NWS4Guy

I'd thikn it would be fine, and just vary based on what it's given. The ECU doesn't know what the power split is anyway, since the center diff is mechanical and controls this, swapping out the stock crown gear for the Statis upgrade is what changes this.

That said, I think this points to the fact that the SP is VERY overengineered for our cars. Stasis doesn't void warranty, they are coupled with Audi on these upgrades, which means Audi stands by the decision to move 33% more power to the rear wheels, meaning a drastic increase in the amount of torque the SP will see under hard driving conditions as it moves the now 80% power to one wheel, rather than 60%. This without needing to upgrade the SP in terms of the internal clutches, gears, or half axles.

Impressive!

westwest888

I posted this on the BMW forum, trying to explain how I think this system works. Let me know if I'm wrong anywhere:

ZF developed all of these parts (differentials and transmissions), and as you'd expect they are used in VW/Audi/Porsche/BMW models. People with earlier generation quattro's (like that one understeering in the video) have swapped the open rear differential for the LSD in the Porsche 944 to avoid that.

Here's a view from 50,000 feet on quattro: right behind the manual transmission there's a torsen (torque sensing) center differential which can redirect torque from front to rear. The factory bias on an S4 is 2:1, meaning you can split the torque 64/36 or 36/64 at maximum, or anything in between. It's supposed to start at 50/50, but since the car has 55% of it's weight over the nose it starts at 55/45. So when you launch the car really hard, torque flows almost evenly to the front and rear wheels until the front wheels lose traction (they'll chirp). Then that center differential starts sending all of the additional force to the rear wheels, up to 2x what the front is getting. So if you're revving 4000 RPM and making 300 ft/lb of torque, it'll send 100 ft/lb to the front differential and 200 ft/lb to the rear (assuming the tires can generate enough friction to handle it).

For $500 STaSIS (like Dinan for Audi) will change your center diff to a 4:1, meaning you can split the power 20/80 or 80/20, or anything inbetween. Allegedly, the RS4 and RS5 have this stock.

There's two more differentials. The front diff is open on all Audi/VW cars, and the rear diff is open on almost all Audi/VW cars. If you're on a track and you lift a wheel, you're pretty much screwed. Let's say you get on the power and the inside front tire lifts on a left turn. Now your front axle can only take 10 ft/lb of torque. You've got that multiplier so your rear axle can take 20 ft/lb of torque. The weight will transfer forward under loss of power, the wheel comes back down, and you continue (albeit slower).

Here's where it gets interesting for the S4 with the sports differential. The rear diff can electronically clamp down on either shaft (left or right) with infinite bias. Let's say 200 ft/lb of torque ends up at the rear differential and the car is turning. It can send 150 ft/lb to the outside rear wheel. This oversteering bias will step the rear end out, and should keep the diagonal corner of the car (the front inside wheel) planted. And avoid the situation above.

Here's an alternate scenario. You do a hard launch, the front wheels chirp, any additional twist being generated is sent to the rear (up to the 2:1 bias), but you have an open diff and it overwhelms one of the rear wheels so your torque goes to heaven and you go nowhere fast. With the sport differential, that power will make it to the ground for sure.

Anyway, this is all armchair high performance driving. When I get this thing on a wet skidpad I'll let you know what it does.

supraman66

Love all of this engineering stuff!

Has anybody installed the Stasis center differential? I bet that would have made the video presentation much more interesting as it should/could step the rear out easier.

This is if it doesn't start to transfer torque to the front, sliding it with the rear, hence no oversteer (doubt this is the case).

My point is....... has anybody installed the stasis diff?

NWS4Guy

None that I know of with Stasis yet no.

As for the RS5, it doesn't have this crown gear, it has something a little more special from my understanding. For one, the RS5 is not a 80/20 split, it's a range from the "normal driving split" of 60/40 rear bias like the S4, to as much as 70% power to the front, and 85% to the rear depending on settings and conditions.

Also of note, the RS5 center diff has a clutch pack, like the Sport Diff, which allows the change of this torque.

http://import2race.com/index.php?opt...:news&Itemid=2


The Stasis Crown gear just changes the ratio overall, and has nothing to actuate the change. Stasis is just a chane in the gearing to put the split to 80/20.

http://stasisengineering.com/sigSeri...4&category=lsd

2010S4Fan

Does anyone know the cost of installing the Stasis Hi Bias Torsen Differential at a dealership?

supraman66

Good point. I was confusing the two. The statis is a static 80%. Damn near a rear driver at that value.

Thanks for setting me straight!

NWS4Guy

I'm curious how this will drive in inclimate weather. I know in the dry it would be a hoot, as I know with the Sport Diff in Dynamic you could easily step the reat out

I think somewhere between 65/35 and 70/30 would be a nice happy medium between this and stock. I don't think that the kinds of snow your state and mine sees would make me comfortable with an 80/20 static setup is all.

supraman66

I couldn't agree with you more! I'm worried that I will become the worlds more expensive snow plow with as much snow as we can get overnight!

Given the the new Audi Center Diff is not "electronicly" activated, I'm curious if you would be able to upgrade ours with the new version offered in the new S5. You know, when it actually comes to market. Then you can have your 80% and snow too!

supraman66

[QUOTE=NWS4Guy;23945919]
As for the RS5, it doesn't have this crown gear, it has something a little more special from my understanding. For one, the RS5 is not a 80/20 split, it's a range from the "normal driving split" of 60/40 rear bias like the S4, to as much as 70% power to the front, and 85% to the rear depending on settings and conditions.

Also of note, the RS5 center diff has a clutch pack, like the Sport Diff, which allows the change of this torque.

http://import2race.com/index.php?opt...:news&Itemid=2

So to answer the question above, the S4 is a 60/40 split all the time due to the absence of a clutch pack. So if your back wheels are off the ground (your screwed.......... Ha Ha) the most you can hope for is 40% of the power to be transfered to the front wheels.

Correct, or am I missing something?

westwest888

[QUOTE=supraman66;23946152]Reread my post about five up from here.

Incorrect. First off, it's not 40/60 or 60/40. That's marketing terminology. It's a 2:1 torque sensing (torsen) differential, which means it can bias power anything from 66.6/33.3 to 33.3/66.6. This includes 50/50, 51/49, 40/60, etc. When the car is at rest the limited slip torsen center diff is operating in open mode at 55/45 because this is the weight distribution. So if you launch granny soft you're 55/45 for power split. If you launch medium you'll be about 50/50. If you launch hard you'll shift a lot of weight to the rear, the differential will lock up, and you'll get a full 33.3/66.6 launch. As you go from 150 ft/lb of engine output (50 front, 100 rear), for each 1 additional ft/lb that gets sent to the front wheels, 2 ft/lb is sent to the rear wheels all the way up to 325 ft/lb (375 ft/lb if you're rocking STaSIS software).

There's three kinds of differentials: open, limited slip, locked. The front differential is open. The rear differential is limited slip if you get the sports rear differential or Audi Drive select; open rear differential if you do not get one of these options. The center differential, which we're talking about right now, is limited slip. One type of limited slip differential is a torsen (S4). Another type is a clutch pack (RS4/RS5). They both do the same thing in principal. The torsen never wears out; the clutch pack will wear out if you take it to the track. The clutches let a certain amount of slip happen then lock up, depending on the ramping and preload. The torsen loses power if the front or rear wheels leave the ground. In reality you'll never lift the entire rear axle or the entire front axle. But if you have an open differential in the front or the rear and you lift one wheel, you're done for. The sports differential takes care of the rear axle, and partially takes care of the front axle by stepping out the rear of the car enough that you *should* never lift the inside front wheel.

So at let's say 3000 RPM with a STaSIS software tune and the stock 2:1 "40/60" differential, at full tilt you're splitting 375 ft/lb = 125 ft/lb front, 250 ft/lb rear.

At 3000 RPM with a STaSIS software tune and the STaSIS 4:1 "20/80" differential, at full tilt you're splitting 375 ft/lb = 75 ft/lb front, 300 ft/lb rear. I'm not sure if the rear drive shaft is engineered to take 300 ft/lb (between the center differential and the rear differential). Nor am I sure that either of the rear half shafts (what connects a rear wheel to the rear differential) is engineered to take the full onslaught of 300 ft/lb. Normally it would be split 150 ft/lb between the rear wheels. But if you have the sports rear differential it can send the full 300 ft/lb to the outside wheel.

http://www.youtube.com/watch?v=fSEkUV7torQ