B8.5 S4 torque vectoring (is sports diff still a 'must have')

[AZT]

From what I have read -

Quattro V (in B8 S4) is Torsen-based and first introduced torque vectoring via the rear sports differential option, which most here still say is a MUST HAVE when buying an S4.

Quattro VI (first introduced in the 2010 RS5 and later 2013+ B8.5 S-tronic S4) added a new crown differential which enabled the integration of a new four-wheel electronic torque vectoring system via ESC (with or without sports differential). I have also seen the 2015 A4/S4 brochure advertise ESC based torque vectoring separate from the optional sports differential, so pretty sure it is 'there'.

Excerpts from an article when this first came out in the RS5:


Audi’s RS5 coupé is the first model to combine the firm’s crown gear centre differential with electronic four-wheel torque vectoring.

The torque vectoring system is similar to systems used in some powerful front-wheel drive cars, except that Audi’s technology acts on all four wheels. If slip is detected on the more lightly-loaded inside tyres during cornering, they are braked slightly. This generates a yaw moment to stabilise the car.

Audi’s torque vectoring rear differential, which uses an electro-hydraulic actuator to actively redistribute drive torque at the rear axle, is an option on the RS5 and works with the brake-based system, but at a higher threshold.


I won't deny under certain conditions the sports diff is superior, but for everyday driving, is the sports diff still a 'must have' option for newer B8.5 S4s if the above is true? Curious to hear any comments.

[SteveYem]

I would get the sport diff, no question. Take it from me, a guy who had a standard diff B8 and traded in for a sport diff B8.5. Both of mine are/were 6MT, though, so I can't speak to the crown center diff and ESP-driven torque vectoring function. Also keep in mind that the RS5 has the ESP-driven torque vectoring as well as the rear sport differential.

Finally - the ESP-driven torque vectoring feature says it works when slip is detected at one or more wheels. It actively brakes where necessary to maintain traction. Meanwhile the sport diff does not actively brake - instead it overdrives the outer rear wheel to keep the front of the car on-line through the turn, i.e. counteracts the car's natural tendency to understeer. Sounds to me that the ESP torque vectoring is a safety feature, while the sport differential is a performance feature (although I maintain that it has its merits as a safety feature in slippery conditions as well).

[cfritos]

Yes..

[Dr GP]

YES!!! It is THE option that makes this car special.

[SteveYem]

While I am a proponent of the sport differential, to be completely fair it is worth remember that while it is an $1100 option, it costs much more to repair/replace (except for leaking seals which would be less expensive). I remember reading a couple years back where an Audizine member in Canada had a sport diff equipped car which took an impact from a block of ice or large rock on the road directly to the sport diff unit. I believe it struck and dented the hydraulic actuator unit that is mounted adjacent to the differential itself. If I remember correctly his sport diff ceased to function (he could still drive the car, but no more torque vectoring) and he had a Sport Diff Fault code. Audi quoted him something like $11000 to replace the unit.

So buyer beware. It's a neat feature and a bargain at $1100, and although it is probably not common to run into a scenario as described above, it is possible. There is no protective belly pan or cover for the relatively fragile actuator unit on the sport diff. It's a nice feature to have, yet another gadget to possibly fail someday.

[rymofo]

I would get the sport diff, no question. Take it from me, a guy who had a standard diff B8 and traded in for a sport diff B8.5.

Same, 2010 to a 2014. Definitely a fun and noticeable feature

[B8 S4]

Wouldnt have bought the car without it

[Okan509]

love my car. no sport diff.

couldn't care less about not having it, it hasn't even crossed my mind.

[nick71692]

my 2012 had it, my 2014 does not...I can honestly say I haven't thought about it once nor miss it.

[HedonismBot]

If you plan on selling the car at some point in the future then it would be silly to not get the Sports Diff. It's one of those rare options that you can recoup 100% of it's cost (maybe even more) during resale.

[Diocletian]

My 2013 does not have it, my last 2 cars I've owned, 2012 Evo X and a 2009 370z, both had one. I honestly don't notice a major difference, not enough to be concerned or care to be honest. I like my car how it is, just wish I had ActiveKey and nav for the convenience!

[dubspeed22]

To answer your question. No. It is not a must have. Is it a 'nice to have' sure. But you will be moreee then happy with a 13'+ s4 because I sure am!! My must haves were black w black alcentra, carbon, nav, smart key, and certified. And that's exactly what I got :D

My Nice to haves would be sport diff and 19's..

As you said it's a daily driver like it is for me. Come on let's be real you don't need it nor will you even notice not having it. Hope this helps. GL

[CaptSlow]

Most of the time I'm driving I don't notice it. But when I push it hard, the car feels like it's on rails. It is noticeable. It's a matter of preference.

[B8 S4]

Also factor in the increased cost of tires since it is hard to resist getting on the throttle in turns with SD

[riegeraudi]

If you drive hard in the corners than yes it is a must have. If you like only to drive fast in a straight line than no it is not a must have.
You don't have to bring it to the track to notice the difference either.
The sport diff pays for itself when you go to trade the car in.
Get an A4 and chip it if you don't get the sport diff. Lighter engine and will have less understeer than the S4 without a sport diff.

[AZT]

Thanks for the general opinions but would like to see more discussion on my specific point - that the B8.5 Quattro VI added its own standard torque vectoring solution that (like a sports diff) will result in more torque applied to the outside wheels than the inside wheels in a corner. So now in that context, will the sports diff make less difference than it used to?

SteveYem made some great points that this ESP solution may be focused on safety vs performance, but I wonder if there is overlap there in everyday driving. This video (skips to 4:30) explains how this torque vectoring in Quattro VI works (without the sports differential):

https://www.youtube.com/watch?featur...SwOrkA#t=4m30s

[BLACKS4-10]

No sport diff here and I had my car for 5 years with upgraded tires,intake,exhaust,front brakes and drive train stabilizer.Handles awesome in any situation and still love it...

[Sales@DriveAuto]

Definitely worth it.

[Diocletian]

I honestly did not know the 2013+ S4s had this feature on the S Tronic. That would be awesome if I had it, because I think my car is damn good in hard turning and quite predictable and fun! Only thing I need to get rid of is the body roll, but I will deal with that AFTER my stage 1 tune lol

[TexasDfwS4]

When you need it is when you'll be thankful you have it. I needed it once on my 2012 and paid the conspicuous for not having it.

[Bmil128]

While cross shopping dealers for my '14 every sales person told me unless you track it you won't notice the difference, and I have no complaints. The only Glacier White one I could find in a reasonable distance that had the B&O and MMI did not have it but I'm more than happy with the handling.

[Dr GP]

While cross shopping dealers for my '14 every sales person told me unless you track it you won't notice the difference, and I have no complaints. The only Glacier White one I could find in a reasonable distance that had the B&O and MMI did not have it but I'm more than happy with the handling.

Unfortunately, Sales associates are not the best source for accurate info. When I originally ordered my 2010, I was on the fence about the SD. My SA was a long time Audi aficionado. He said I would not need the SD. Fortunately, I decided to order it. IMHO, it was a good decision. Have since had 2 more with SD. Wouldn't have one without it. That being said, if you never drive an s4 with the SD, you will never know what you are missing. I have never driven an s4 with ADS. Don't miss not having it. Perhaps if I drove one with ADS, I might change my mind. BUT, an S4 without ADS, SD, Nav, B&O is still a great car. The options are icing on the cake.

[S4$Bill]

Most of the time I'm driving I don't notice it. But when I push it hard, the car feels like it's on rails. It is noticeable. It's a matter of preference.

This is the perfect description of the sport diff to me. But it's so much fun when you get on it in a corner.

[superswiss]

Interesting question. I don't have an answer, but I can tell you this. In the two years I had my RS5 (which as you stated has both), I experienced the brake-based torque vectoring working exactly once, while I regularly feel the sport diff working. I love to corner hard on the back roads here.

Here's my experience. I went on a spirited drive and came upon a corner in the shadow of the trees. The road was clearly wet, so I stepped on the brakes to shave off enough speed, expecting that the water mixed with the gunk from the trees made for a pretty slippery situation. As I entered the corner I let off the brakes, but at that point I could clearly feel the front inner brake not releasing and literally pulling the nose of the car into the bend. Shortly before the apex I gave it throttle and the car went into a controlled drift clearly aided by the sport differential. I had to counter steer, but pretty much kept drifting around the corner until the road straighten out again. I had clearly underestimated just how slippery the road was, but it was probably the most exciting corner ever in this car. It just held on despite the drift and it never felt like I was gonna fly off the road. It sure was fun, though. I probably should also mention that I had ESP in Sport mode.

So, I don't know how active the brake-based torque vectoring is during cornering on dry roads, but it sure kicks in if the road is slippery. The SD on the other hand constantly helps rotating the car and if you love to corner hard you'll feel it working.

[Subsonic]

Most of you are completely ignoring his actual question. I'm not in the market for a new S4 but I'm curious too.

Thanks for the general opinions but would like to see more discussion on my specific point - that the B8.5 Quattro VI added its own standard torque vectoring solution that (like a sports diff) will result in more torque applied to the outside wheels than the inside wheels in a corner. So now in that context, will the sports diff make less difference than it used to?

[SteveYem]

Thanks for the general opinions but would like to see more discussion on my specific point - that the B8.5 Quattro VI added its own standard torque vectoring solution that (like a sports diff) will result in more torque applied to the outside wheels than the inside wheels in a corner. So now in that context, will the sports diff make less difference than it used to?

SteveYem made some great points that this ESP solution may be focused on safety vs performance, but I wonder if there is overlap there in everyday driving. This video (skips to 4:30) explains how this torque vectoring in Quattro VI works (without the sports differential):

https://www.youtube.com/watch?featur...SwOrkA#t=4m30s

If I understand it correctly, the crown gear center differential is for front to rear torque distribution, while the sport differential is for rear wheel side to side torque distribution. It seems to me that the crown gear center diff, 4-wheel torque vectoring (i.e. active braking at each wheel), and rear sport differential are 3 stand-alone features.

In any case, I believe a distinguishing difference between the active-braking 4 wheel torque vectoring and the sport differential torque vectoring is that the active braking method brakes the inner wheel while braking lightly or not at all the outer wheel, so relatively there is more torque at the outer wheel but some of the rotational energy from the engine's flywheel is being burned off by the inner wheel's brake. Meanwhile the sport differential takes all of the flywheel energy (minus the drivetrain efficiency loss of course) and distributes it side to side but you get all that torque instead of burning it off at the brakes. (Someone please correct me if I'm wrong).

[superswiss]

If I understand it correctly, the crown gear center differential is for front to rear torque distribution, while the sport differential is for rear wheel side to side torque distribution. It seems to me that the crown gear center diff, 4-wheel torque vectoring (i.e. active braking at each wheel), and rear sport differential are 3 stand-alone features.

In any case, I believe a distinguishing difference between the active-braking 4 wheel torque vectoring and the sport differential torque vectoring is that the active braking method brakes the inner wheel while braking lightly or not at all the outer wheel, so relatively there is more torque at the outer wheel but some of the rotational energy from the engine's flywheel is being burned off by the inner wheel's brake. Meanwhile the sport differential takes all of the flywheel energy (minus the drivetrain efficiency loss of course) and distributes it side to side but you get all that torque instead of burning it off at the brakes. (Someone please correct me if I'm wrong).

That's not quite how I understand the mechanics involved here. Applying the brakes on the inner wheel subtracts torque from the inner wheel. The subtracted torque is instantly redirected to the other wheel through normal diff operation. That's what a diff does. Torque's not lost. There is certainly kinetic energy being converted to heat, but the torque is simply redirected through the diff to the outer wheel.

The SD on the other hand adds torque to the outer wheel, without taking it from the inner wheel. Think of it as a similar effect to the torque multiplication that takes place in a torque converter. The SD has a second half shaft on each side that rotates 10% faster than the main shaft and a clutch pack transfers more or less of that additional torque to the main shaft as needed. Also, the SD only acts on the rear axle, while the brake-based torque vectoring also acts on the front axle.

[blackfunk]

That's not quite how I understand the mechanics involved here. Applying the brakes on the inner wheel subtracts torque from the inner wheel. The subtracted torque is instantly redirected to the other wheel through normal diff operation. That's what a diff does. Torque's not lost. There is certainly kinetic energy being converted to heat, but the torque is simply redirected through the diff to the outer wheel.

The SD on the other hand adds torque to the outer wheel, without taking it from the inner wheel. Think of it as a similar effect to the torque multiplication that takes place in a torque converter. The SD has a second half shaft on each side that rotates 10% faster than the main shaft and a clutch pack transfers more or less of that additional torque to the main shaft as needed. Also, the SD only acts on the rear axle, while the brake-based torque vectoring also acts on the front axle.

This is correct. The way torque works as the brakes are applied on the side loosing traction its transferred to the wheels that grip. Assuming there's always 100% of torque sent from the engine and its split 50-50 side to side and the left side can only use 20% then the remaining 30% gets routed to the wheels with contact. Not talking about drive train losses, surface mu etc.

The SD then takes whatever power is send to the rear and apportions it across the rear wheels to the one with the most grip. It’s a system within a system. According to Audi literature the brakes can even intervene across the rear axle if needed - I'm not sure where I saw this but I'll try to look it up.

[LINDW4LL]

I don't think that the ABS/ESP-related torque vectoring can produce an effect that's comparable to the Rear Sports Diff.

Regarding worth: realistically, the S4 is still a fantastic car without the sports diff. While you can feel it on a daily basis and it really does change the enjoyment of the car, there is not a massive difference in measurable performance without it. Torque vectoring differentials are a pretty recent development and are still only available on a handful of cars. The B7 RS4 is a fantastic car and doesn't have torque vectoring. The E9X M3 is a fantastic car, again no torque vectoring. Same with hundreds of other sports cars.


I think most of us say it's a must-have just because of how much better the car feels with it and no one wants to buy a non-SD car knowing that the dynamics are so much better when equipped with it. I struggled with this about a month ago as I found a non-SD car for $4,000 less than the next comparable car with the Sports Diff (a 9% difference in price- both pre owned) but I know I'd always regret not getting it.

[superswiss]

The B7 RS4 is a fantastic car and doesn't have torque vectoring.

Has significant more understeer than a B8/.5 with SD, though.

The E9X M3 is a fantastic car, again no torque vectoring.

But it has an LSD.

Agree with your general point, though. Torque vectoring is a recent improvement that's being employed across the board. It works great on FWD cars to get rid of torque steer and even the new M3/4 now has an available torque vectoring rear diff, so does Porsche and others.

However, if all one does is getting groceries and be stuck in traffic on the commute to work the SD becomes a questionable expense.

[anerbe]

That's not quite how I understand the mechanics involved here. Applying the brakes on the inner wheel subtracts torque from the inner wheel. The subtracted torque is instantly redirected to the other wheel through normal diff operation. That's what a diff does. Torque's not lost. There is certainly kinetic energy being converted to heat, but the torque is simply redirected through the diff to the outer wheel.

Not sure on your conclusion that torque is not lost. Torque is not lost to the axle shafts, but torque is lost to the road with a brake based LSD.

Remember, calculating torque at the road surface is a relationship between the force applied on a tire, and the friction of the ground to move the tire in relation to the ground. An open differential always splits torque evenly between both sides. When one side has a lower available torque to place on the ground before it slips (inside wheel, wet/ice on one side), the torque applied to both sides decreases by that amount. For instance in dry conditions going straight, each side gets 50/50. One side will not see more than 50%. If one side is on a wet/slippery surface where only 10% torque is available to put on the ground, both sides will see 10%, total 20% of the torque. When one side is on ice / 0%, both sides get 0% applied to the ground (traction side gets no torque).

If one wheel is on ice with a brake based LSD (ice is extreme version of taking a turn with throttle), the system will brake complete the side on ice stopping the spin, going from 0% to 50% on that axle, allowing the other 50% to go to the side with traction. Therefore, you only get 50% on the side not on ice, half the acceleration power.

Another example: If you are on a turn, where the inside wheel can only apply 20% of the traction available to the ground before losing traction, open diff car will only apply 20% on each side, total of 40%, before the inside tire starts spinning. If the system brakes the inside wheel, the brakes can add up to an additional 30% torque to the left axle, totaling 50% to that side, allowing the full 50% to be applied to the outside wheel. Therefore, your torque breakdown is: Inside Traction - 20%, Inside Brakes - 30%, Outside Traction - 50%, so total power to ground is 70%.

Torsen can do ratios of torque such as 5:1, allowing torque to be sent to the side with more traction up to a certain amount. That means that the inside/less traction tire can see as low as ~15% torque, causing the outer wheel to see 85%, totaling 100%. From what I read about the rear sport diff, in most conditions, it can essentially send all power to the side with more traction, so no torque split. It can send pretty much 100% to one side, maximizing available traction in any situation, and even overdriving the outer wheel, where the inside isn't even at a level where traction is an issue (perhaps causing traction to be an issue to the outside). That's why the rear comes out so nicely on a SD car.

From what I understand, this is why active/mechanical systems are better than the brake based systems. On the turn example, Open Diff would see 40% power to the ground, Brake Based would see 70%, and torsen/clutch/system based would see 100%.

[Works2shoot]

If you drive hard in the corners than yes it is a must have. If you like only to drive fast in a straight line than no it is not a must have.
You don't have to bring it to the track to notice the difference either.
The sport diff pays for itself when you go to trade the car in.
Get an A4 and chip it if you don't get the sport diff. Lighter engine and will have less understeer than the S4 without a sport diff.

You're saying that without a sport dif, the S4 isn't worth owning? Give me a break.

[SteveYem]

Not sure on your conclusion that torque is not lost. Torque is not lost to the axle shafts, but torque is lost to the road with a brake based LSD.

Remember, calculating torque at the road surface is a relationship between the force applied on a tire, and the friction of the ground to move the tire in relation to the ground. An open differential always splits torque evenly between both sides. When one side has a lower available torque to place on the ground before it slips (inside wheel, wet/ice on one side), the torque applied to both sides decreases by that amount. For instance in dry conditions going straight, each side gets 50/50. One side will not see more than 50%. If one side is on a wet/slippery surface where only 10% torque is available to put on the ground, both sides will see 10%, total 20% of the torque. When one side is on ice / 0%, both sides get 0% applied to the ground (traction side gets no torque).

If one wheel is on ice with a brake based LSD (ice is extreme version of taking a turn with throttle), the system will brake complete the side on ice stopping the spin, going from 0% to 50% on that axle, allowing the other 50% to go to the side with traction. Therefore, you only get 50% on the side not on ice, half the acceleration power.

Another example: If you are on a turn, where the inside wheel can only apply 20% of the traction available to the ground before losing traction, open diff car will only apply 20% on each side, total of 40%, before the inside tire starts spinning. If the system brakes the inside wheel, the brakes can add up to an additional 30% torque to the left axle, totaling 50% to that side, allowing the full 50% to be applied to the outside wheel. Therefore, your torque breakdown is: Inside Traction - 20%, Inside Brakes - 30%, Outside Traction - 50%, so total power to ground is 70%.

Torsen can do ratios of torque such as 5:1, allowing torque to be sent to the side with more traction up to a certain amount. That means that the inside/less traction tire can see as low as ~15% torque, causing the outer wheel to see 85%, totaling 100%. From what I read about the rear sport diff, in most conditions, it can essentially send all power to the side with more traction, so no torque split. It can send pretty much 100% to one side, maximizing available traction in any situation, and even overdriving the outer wheel, where the inside isn't even at a level where traction is an issue (perhaps causing traction to be an issue to the outside). That's why the rear comes out so nicely on a SD car.

From what I understand, this is why active/mechanical systems are better than the brake based systems. On the turn example, Open Diff would see 40% power to the ground, Brake Based would see 70%, and torsen/clutch/system based would see 100%.

This is my understanding as well.
From an energy balance point of view, assuming a constant engine torque output- the active braking torque vectoring approach converts rotational energy (torque) to heat by braking before it gets to the road surface, while the sport differential distributes all of the available rotational energy without converting any to heat.

[SteveYem]

You're saying that without a sport dif, the S4 isn't worth owning? Give me a break.

Agreed, it gets blown out of proportion a bit around here. I owned a B8 without sport diff and thought it was a great car. I would still have it if it weren't for some other reasons that drove me to trade in for a B8.5. Sport diff was one reason, but was not the only motivator.

[johnstah]

Get the sport diff. For a stock car with no sport diff, the body roll is very noticeable and you won't enjoy the car when taking a sharp corner, it will not be "riding on rails".

I bought my car without one and I regretted it. I had to upgrade front/rear sways,endlinks and alu kreuz as they made a huge difference in body roll and gripping the road going into a turn.

[anerbe]

This is my understanding as well.
From an energy balance point of view, assuming a constant engine torque output- the active braking torque vectoring approach converts rotational energy (torque) to heat by braking before it gets to the road surface, while the sport differential distributes all of the available rotational energy without converting any to heat.

Yes, brake based reduces total torque to the ground, lost thru braking. You will only see 100% traction forces available going straight with equal traction available both sides. Any other condition (turning/reduced traction surface variation left to right), there will be a loss of power going to the ground. Mechanical/Viscous types see less reduction, active diff technology won't see this issue at all if the more traction side can accept 100% of the power to that axle.

[jygesq]

Get the sport diff. For a stock car with no sport diff, the body roll is very noticeable and you won't enjoy the car when taking a sharp corner, it will not be "riding on rails".

I bought my car without one and I regretted it. I had to upgrade front/rear sways,endlinks and alu kreuz as they made a huge difference in body roll and gripping the road going into a turn.

the "riding on rails " description is accurate. That is how I describe my 2015 s4. To get the most out of this car order:

Sport diff, Tech package & B.& O. and for me S-tronic. If you want 6sp MT, that is your choice, I prefer S-tronic. That discussion has been flogged to death

[anerbe]

Not considering normal losses, the sum of the torque on either side of a diff is always 100% of whatever the input torque is. Also, torque in an open diff goes to the side with least resistance. So if one side is on ice (zero friction) and no EDL or torque vectoring is present, 100% of the torque goes to the wheel on ice and the wheel with grip gets nothing. If the wheel with no grip is now slowed down or even brought to a stop with the help of the breaks, the path of least resistance becomes the wheel with grip, so the torque goes to that, all of it if nothing is going to the slipping wheel.

The crown gear center diff works the same way, except it is self-locking. If the front loses traction, the clutch pack in the center diff engages and locks the diff to send more torque to the rear, but again, torque never gets lost. The sum always adds up to 100%.

That doesn't necessarily mean that the remaining tire(s) manage to transfer all that torque to the road. That's a different issue. If the tire with grip can't transfer all the torque, then that wheels starts to slip as well and then the system starts doing a dance of shifting torque back and forward until both wheels have grip again.

Open diff with brakes will not send 100% of the power to the side with traction. 50% is sent combating a locked wheel from the brakes, other 50% goes to the traction side if all 50% can be applied to the ground. Active Diff can disengage the axle on ice completely, sending 100% of the torque to the traction side. Torsens, by mechanical structure/layout, prevents variation in wheel speed, so it has the ability to go past a 50/50 torque split. You will not get more than 50% torque applied to a half shaft with an open diff, with or without brake LSD.

Some info on diffs:

http://en.wikipedia.org/wiki/Differe...ical_device%29
"section - loss of traction"

http://auto.howstuffworks.com/differential3.htm

There's always equal torque applied to both half shafts.

[superswiss]

Open diff with brakes will not send 100% of the power to the side with traction. 50% is sent combating a locked wheel from the brakes, other 50% goes to the traction side if all 50% can be applied to the ground. Active Diff can disengage the axle on ice completely, sending 100% of the torque to the traction side. Torsens, by mechanical structure/layout, prevents variation in wheel speed, so it has the ability to go past a 50/50 torque split. You will not get more than 50% torque applied to a half shaft with an open diff, with or without brake LSD.

Some info on diffs:

http://en.wikipedia.org/wiki/Differe...ical_device%29
"section - loss of traction"

http://auto.howstuffworks.com/differential3.htm

There's always equal torque applied to both half shafts.

Looks like I got some of my wires crossed. You're correct, I totally left combating the brake force out of the picture.

[anerbe]

No worries, it's harder to conceptualize. Whenever I revisit the topic, I initially get confused on how the system works in my head, before I remember getting this stuff pounded in my brain during BSME classes.