replacing stock WRX limited slip (center diff)
01-06-2004, 07:31 AM
#16
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Join Date: Feb 2003
Location: 12.9 / 105+
Posts: 488
Car Info: black my03 5mt wrx s/w
take out the VC unit on the stock wrx center diff and you get a straight 50:50 split. dry pavement, hit the gas, the torque is evenly distributed. the turning force transmitted to the front wheels will be exactly equal to that at the back.
what happens when we put the front tires on ice?
same thing. how much torque can the front wheels handle now? a very small amount before breaking loose. this same very small amount of torque will be transmitted to the rear wheels. this will result in you barely moving.
if you produce more torque with the engine and the front wheels break loose, then the two halves of the differential will experience a speed difference, since the rear wheels are not spinning.
now, the VC in the center diff. normally takes care of this circumstance. as soon as the relative speeds of the front and rear driveshafts differ, the VC starts to clamp up. the differential is no longer an open diff, and as a result a different torque distribution occcurs... more torque will actually go to the rear, since it is not slipping. the car will move.
ok, how does the open cusco split torque differently front and rear?
it's all in the gearing. in our previous open diff case, we had equal size gears on the front and rear propellor shafts. thus, they received equal torque from the spider gears in the differential carrier. what happens if we make one gear larger than the other? the torque will be unequally split, according to the mechanical gear ratio.
now, the problem with the open cusco and an open front arises when one front wheel has no traction. any fraction or multiple of zero is zero, so you will have zero torque going to the rear. the one front tire will spin mightily and the car will go nowhere.
i hope this helps explain things a bit better. let me know if you have additional questions.
ken
what happens when we put the front tires on ice?
same thing. how much torque can the front wheels handle now? a very small amount before breaking loose. this same very small amount of torque will be transmitted to the rear wheels. this will result in you barely moving.
if you produce more torque with the engine and the front wheels break loose, then the two halves of the differential will experience a speed difference, since the rear wheels are not spinning.
now, the VC in the center diff. normally takes care of this circumstance. as soon as the relative speeds of the front and rear driveshafts differ, the VC starts to clamp up. the differential is no longer an open diff, and as a result a different torque distribution occcurs... more torque will actually go to the rear, since it is not slipping. the car will move.
ok, how does the open cusco split torque differently front and rear?
it's all in the gearing. in our previous open diff case, we had equal size gears on the front and rear propellor shafts. thus, they received equal torque from the spider gears in the differential carrier. what happens if we make one gear larger than the other? the torque will be unequally split, according to the mechanical gear ratio.
now, the problem with the open cusco and an open front arises when one front wheel has no traction. any fraction or multiple of zero is zero, so you will have zero torque going to the rear. the one front tire will spin mightily and the car will go nowhere.
i hope this helps explain things a bit better. let me know if you have additional questions.
ken
09-25-2012, 03:38 PM
#18
Registered User
Join Date: May 2011
Location: Bay Area, CA
Posts: 122
Car Info: 2003 Aspen White Subaru Impreza WRX Sedan
Revival from the dead!!
This is the only thread that I have found so far that addresses this issue... There is currently a guy on my local craigslist that is selling an STi center diff. I have a 2003 WRX sedan that I currently have a lift and off-road tires installed. It sees dirt, mud and snow more often than most sedans but It also commutes about 30 miles on the freeway daily. I have swift sport lowering springs and street tires/wheels that I will be installing soon for my commute but the off-road package comes back for road trips. I am always looking for projects and simple MODs. 
He mentioned that it would bring a WRX to STi specs. He does not mention what year it came off of and is selling this unit for $450 as he bought it for a spare on his autocross vehicle for $1500 and will no longer be autocrossing his car so is selling it now. First off:
-Is it true that using this center diff would bring the diff gearing to STi spec?
-What are the benefits to having this on a WRX?
-What else is required (besides diff) to to an exchange from the stock WRX to an STi center diff?
-Is this diff the one that is controlled by the DCCD switch on STis and do I need that switch/harness to make it work?
-After all is said and done, is it actually worth doing on a car with my setup?
He mentioned that it would bring a WRX to STi specs. He does not mention what year it came off of and is selling this unit for $450 as he bought it for a spare on his autocross vehicle for $1500 and will no longer be autocrossing his car so is selling it now. First off:-Is it true that using this center diff would bring the diff gearing to STi spec?
-What are the benefits to having this on a WRX?
-What else is required (besides diff) to to an exchange from the stock WRX to an STi center diff?
-Is this diff the one that is controlled by the DCCD switch on STis and do I need that switch/harness to make it work?
-After all is said and done, is it actually worth doing on a car with my setup?
09-25-2012, 04:11 PM
#20
Registered User
iTrader: (17)
Join Date: Oct 2008
Location: Bay Area
Posts: 2,178
Car Info: 07lmtd 6spd wrx eq tuned
This is the only thread that I have found so far that addresses this issue... There is currently a guy on my local craigslist that is selling an STi center diff. I have a 2003 WRX sedan that I currently have a lift and off-road tires installed. It sees dirt, mud and snow more often than most sedans but It also commutes about 30 miles on the freeway daily. I have swift sport lowering springs and street tires/wheels that I will be installing soon for my commute but the off-road package comes back for road trips. I am always looking for projects and simple MODs. He mentioned that it would bring a WRX to STi specs. He does not mention what year it came off of and is selling this unit for $450 as he bought it for a spare on his autocross vehicle for $1500 and will no longer be autocrossing his car so is selling it now. First off:
-Is it true that using this center diff would bring the diff gearing to STi spec?
-What are the benefits to having this on a WRX?
-What else is required (besides diff) to to an exchange from the stock WRX to an STi center diff?
-Is this diff the one that is controlled by the DCCD switch on STis and do I need that switch/harness to make it work?
-After all is said and done, is it actually worth doing on a car with my setup?
He mentioned that it would bring a WRX to STi specs. He does not mention what year it came off of and is selling this unit for $450 as he bought it for a spare on his autocross vehicle for $1500 and will no longer be autocrossing his car so is selling it now. First off:-Is it true that using this center diff would bring the diff gearing to STi spec?
-What are the benefits to having this on a WRX?
-What else is required (besides diff) to to an exchange from the stock WRX to an STi center diff?
-Is this diff the one that is controlled by the DCCD switch on STis and do I need that switch/harness to make it work?
-After all is said and done, is it actually worth doing on a car with my setup?
09-27-2012, 12:49 PM
#21
If in doubt, FLAT OUT
iTrader: (33)
Join Date: Oct 2010
Location: Nor Cal SJ
Posts: 7,035
Car Info: 2010 Hatch, 2011 Sedan
I didn't even read this lol... You have an 03, the transmission isn't the strongest to beat on. Using that C.diff will not bring your gearing to STI specs... My advice? Get a STI6spd that comes with a nice center diff and super strong gears. Your 5spd will blow up in flames and burn all the animals in the forest, like in Bambi...ok not really, but it will break.trust me I know.
It sounds like the guy is offering up a DCCD, which is different than the WRX, but it will NOT bring the WRX up to STI spec with the exception of the DCCD.
01-30-2014, 03:21 PM
#22
VIP Member
Join Date: May 2006
Location: Plano, TX
Posts: 2
Car Info: 2002 WRX Wagon
Thread revival...
I have a 2002 WRX with a JDM 5 speed - stock diffs.
I had thought that the Cusco Tarmac would help reduce the front wheel spin I experience when accelerating out of a corner and was thinking that it would give me more rear wheel bias which would make the car handle a little more like a RWD and be more tail happy.
It sounds like this may not be the case unless I get the 35/65 center AND a front LSD. Is this correct?
Will a front LSD help with the inside wheel spin when accelerating out of corners?
Thanks,
Greg
I have a 2002 WRX with a JDM 5 speed - stock diffs.
I had thought that the Cusco Tarmac would help reduce the front wheel spin I experience when accelerating out of a corner and was thinking that it would give me more rear wheel bias which would make the car handle a little more like a RWD and be more tail happy.
It sounds like this may not be the case unless I get the 35/65 center AND a front LSD. Is this correct?
Will a front LSD help with the inside wheel spin when accelerating out of corners?
Thanks,
Greg
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