cusco. Smooth engagement, and it's quieter than the Cusco was. Very happy with it so far
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Old 05-23-2012, 10:07 PM   #22
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Originally Posted by pignoseSTI View Post
I really liked my cusco rs 1.5 way as it totally transformed the car from stock, only downside was the sucker blew up. Broke a spider gear. Went with a Kaaz because they are in Socal and have all parts readily available to rebuild when the time comes. Support is great with the Kaaz as well. Put the super Q in and it feels just like my Cusco. Smooth engagement, and it's quieter than the Cusco was. Very happy with it so far
What's the fluid changes like? Anything like I described above about the Carbonetic? Any special care and feeding needed by either the Cusco or Kaaz?
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Old 05-23-2012, 10:10 PM   #23
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Quote:
Originally Posted by xcdhridr View Post
Just found it for under $900 shipped (after coupon).

Here is the site and info for the Modena...

http://www.flatironstuning.com/p-197...02-07-wrx.aspx

I'm fortunate enough to be local to the US distributor and his knowledge about gears and differentials helped me decide what diff I wanted.
Is that coupon available for others?

So the modena is a torsen.
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Old 05-23-2012, 11:08 PM   #24
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I may be wrong but I found a price difference for a wrx lsd opposed to a sti? Could the $900 lsd be for r160's
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Old 05-23-2012, 11:13 PM   #25
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I may be wrong but I found a price difference for a wrx lsd opposed to a sti? Could the $900 lsd be for r160's
Good point. I looked and found some off of google for $980, so after coupon I could definitely see the R160 flavor being $900.
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