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Sierra 'LSD' Diffs - are they truly LSD?
plentywahalla - 17/12/09 at 05:10 PM

This is probably an idiot question but here goes.

I am after a limited slip diff for my book type independant rear end.

I see many Sierra and Escort Cosworth diffs advertised on eBay which state that they are LSD. Some have pics with the backplate off to show the condition. What you can see appears to be a standard open diff.

Are they properly LSD? How do they work as there appears to be nothing to limit the free rotation of the planet gears.

This is a pc lifted off an ad

mookaloid - 17/12/09 at 05:13 PM

The sierra LSD is a viscious coupling type so it looks like an open diff but the back of the crown wheel and the carrier are thicker than on an open diff to accomodate the viscious bit.

minitici - 17/12/09 at 05:13 PM

The LSD is a viscous coupling which hides in the split crownwheel flange on the left hand side.
The viscous coupling connects the two output shafts and the housing via splines on the longer LH output shaft.

t16turbotone - 17/12/09 at 05:17 PM

Viscous LSD's do not have clutch packs at all, they rely 100% on viscous fluid coupling. That is, they don't have frictional metal to metal clutching, they have discs that rotate close to each other but rely 100% on the viscous fluid to do the coupling after enough sheer force is built up within the fluid, they don't press against each other.

plentywahalla - 17/12/09 at 06:29 PM

... for all that.

I can see how it works now. Does that make it as good as a quaife ATB type?

What is the best option for my IRS with a Dry sumped Zetec on throttle bodies aiming at 180 to 200 brake.

Do I buy a standard diff and fit a quaife ATB or a sierra viscous type which works out at half the cost.

minitici - 17/12/09 at 07:10 PM

quote:
Originally posted by plentywahalla
... for all that.

I can see how it works now. Does that make it as good as a quaife ATB type?

What is the best option for my IRS with a Dry sumped Zetec on throttle bodies aiming at 180 to 200 brake.

Do I buy a standard diff and fit a quaife ATB or a sierra viscous type which works out at half the cost.


In some instances the Sierra viscous diff can be better than a Quaife ATB. For instance with a Quaife if you get one wheel in the air or on really slippy ground - you will not get any drive to the wheel with grip. With the viscous diff, the differential speed between the axles is sensed by the viscous coupling and the diff will progressively lock up and still give you drive.

mark chandler - 17/12/09 at 09:18 PM

My old high mileage LSD works a treat, plenty of grip near the edge, if booted both wheels spin up together.

I have had cars with plated diffs, these have clonked in knocking you sideways, with viscous you do not notice them, just the extra pull as you boot through a bend.

An expensive diff correctly set up for the car would be better, or people would not make them. However would I notice the difference, probally not a decent set of tyres would do this.