[Vudoo4u2]

I'll throw it out there for you, I have an i, Ive driven multiple xi's:

335i is much more...nimble

335xi is much more of a safety net for the avg driver


When it comes to adding a chip, I realize some people see the traction light blinking all day...but I say, learn to drive, learn to slip the clutch properly, and get some legit tires so you can grip the road, and you wont have any issues with traction. Turn the DTC/DSC "off" by holding it down until it gives you the *WARNING EXCLAMATION!* telling you that you're driving at your own risk. The xi might get a better launch for most drivers, but I'll say that in the i, even with a tune, a great launch is entirely possible if you really learn the car. The xi just makes it easier for the avg driver to do it, theres little effort involved.

Learn the car well, and the i will treat you right. In the winter, throw some snows on there and take it easy. The i with good snows is pretty much as good as the xi with all seasons if not better. The true sport suspension makes a big difference in handling feel

Unless you live in a mountainous area, hit up the i, and you wont regret the fuel savings or the fun factor.

If you get an xi, its just an extra safety blanket for poor conditions outside, personally, Id rather have a car thats 200 lbs lighter and just learn to drive better/safer in more dangerous conditions. I dont care about cornering forces when its wet outside, who's running hotlaps in the rain?

[Lassaxi]

Originally Posted by roamer

HOWEVER, the xi is NOT 4wd, it is 2wd which can be allocated among 4 wheels.

I'm not sure you meant to imply this, but I just wanted to make sure everyone is clear that xDrive is not an "on-demand" AWD system, which would be a fundamentally 2WD system. xDrive is a full-time AWD system that continuously shifts torque between the front and rear axles as needed. There are very few instances where the F/R torque split is maintained at 0:100 or 100:0.

[roamer]

I'm not sure you meant to imply this, but I just wanted to make sure everyone is clear that xDrive is not an "on-demand" AWD system, which would be a fundamentally 2WD system. xDrive is a full-time AWD system that continuously shifts torque between the front and rear axles as needed. There are very few instances where the F/R torque split is maintained at 0:100 or 100:0.

What I meant was that the xi system is incapable of proportiong power to more than 2 wheels (1 at each axle) at a time. BMW uses the abs system to selectively brake a spinning wheel so that the open diff sends the power to the other side. I understand that power can be proportioned in varying degrees from front-to-rear, but it cannot proportionately share power side-to-side. This means that you cannot have any more than 2 wheels under power at any point in time. It can move it around quite a bit, but still cannot actually use more than 2 at a time.

[uhaulball]

i have a 330xi. ive also driven a 328i loaner. a very simple difference i can feel is that i love to make sharp turns all the time without slowing down at all. in my 330xi it takes the corner sweet. with the 328i it plows a little and the tire squeaks in the front. i thought i was giving it too much gas but then in my 330xi i never had this issue no matter how much gas i gave.

[silow]

Originally Posted by roamer

What I meant was that the xi system is incapable of proportiong power to more than 2 wheels (1 at each axle) at a time. BMW uses the abs system to selectively brake a spinning wheel so that the open diff sends the power to the other side. I understand that power can be proportioned in varying degrees from front-to-rear, but it cannot proportionately share power side-to-side. This means that you cannot have any more than 2 wheels under power at any point in time. It can move it around quite a bit, but still cannot actually use more than 2 at a time.

You're understanding of open differentials is incorrect. With an open differential total torque applied to the axles connected to the differential is limited by the axle with the least traction. That is why xDrive (and other similar systems) will apply brakes to the slipping wheel to "increase traction" and increase overall torque applied. Also Torque is indeed applied equally side-to-side otherwise you somehow have a differential that is capable of yaw control

Power is indeed apply to all 4 wheels. The purpose of an open differential is to evenly distribute power to both axles while allowing it to rotate at different speeds.

[awdonpsi]

Originally Posted by Mid_West

Have to throw my $0.02 in here since I just read 65 posts!

I have an i and with a tune I get zero traction. I wish I had an Xi...then again Im back to stock and the i is fine for me. So, I agree with the previous poster who mentioned tuning as the deciding factor. Forget about the snow or rain! Its all about power to the wheels. On the i with a tune you will continually do one-wheel-squealies! You will also get tired of seeing the traction light blinking.

Go for the Xi and pick up a JB3, Procede, Dinan or whatever your flavor and enjoy the grip you get at every stop light!

Could not agree more. My car with SSTT goes 0-60 in 4.4, 1/4 mile in 13 flat, and gets 28 mpg on the highway. To me, wheelspin is embarrassing (not to mention ridiculously inefficient) launching from a stoplight. I love being able to floor it with no drama and just be gone and the guy beside me is sweating trying to get the launch just right. I guess I'm just lazy, and I know I'm nobody's race driver, so it works for me.

[Lassaxi]

Originally Posted by roamer

What I meant was that the xi system is incapable of proportiong power to more than 2 wheels (1 at each axle) at a time. BMW uses the abs system to selectively brake a spinning wheel so that the open diff sends the power to the other side. I understand that power can be proportioned in varying degrees from front-to-rear, but it cannot proportionately share power side-to-side. This means that you cannot have any more than 2 wheels under power at any point in time. It can move it around quite a bit, but still cannot actually use more than 2 at a time.

Yeah, this is wrong. What silow wrote is correct. People write all the time that a BMW with RWD really only has "one wheel drive," but that reflects an incorrect understanding of how a differential works. The only time that statement is true is when one of the rear wheels is on ice or in up in the air.

An open differential always sends the same amount of torque to each wheel on a driven axle. If both wheels on the axle have full traction, the L/R torque split is 50:50. If one wheel slips, the torque split is still 50:50, but all of the force coming in from the engine will be blown off on the spinning wheel (which requires very little torque to spin real fast). When DSC applies the brakes to slow the spinning wheel, resistance to wheelspin on the slipping wheel increases, and the differential will send more torque to BOTH wheels.

[roamer]

Addmittedly my statements were an oversimplification, however, when there is an imbalance of traction, the open diff sends power to the WRONG wheel, namely, the one that is already slipping. Further, the design of the open differential is such that it sends power to the wheel with less resistance, which, when you need traction, is the exact OPPOSITE of what you want/need. The reason why open diffs have single-wheel burnouts is because the diff keeps sending power to the axle with less resistance. Once one wheel looses traction, you are sunk (unless power is reduced).

The x-drive system tries to counter this by creating resistance through the ABS system to shift the power over to the other side. In contrast, mechanical LSDs are biased toward sending power where there IS traction and then proportioning it if slippage occurs. The open diff cannot do this, it just spirals downward.

Here is a great link explaining how the systems work: http://www.houseofthud.com/differentials.htm

The operation of the open diffs is evident when you drive an xi w/wo the DTC on. The car has greater cornering capabilities with the DTC on not only because you have computer intervention, but because the mechanical limitations of the diffs are MITIGATED by the tc. However, this is still a band-aid approach as compared to mechanical/hydraulic LSD units. The system's flaw is that it has to REDUCE power to overcome the open diff, rather than proportioning the power.

So can the xi actually send power to 4 wheels, YES, under full-traction conditions. However, once traction is lost at a wheel, the power shifts to that side unless it is reduced to below the threshold of traction. My statements were relative to the extreme traction conditions (>10/10ths).

[Lassaxi]

Originally Posted by roamer

Addmittedly my statements were an oversimplification, however, when there is an imbalance of traction, the open diff sends power to the WRONG wheel, namely, the one that is already slipping. Further, the design of the open differential is such that it sends power to the wheel with less resistance, which, when you need traction, is the exact OPPOSITE of what you want/need. The reason why open diffs have single-wheel burnouts is because the diff keeps sending power to the axle with less resistance. Once one wheel looses traction, you are sunk (unless power is reduced).

Okie dokie, but this is not the same thing as saying that xDrive can only send power to 2 wheels (one on each axle) at a time, which is what you wrote earlier.

The x-drive system tries to counter this by creating resistance through the ABS system to shift the power over to the other side. In contrast, mechanical LSDs are biased toward sending power where there IS traction and then proportioning it if slippage occurs. The open diff cannot do this, it just spirals downward.

It's actually the DSC system that does this, not xDrive. This happens whether you drive an i or xi.

Here is a great link explaining how the systems work: http://www.houseofthud.com/differentials.htm

The operation of the open diffs is evident when you drive an xi w/wo the DTC on. The car has greater cornering capabilities with the DTC on not only because you have computer intervention, but because the mechanical limitations of the diffs are MITIGATED by the tc. However, this is still a band-aid approach as compared to mechanical/hydraulic LSD units. The system's flaw is that it has to REDUCE power to overcome the open diff, rather than proportioning the power.

So can the xi actually send power to 4 wheels, YES, under full-traction conditions. However, once traction is lost at a wheel, the power shifts to that side unless it is reduced to below the threshold of traction. My statements were relative to the extreme traction conditions (>10/10ths).

This is all more or less right, except that a truer statement would be that xDrive sends power to all 4 wheels until one or more of those wheels has zero traction. Since the total amount of torque available to either wheel on a given axle is limited to the amount of torque that the wheel with less traction can accept, both wheels will continue to receive some power until one of them has zero traction.

Also, whether you consider DSC/DTC to be a band-aid approach depends on why you want an LSD. If you want it for performance purposes, then yes, DSC/DTC is suboptimal. If safety is your primary concern, then DSC/DTC is better because its interventions tend to keep the car within its handling limits. An LSD will not prevent you from losing control of your car, but DSC can.

[nicknaz]

People consider a xi BMW should try a WRX/STi/EVO or perspective before making the purchase decision.

front and rear LSD combined with Subaru/Mitsubishi experience at fine tuning the handling balance of AWD car is far superior to BMW IMHO. I think BMW's core competency is RWD NA vehicles

[silow]

Suburu, Mitsubishi, and Audi definitely have more experience in the AWD game and have more sophisticated systems.

Roamer you're explaination is still technically incorrect. Power isn't sent to the 1 wheel that loses traction. Cars doing 1 wheel burnouts still manage to move forward right? Because torque is still equally applied to the wheel that has traction. What really happens is that overall torque is limited by the slipping wheel with an open differential.

You still get individual wheel spin even with mechanical LSDs, although to a much lesser extent. The purpose of an LSD is to limit the rotational difference between the 2 axles thus allowing them to spin at different rates during turns, but locking up once the rotational difference surpasses a specific ratio. Even in this case torque is STILL equally applied side-to-side.

xDrive emulates the LSDs effect by litterally limiting the rotational difference between two wheels with the brake system. Mercedes' 4matic system is even more simple. Their center differential is open and they control front/rear torque bias by modulating the front/rear brakes.

When a system has the ability to vary torque split side-to-side that's called torque vectoring. The only current production system that's capable of this that I'm aware of is Honda's SH-AWD. However all players in the AWD market are now putting R&D into this technology.

[Lassaxi]

Originally Posted by silow

Suburu, Mitsubishi, and Audi definitely have more experience in the AWD game and have more sophisticated systems.

Very true, but it depends to some extent on which car and which system. For example, Subie uses all manner of different AWD systems (viscous coupling, electronic clutch with rear LSD, electronic clutch with LSD and rear bias, and so on) depending on which car you're talking about. If we're talking about a WRX STi, then, yes, that is a very sophisticated, performance oriented system that is tough to beat. But if we're talking about a 5MT Imprezza (which will have a viscous center coupling) or a 4AT Legacy (which will have the FWD-biased system that Subaru calls "Active AWD") I'd probably prefer xDrive.

Ditto with Audi, but it depends on what you want AWD to do for you. From a performance standpoint (i.e., grip around the corners and performance in slippery conditions), Torsen is definitely the way to go, but if my primary concern was being able to get moving from a standstill on ice, an electornic center clutch pack like xDrive (or Subie's VTD system), probably has a slight edge.

Roamer you're explaination is still technically incorrect. Power isn't sent to the 1 wheel that loses traction. Cars doing 1 wheel burnouts still manage to move forward right? Because torque is still equally applied to the wheel that has traction. What really happens is that overall torque is limited by the slipping wheel with an open differential.

You still get individual wheel spin even with mechanical LSDs, although to a much lesser extent. The purpose of an LSD is to limit the rotational difference between the 2 axles thus allowing them to spin at different rates during turns, but locking up once the rotational difference surpasses a specific ratio. Even in this case torque is STILL equally applied side-to-side.

xDrive emulates the LSDs effect by litterally limiting the rotational difference between two wheels with the brake system. Mercedes' 4matic system is even more simple. Their center differential is open and they control front/rear torque bias by modulating the front/rear brakes.

When a system has the ability to vary torque split side-to-side that's called torque vectoring. The only current production system that's capable of this that I'm aware of is Honda's SH-AWD. However all players in the AWD market are now putting R&D into this technology.

On point.

[nicknaz]

Originally Posted by Lassaxi

But if we're talking about a 5MT Imprezza (which will have a viscous center coupling)

I'd probably prefer xDrive.

Like I said try a WRX at the traction limit before making the purchase decision especially an '02 model (which has the same 20mm rear bar as the STi)

IMHO, there are few cars that easy to balance at the traction limit. Throw a mid corner bump into the mix while cornering at that traction limit and I'm pretty sure you'll be a believer. Most people get thrown off by the body roll but the wheel control is fantastic and the long travel suspension is awesome for straightening out an unfamiliar and imperfect back road

I believe the Evolution with AYC can vector the rear torque from side to side

Personally, I find that Subaru's tuning is biased towards loose/imperfect surfaces and a 'finish all rotation at turn in/big throttle way before the apex' driving style while Mitsubishi is biased for smooth tarmac and feels more like a RWD car

[Lassaxi]

Originally Posted by Nick Nazareno

Like I said try a WRX at the traction limit before making the purchase decision especially an '02 model (which has the same 20mm rear bar as the STi)

IMHO, there are few cars that easy to balance at the traction limit. Throw a mid corner bump into the mix while cornering at that traction limit and I'm pretty sure you'll be a believer. Most people get thrown off by the body roll but the wheel control is fantastic and the long travel suspension is awesome for straightening out an unfamiliar and imperfect back road

I've driven one, and, based on the full package, I agree.

[Nikolas]

The Subaru STi is an amazing car. It is a painful daily driver though. I had one for three years. Before that I had an '03 WRX for 2 years. They are great cars, just not so comfortable on a daily basis. If pure performance and AWD are the criteria, the STI or the EVO are the cars of choice.

[roamer]

Silo and Lassaxi-
You guys are getting caught up in your own technicalities. Someone wrote that while one wheel is spinning the other might get some torque. Great, so while you are trying to get power down, 90% (or whatever proportion) ends up turning the slipping wheel and 10% might get shifted to the wheel with traction. How is this an effective means for getting power down?

However, Lasssaxi's statement that the DTC system is subobtimal for performance driving is my entire point. The DTC-open diff systems on the 335 are subobtimal, which was my reason for stating that the 335xi is superior from a traction standpoint to an i, but the difference lessens when a LSD is added to an i. Add to that the i's weight advantage and better suspension and the i ends up being the better performance platform.

On a somewhat related note, does anyone have concerns about lowering an xi? It seems odd to me that bmw chose not to. Is there a potential for bad driveline angles or too much stress on the front seals?

[Nikolas]

Originally Posted by roamer

Silo and Lassaxi-
It seems odd to me that bmw chose not to. Is there a potential for bad driveline angles or too much stress on the front seals?

This is an issue with any car xi or i. That said, if done properly, it should not be a problem. Simply slamming the car on lowering springs will 99.9% of the time decrease the cars performance capabilities.

I also still hear a ton of speculation with regards to the performance difference between the i and xi. Has there been any proof yet to date? or is everyone still in speculation mode? I know this was argued in the past between the infiniti G35S and the G35X until it was actually tested and to everyone's surprise, the X outperformed the S in handling and lap times. I'm not saying the xi is faster than the i, I'm just looking for some real world comparison that actually shows an i can outhandle an xi.

[Lassaxi]

Originally Posted by roamer

Silo and Lassaxi-
You guys are getting caught up in your own technicalities. Someone wrote that while one wheel is spinning the other might get some torque. Great, so while you are trying to get power down, 90% (or whatever proportion) ends up turning the slipping wheel and 10% might get shifted to the wheel with traction. How is this an effective means for getting power down?

I agree, it's not, and I don't have any issue with the ultimate effect of what you're describing. The only place where you're getting caught up is when you're trying to explain why this is not an effective means for getting power down. You keep saying things like "So can the xi actually send power to 4 wheels, YES, under full-traction conditions" and "90% (or whatever proportion) ends up turning the slipping wheel and 10% might get shifted to the wheel with traction."

All we're trying to tell you is that an open diff sends the same amount of torque to both wheels on a driven axle. You're right that the practical effect of that is that when one wheel spins, the other wheel doesn't get as much power as it needs. But that does not mean, as you wrote earlier, that "the xi system is incapable of proportiong power to more than 2 wheels (1 at each axle) at a time." That's all we're saying. Otherwise, I think we're all in agreement here.

On a somewhat related note, does anyone have concerns about lowering an xi? It seems odd to me that bmw chose not to. Is there a potential for bad driveline angles or too much stress on the front seals?

for this one. There are various theories about why the xi models are higher than the i models. The theories range from snow clearance issues, to concerns about undue stress on the front suspension, and other things. Nonetheless, many people have successfully lowered their xis, although some have reported uncreased vibration and/or harshenss after doing so.

[M4TW]

You can get along just fine in the winter with the RWD i and a good set of snows. You can also get along fine in the summer with a Honda Civic and a good set of summers.

If you want to do more than just get along fine but would also like to have FUN, and four seasons of it, the xi is your ride.

[M4TW]

Btw, I'm picking up my modestly lowered (KW var. 3) 335xi from the shop today

I think the xi one-size-fits-all stock suspension is simply a product of economics and (as much as us xi aficionados hate to admit it) BMW SUV pedigree.

[T Bone]

Just get the XI, it has one more letter so it must therefore be better

[silow]

Originally Posted by Lassaxi

I agree, it's not, and I don't have any issue with the ultimate effect of what you're describing. The only place where you're getting caught up is when you're trying to explain why this is not an effective means for getting power down. You keep saying things like "So can the xi actually send power to 4 wheels, YES, under full-traction conditions" and "90% (or whatever proportion) ends up turning the slipping wheel and 10% might get shifted to the wheel with traction."

All we're trying to tell you is that an open diff sends the same amount of torque to both wheels on a driven axle. You're right that the practical effect of that is that when one wheel spins, the other wheel doesn't get as much power as it needs. But that does not mean, as you wrote earlier, that "the xi system is incapable of proportiong power to more than 2 wheels (1 at each axle) at a time." That's all we're saying. Otherwise, I think we're all in agreement here.

Well said. I don't like open diffs at all, but saying that any AWD system with open diffs only powers 2 wheels is completely wrong. AWD with open diffs will still have better straightline traction than WWD, wrong wheel drive (a term we use in the DSM world ), equipped with a LSD - on a normal car that is, don't go comparing top fuel dragsters :P

If you live where it snows then you might want to consider an XI. You can make do with good snow tires with RWD but if you've ever driven in the snow with AWD it's totally different. From a performance standpoint xDrive definitely isn't a performance AWD system like Mitsu's. However I was sold on AWD when I first took out my Eclipse GSX in 15 inches of snow.