Chknkatsu

After searching on EvoM, google, etc. i still cant find an answer

on an '03-'04 evo8, what is the normal torque split while you're driving in a straight line

from what it seems like a viscous coupling would keep most of the power to the front until it starts to spin/slip too much and the disks will start to catch. do i have the right idea?

please clarify, sorry if its in the wrong forum. im looking into getting an evo soon and im trying to get as much info as possible.

also, do evo owners experience problems with their VCD/VCU?

RoadSpike

iTrader: (5)

I'm relatively certain that the center diff in those cars have no bias what so ever. The center diff should be simply an open diff and split the torque 50/50. So whatever has less grip spins.

We aren't like subies which have a gear biased center diff of 70/30 and then locking plates to lock the diff into 50/50 mode. The helical gear set in the non acd basically only stop slip between the front wheels only.

I've heard of center diff upgrades but i have no idea what their torque bias is.

EvoJoeIX

iTrader: (16)

^exactly right

Chknkatsu

oh i understand. if the viscous coupling only determines the slip between the front wheels then why is your VCD/VCU called the center diff

from what i see in this diagram the front diff is on the left and continues on into the transfercase. and the viscous coupling manages how much torque goes into the pinion.

that is all im absorbing from this diagram

since the viscous coupling has no actual mechanical locking system (instead it contains fluid with high viscosity) i'd imagine there is a lot of power lose going to the rear

barneyb

Viscous coupling sounds like a pretty lame device. However it does lock. The Japanese term for this is hump mode (really).

The viscous coupling mechanically senses any difference between the rpm of the front and rear axles. If there is a difference and it continues for a short period of time, the viscous coupling locks. In other words, if the car is stuck in snow and the rear has no traction, the coupling locks causing the front axle to turn too. So while the device is not quick acting it will get you out of an otherwise stuck situation.

Other than that, what is written above about a 50/50 torque split is true.

Chknkatsu

ah ok i just took another thorough look at the diagram. i made a mistake on which one is the center and front diff.

so from what i understand now, the center diff receives the torque and initially the torque is split 50:50 over the spider gears.

from what i've gathered;

if one of the rear or front wheels slip, all of a sudden, depending on the situation if the ring gear OR torque tube end up spinning faster, the viscous coupling will suddenly catch to transfer power evenly?

barneyb

There are plates splined to each side of the center diff and if the motion doesn't match they start moving past each other. This forces the viscous fluid out from between the plates and they make metal to metal contact.

The only thing you have wrong is the suddenly part. I remember a guy with a DSM who had pulled the transfer case. A DSM has both the front and center differentials in the transmission. All the transfer case has in it is 90 degree bevel gears to turn the drive to the rear. He wrote to the DSM forum - should you be able to drive around this way because at first I can't but if I keep on the gas pretty soon the car starts moving?

At first only the shaft that should have been in the transfer case was spinning but after some seconds the viscous coupling locked enough so that he could drive. We advised him to stop doing that or burn out the viscous coupling.

Chknkatsu

ah i get it. so he kinda turned the car into FWD?

barneyb

Yup

Ultraperio

In a viscous coupling there is never any metal to metal contact (that is, if things are running correctly) between the two sides of the coupling. The lock, and subsequent torque transfer, from a viscious coupling comes from the variable viscosity fluid between the plates in the coupling. Tight tolerances of the plates creates friction when they rotate at different speeds. This friction causes heat, increasing the temperature of the fluid. As the fluids temperature increases so does its viscosity. At a certain point the viscosity of the fluid (hopefully) overcomes the torque difference between either side of the coupling and effectively 'locks' the coupling together. There is never a full 'lock' in a viscous coupling simply because it is still a fluid coupling. Even in a locked configuration the highly viscous fluid will still allow the plates to rotate past each other. In the case of a viscous center differential this means the power will still flow through the path of least resistance (the slipping wheels) but the coupling will enable a partial power split to force the wheels with traction to rotate. The amount of split in a 'locked' viscious coupling is dependent on the torque being input, the tolerances between the plates, the temperature range it is operating, and the viscosity of the fluid in this temperature range.

In the case of the AWD DSM converted to FWD. The car only would start rolling after the fluid reached a temperature sufficient to overcome the torque differential between the freewheeling rear wheel output and the torque being sent by the engine, which then allowed the front wheels to become partially powered with, I'd wager, a very large amount of waste through the rear output even after the coupling 'locked'. The danger in this scenario is overheating the fluid which will cause it to start to break down, reducing its viscosity in all temperature ranges and reducing the effectiveness of the coupling as a whole.

barneyb

"Hump is a phenomenon specific to viscous couplings although it does not occur under normal operating conditions. This phenomenon occurs when the silicone oil temperature has risen due to sustained differential action. Normally silicone oil fills the space between the inner and outer plates, preventing their direct coupling. When silicon oil expands at a high temperature to such a degree as to develop abnormally high pressure between the plates (normal thermal expansion is absorbed by compression of air mixed in silicon oil), silicone oil escapes from between the plates. As a result, the plates couple directly, causing abrupt torque transmission. When the viscous coupling is directly coupled in this way, a rotating speed difference does not exist, and then silicon oil temperature drops and normal function is restored."

In other words, it locks.

page 21-14 Laser/Talon Technical Information Manual

Ultraperio

A coupling that makes actual contact between the differential speed plates is not a true viscous coupling by definition. A viscous coupling, as its name implies, transfers torque through the viscosity of a fluid. What you (and the technical manual you've referenced) described is a type of hybrid viscous/clutch coupling. Acting as a viscous coupling most of the time until large differentials of plate speed occur.

Even this type of coupling cannot produce a solid reliable 'lock' as constant differential plate speed is required to maintain the pressure required to keep the plates locked together in a clutch type configuration. If the plates are in contact and the coupling is clutch locked, pressure is not being generated and thus the lock is lost. What this means is there is still constant slip even in this type of 'fully locking' hybrid coupling, if not as much as in a true viscous coupling.