Canon20D Posted April 9, 2009 Share Posted April 9, 2009 I dont know why, but for a long time i though that the LGT had an open diff and would spin one wheel under certain circumstances. but then I was thinking that maybe I was getting confused from all the old info on the 2.5RS I used to have, vs WRX, vs STi, VS LGT... So what is it? (i searched but didnt find anything USEFUL) Link to comment Share on other sites More sharing options...
Underdog Posted April 10, 2009 Share Posted April 10, 2009 The 5MT GT has viscous center and rear differentials. The front differential is open. The torque split is 50:50 front to rear with the 5MT setup. The Crimson Dynamo Link to comment Share on other sites More sharing options...
Canon20D Posted April 10, 2009 Author Share Posted April 10, 2009 So the center and rear are Viscous limited slip diffs? Im not sure I still understand all the options of the differential types.. I understand a lot more about how the power is made than how its transferred to the wheels Link to comment Share on other sites More sharing options...
Underdog Posted April 10, 2009 Share Posted April 10, 2009 Sorry, yes, they are viscous limited slip differentials. There is a silicon-based fluid in the diff which thickens as it heats up. On each axle (be it left & right or front & rear) there are perforated plates. As the axles spin, the plates spin. If the axles are spinning at different speeds the fluid is sheared and heats up. The thickening of the fluid results in torque being transferred from the slipping wheel to the wheel with traction. (This is an oversimplification and doesn't describe precisely how the diff works, but it does provide a basis for understanding. For a more elaborate explanation, just ask.) If there is a very large difference in rotating speed the diff will go into "hump mode" where it acts as a locked axle, meaning both axles spin at the exact same speed. This doesn't last long though, because as soon as it enters "hump mode" the shear is eliminated and the axles unlock. Viscous diffs are low-service and good for daily-driven cars because of the smooth torque transfer. However, they can wear out over time (the fluid breaks down) and they do not respond quickly enough nor do they transfer enough torque consistently for real performance driving. In those situations you would want either a helical or clutch type diff, depending on the application. The Crimson Dynamo Link to comment Share on other sites More sharing options...
goneskiian Posted April 14, 2009 Share Posted April 14, 2009 http://auto.howstuffworks.com/differential.htm Link to comment Share on other sites More sharing options...
JoeFromPA Posted April 14, 2009 Share Posted April 14, 2009 Underdog - Just curious, but what would you expect the life of the fluid to be in a viscous diff with typically gentle driving. 250k and beyond? Joe Link to comment Share on other sites More sharing options...
Underdog Posted April 14, 2009 Share Posted April 14, 2009 No, I would suspect much less. I have read somewhere that it takes ~60k before the fluid starts breaking down, although I forget where I read it. As the fluid degrades the amount of shear/transfer is reduced. As with most things, the harder you are on the diffs the quicker they will start to break down. Fortunately, the failure mode for these results in an open diff and is still perfectly driveable. This is one of the things pushing me towards an eventual 6MT swap w/ helicals, or keep the 5MT and put in helical front and rear diffs and an open Cusco tarmac center diff which shifts the torque bias from 50:50 to 65:35. The Crimson Dynamo Link to comment Share on other sites More sharing options...
whitetiger Posted April 14, 2009 Share Posted April 14, 2009 the 5spd LGT and WRX both have VLSD in the center and rear. the viscous couplings are reactive and do not work that well in performance applications. The 07-09 spec b's cmoe with a torsen rear which works much better. Only on the sti do you get true helical or clutch type diffs in all 3 spots. Link to comment Share on other sites More sharing options...
whitetiger Posted April 14, 2009 Share Posted April 14, 2009 No, I would suspect much less. I have read somewhere that it takes ~60k before the fluid starts breaking down, although I forget where I read it. As the fluid degrades the amount of shear/transfer is reduced. As with most things, the harder you are on the diffs the quicker they will start to break down. Fortunately, the failure mode for these results in an open diff and is still perfectly driveable. This is one of the things pushing me towards an eventual 6MT swap w/ helicals, or keep the 5MT and put in helical front and rear diffs and an open Cusco tarmac center diff which shifts the torque bias from 50:50 to 65:35. BTW PPG makes a helical center diff for the 5spd which is much better for street/inclimate weather applications and it does not induce understeer. Better than the open biased tarmac diff IMO. Link to comment Share on other sites More sharing options...
Underdog Posted April 14, 2009 Share Posted April 14, 2009 the 5spd LGT and WRX both have VLSD in the center and rear. the viscous couplings are reactive and do not work that well in performance applications. The 07-09 spec b's cmoe with a torsen rear which works much better. Only on the sti do you get true helical or clutch type diffs in all 3 spots. Agreed, performance is a big disadvantage of the VLSD but they are very linear and predictable which is great for the average DD (especially in wintry climates). The Crimson Dynamo Link to comment Share on other sites More sharing options...
Underdog Posted April 14, 2009 Share Posted April 14, 2009 BTW PPG makes a helical center diff for the 5spd which is much better for street/inclimate weather applications and it does not induce understeer. Better than the open biased tarmac diff IMO. I was not aware. Thanks for the heads up! (The transmission/diff, actually the whole powertrain is a few years off for me...) The Crimson Dynamo Link to comment Share on other sites More sharing options...
whitetiger Posted April 14, 2009 Share Posted April 14, 2009 Agreed, performance is a big disadvantage of the VLSD but they are very linear and predictable which is great for the average DD (especially in wintry climates). Precisely, but there are a few areas where they dont work so well. If you drive over a snow bank and get stuck where one side the car's wheels are off the ground, the rear VLSD will not beable to transfer enough power to the other side to move the car. Combide that with the open front...FAIL. In that same situation, with a toren/helical, you can apply the brake slightly to give the wheel thats in the air some resistance, this will activate the diff and power will be transfered to the wheel on the ground. Link to comment Share on other sites More sharing options...
Underdog Posted April 14, 2009 Share Posted April 14, 2009 Precisely, but there are a few areas where they dont work so well. If you drive over a snow bank and get stuck where one side the car's wheels are off the ground, the rear VLSD will not beable to transfer enough power to the other side to move the car. Combide that with the open front...FAIL. True. Getting into that situation on public roads... FAIL. The PPG site doesn't have a bunch of info, but I would love to have 3x torsens! The Crimson Dynamo Link to comment Share on other sites More sharing options...
whitetiger Posted April 14, 2009 Share Posted April 14, 2009 I only found out about the PPG center unit through ralispec when i asked hem about our options. IIRC its $1800 for the part. Not cheap, but it will be mine soon. Link to comment Share on other sites More sharing options...
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