gmonsen

Can anyone tell me what the effect of an open differential would be on off-throttle oversteer? The 3.6 Turbo I bought has a Quaife differential that nominally provides 50/50 power distribution on both acceleration and deceleration with up to 80% going to the wheel with the most traction in a corner. The stock differential would lock 100% on deceleration. I don;t know enough to know whether this Quaife differential would cause the dreaded off-throttle oversteer (read bad doughnuts). -Gordon

Cupcar

The high locking factor on trailing throttle in the differential the factory installed in the Turbo and 964 Euro RS was to moderate the trailing throttle oversteer characteristic of the cars. It does this by essentially locking together the rear wheels on trailing throttle so they must turn at the same speed .

This is because if the car is to turn (oversteer), then the inside rear wheel must be allowed to turn slower than the outside, but the 100% differential blocks this variation in wheel speed, effectively stabilizing the car into a straight line.

So your Turbo may be a little more squirelly with trailing throttle oversteer with the Quaife. I doubt this will be much of a problem though because, I'm assuming your car is a USA Turbo, which does not have the modified rear trailing arms to block the Weissach axel effect of toe in under trailing throttle the Euro turbo does.

The Euro RS and Euro Turbo have trailing arms in the rear with the Weissach axel effect designed out so Porsche turned to the differential to help stabilize the car. <img border="0" alt="[typing]" title="" src="graemlins/yltype.gif" />

gmonsen

Thanks. You are correct. It is a US Turbo and I'm pleased to hear there won;t be that much of an effect. I still am researching the Quaife to better understand exactly what it does. I know its a very different setup than the stock unit. -Gordon

Dave R.

Gordon,

Are you sure the Quaife provides torque biasing on deceleration?

About a year ago a representative in their tech department told me that the Quaife provides torque biasing on acceleration, but acts like an open differential on deceleration (in contrast to the Torsen design, which biases torque in both directions).

Good luck in your researching.

gmonsen

Dave, I spoke to Quaife today and they said that on deceleration it would behave much the same as the stock Porsche unit. I believe that it does act like an open differential, though it is technically not one. Though the guy was a bit full of pride in Quaife, he was absolutely certain that I would not only have no off-throttle oversteer, but that the car would have more traction than the stock unit. I don't know enough about differentials to comment. -Gordon

1AS

S'cuse me, but I think Cupcar has it wrong. If the differential locks, the car must slide, since you can't have differing inside and outside wheel speeds. At slow speeds it will slightly hop as it turns, but at higher speeds, the rear slides. I have two cars with Detroit lockers which I believe behave opposite of what has been stated. The differentials lock on acceleration and open on coast.
Locking the rear on acceleration makes for a very easy-to-control drift. AS

Dave R.

Gordon,

Thanks for the data point.

-Dave

Edward

Gordon,

Has Quaife changed the way they do things? My understanding of their design is that it biases torque on accel, but NOT on decel (thus behaves like an "open" diff on decel = MORE prone to oversteer than a diff WITH some lockup on decel). This has been my understanding for some time now, unless they've changed their LSDs. Perhaps the rep you spoke to was not as knowledgable as he would like to think, or that I'm flat wrong. He also contradicts himself by saying that it (the Quaife) behaves like an open diff on decel, but behaves like the stock LSD (Porsche's LSDs have a degree of lock up on decel)! Plus, though I admittedly know nothing about the 3.6T's LSD characteristics, are you SURE that the stock one locks 100% on decel. This doesn't sound right at all. BTW, forgive my lurking on this board

Edward

Cupcar

The differential lock in some of the 911 differentials varies whether the car is accelerating or decelerating. This is accomplished by variable angles on the acceleration ramps supporting the pinion gears. The Turbo differential is 100% or deceleration but only 20% on acceleration. So, as soon as you go to neutral throttle after braking for a corner, the differential locking factor goes to 20% and the 100% effect goes away as you enter the corner. It's true if the differential remained locked up the car would not turn in at all unless the wheels started to slide.

Pictured is a Cusco differential from Japan for the 350Z (only pic I could find). The internal clutch plates and drive pieces only are shown, the case is not. It is similar in design to the Porsche. Note the "D" shaped pinion gear shaft in the center in the sort of "D" or triangle shaped hole located between the horizontal coil springs. This is the acceleration ramp system. This "D" shape gives the differential a high locking factor in one direction and a low locking factor in the other. It does this because as you can see the nose of the "D" if driven down in the picture would separate the drive pieces increasing lock effect. Moving the back of the "D" up in the opposite direction would have no effect on the drive pieces, the only force promoting lock in this case would be the preload force of the horizontal coil springs (Porsche design does not use coil springs for preload though)

Also note the "&lt;&gt;" shaped hole above and below the "D" shaped hole. Installation of a round pinion shaft in this hole would give the differential identical locking characteristic on braking and accelerating. The Porsche diffs don't have this assembly option built into their design. This diff is apparently going to be one of the dealer available NISMO parts to hot rod the 350Z along with some other stuff. Would be nice if PCNA were forthcoming with such a parts catalog.

<img src="http://boards.rennlist.com/upload/LSD4.jpg" alt=" - " />