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Sierra Lsd Diff


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All thank you for your feedback and the instructions. My reasons, I have more power now and thought more traction and possibly better brakes would be beneficial. Im a sensible driver and would like to do the odd track day.

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All thank you for your feedback and the instructions. My reasons, I have more power now and thought more traction and possibly better brakes would be beneficial. Im a sensible driver and would like to do the odd track day.

 

First thing I would ask yourself is, is your suspension in tip-top condition, with properly dialled-in settings, meaning the car is handling somewhere near as good as it can? Because if you've got a low-grip or badly-handling situation because of suspension issues, an LSD is going to do nothing but exaccerbate the problem. They can turn a balanced car into an understeery mess as the back end tries to push the front wide on the power, unless you have enough power and torque on tap to "break" traction evenly across the newly-grippy back end. So don't assume an LSD is some panacea for a badly-handling car.

 

Same goes for tyres. If you've got half-decent grippy tyres, then cool, but if not then a better solution to improving grip could be down that approach. You're either losing grip because your tyre and suspension combination simply can't put the torque to the ground, or because your not keeping the contact patches down on the tarmac in turns. An LSD isn't going to do diddly squat about the latter problem, you're still going to lose that grip under the tyre, it just means now you're not going to spin the power away because of it.

 

Assuming then that we're happy with the suspension and the tyres, and you've still got a wealth more torque than the back end can cope with, you may opt to pop an LSD of some sorts in there (and I'm including torque-biasing units into this).

 

Depending on the choice, then, you'll notice some traits.

 

The common one - a Ford viscous diff - as mentioned above you'll need to be careful that the viscous fluid is still good in these now-aging diffs. But you can get it changed. Positives are nothing to really wear out, relatively plentiful. A viscous diff will not have the immediacy in action of a mechanical diff, and is not really "tuneable" (short of changing the viscosity of the fluid), so for ultimate track/competition use drivers tend to shy away from viscous-type couplings. You never see them used in motorsport. I wonder if they have a tendency to overheat when worked hard?

 

The Quaife ATB and equivalents are the next step up, but they are not true limited slip devices, as pointed out above - you still need your contact patches good. I personally think these are brilliant road-biased solutions as they work really well if you have good sorted suspension, don't really suffer from wear and are fit-and-forget, and have a more immediate action to them like a conventional LSD.

 

Finally, you've got a plate-type conventional LSD (this is what I have more experience with, and have sitting around waiting to be fitted into the Zero). They are more tuneable so you can get their action how you want them. There action is also more immediate so drivers tend to prefer them. But they require maintenance over time because they use physical clutch plates, that wear out depending on how hard a use they get. As said, it can change the driving dynamics of the car. If you've not got big torque you may find the car understeery under power as the back now digs in and pushes forward. A more agressive driving style combats this. You will learn that driving just on a tickle of throttle at the limit helps a plate diff do it's thing. Not that it's relevant for us lot, but in a front wheel drive car an agressive plate diff can really catch people out, dangerously so if they don't know how to "drive them" properly.

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Just to cut through a bit of the speculation and hopefully add a bit of clarity....

 

The only easily found Sierra viscous LSD is the 3.62 or 3.64 ratio generally 3.62 is a 7 and 3.64 is a 71/2

The 3.92 was a 7 only found on the 2.0L DOHC 4x4

The viscous diff is a softer street LSD, generally they are sealed for life and as they have silicon fluid you can fit and forget, I personally would not fiddle with it as it has a specific sized air bubble in it. I know that oval racers sometimes pump air in to make the diff stiffer.

 

I run an ATB only because it fitted my push in shaft back end.

 

Most of not all Sierra viscous diffs are bolt on.

 

The reality of an LSD on the road is that it benefits the sporty driver, either viscous or ATB, plate diffs are mostly on or off and you can never set it up for all conditions.

 

An LSD will tend to push you straight (understeer) but you can compensate easily in your driving style and/or change your setup a bit, certainly the drive out of corners is great and confidence in the fast corners is a delight.

 

The Sierra viscous diff is the cheapest, easiest to fit and an excellent upgrade for the road.

And of course the bragging rights

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  • 1 month later...

Thank you for the detailed feedback, at this stage whilst investing the noise I am tempted just to swap with a known good diff. I would hate to pay for a new lsd and find the fault is there or worst cause the diff to fail.

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