Im running a torsen lsd so there is always torque running to both wheels until one starts to loose traction then more is transfered to the other instantly. So the viscous coupler to the front will not react faster.

As for the open rear diff not spinning on corner exit then the front would not get power, when this happens(and it does) the car tends to push. Its really accentuated in the wet and very unnerving.

 

I understand the purpose and function of the LSD, but I do have questions as pertains to LSD use in torque biasing AWD cars. It is not unheard of for AWD sports cars to not have a strictly mechanical non-electricly controlled LSD. In fact it seems to be more to the norm.

Let me ask this question from a hypothetical perspective. If the center differential that is in the front differential housing is designed to put torque(or more torque for these purposes) to the front wheels when a rear tire spins than wouldn't it be possible that the LSD when combined with the center differential could cause a possible erratic understeer condition or other difficult to predict behavior? So the front wheels get torque to counter an oversteer scenario when rear traction becomes an issue. So if the rear wheels are locking together and the front wheels have torque couldn't this result in a possible understeer situation? Or does the locking of the rear wheels when an LSD is installed stop the front wheels from getting the increased torque due to increased rear wheel traction?

 

I wouldn't say 520 is a ton of power, but that's my opinion

 

False. First of all there is always power going to the front wheels anyways, and quick as the LSD can lock more torque can be transfered to the front wheels. By the time the LSD locks, and regathers traction in the back, more power is going to the front wheels.

Of course this is if you can get the car into a slide with both AWD and LSD.

But this is what I meant by you agreeing with me, EVERY example you have used is under wet conditions and that's the first thing I said in terms of exceptions.

LSD's don't stop wheel spin, they make it manageable and easily correctable. They transfer power gradually because if they didn't then the wheel that had traction would lose traction too and it wouldn't be any good. The point is that you will be using an LSD and AWD to accomplish the same thing because the LSD (even torque biased ones) will allow the car to slide.

If you think I'm lying take your input shaft out of the car and go drive your car hard around a road course and see if what I say isn't right. Power out of a few turns and see if you can't get your car into a controlled slide. Even the Torque based LSD will allow that, and that's when your AWD will increase power to the front wheels.


BTW I've driven every configuration of 996 TT on track.

AWD - No LSD
RWD - No LSD
AWD - LSD
RWD - LSD




I know it's not, what I'm saying is that it's significantly more than what I have. Exactly how much I can't say. I've driven a 600+ whp too and on NT-01's it didn't have much of an issue with traction either. At my power level I definitely don't need AWD and LSD.

 

So the rear of a 996 turbo is just an open diff?

 

Lol. 600hp is the new 500hp. 500hp is soo 2004.

 

I think it's a must do mod for every seriously track driven Turbo.

Adding LSD to AWD Turbo has been a good investment. Better lap times, faster acceleration; not anymore that annoying one wheel spin. Remember, the front wheel drive is not that powerful...it does not help that much. Some said it's approx. 150Nm that is transferred to front no matter the situation, and that could be true...so LSD is needed.

Understeer? You can sort it out with sways. Actually, perhaps one has to be more alert with the rear not stepping out too suddenly with the LSD, but that is not a problem at all. What I mean is that with open diff, the rear does not come loose as suddenly as most of the time it just spins annoingly the inside tire...and that's slow.

LSD was one of the first recommendations by the famous Manthey-Racing when I brought my car there in 2007...but yes, it already had additional mods then.

 

here is good real info from someone who had it done

 

Please explain how there is always power going to the front through a viscous coupling, love to hear that one

Please do also explain how im waiting for my lsd 'to lockup' when its a torsen unit

Im not agreeing with you regarding you wont be able to tell if it has an lsd on the street. Obviously any low traction condition will show its value, but that certainly doesnt mean in the dry you wont notice it.

 

Basic, I have also heard that the minimum power being sent to the front is 5% not zero. And I think it is because it is a viscous coupler that there is always some transfer of power to the fronts. Not sure how true it is. I agree with you, however, that the LSD will react faster than the viscous coupler WITH PSM off. Not sure what happens with psm on.

I have never heard anyone who got an LSD say that it made things worse in any car. Including the person I quoted above. But I don't really know since I am not in that boat. However, D33PBLUE has just that and has had it for sometime and he gets much better traction in all conditions with his LSD and awd than just awd, and he drives like he was on a track all the time....

 

I have heard that as well but dont believe it, the awd system on a 996tt is very simple. A viscous coupling will not transfer any power unless there is a differential of speed across it, ie a rear wheel spinning faster then a front wheel.

My point and the reason I installed a lsd in the rear is based on my track experience and understanding of the viscous coupler. By removing the single unloaded rear wheel from losing traction, via a lsd, and transfering power to the front causing unwanted push. If both rear wheels lose traction then I want power to the front, and therefore benefit from the additional traction.

I have also run this can in rwd with no lsd and its near hopeless and much slower lap time wise to the awd configuration.

I also do plan on trying it again with the lsd in rwd, but fully intend to leave it awd.

 

It's pretty common knowledge that the 996 TT is not capable of having 100% power going to the rear wheels. Neither can the 997 TT (at least the 997.1) and Porsche clearly acknowledges this. Put your car on an AWD dyno and put it in first gear (or any gear) and watch the front wheels try to turn even without any slippage on the rear wheel.

Like I said, this is not speculation, these are proven facts. There is always power going to the front wheels. You can feel it when you are driving, even on the street when it's not that fast. Power understeer has a distinct different feeling from push.

You are correct in that the 996 TT AWD is very simple, and there is doubt about how much power actually gets transfered. But there is no doubt that there is always power to the front wheels, unless your diff is broken.

Some of you guys must have some crappy roads, or crappy tires, my car has never broken loose on the street unless it was a clutch dump or wet, or Cold MPSC. I will admit that I only have probably 5-6k miles on the street on actual street tires though. But the 996 TT is an understeer machine on dry roads unless you have lots of steering input and lots of throttle input at the same time.

The OP is not having traction or wheel spin issues, so without us turning this into a grudge match let's actually stick to the topic. I have the data from every configuration, which I've tried at one track (Barber) and would be glad to post the cornering speeds.

 

The only way that is possible is if porsche designed a slightly different gear ratio across the viscous coupler to induce slip in the unit. Which I can find anything to support that anywhere.

I've searched through the factory manual and it shows that the rear pinion gear is directly connected to the front output shaft.

The front and rear final drives are the same. So I still cant see how any power could be transfered through the viscous coupler until slip happens at the rear. Your example on a dyno where the fronts try to turn is what im talking about, if the rears turn at a different speed then the fronts will try to stay at the same speed.

 

Basic, no offense meant, but you don't understand how the viscous coupling works if you think that power only goes to the front if there is slippage. I recommend you do a search and see. Porsche says there is always a minimum of 5% going to the front, and at 150mph there is about 30%. The viscous coupling works based on internal rotation and heat that causes the special silicon fluid to harden and engage, not slippage.

 

No offense taken, here's a link to read

http://en.m.wikipedia.org/wiki/Viscous_coupling_unit

There must be a difference in rotation, and if it is truely 5% and 30% at the higher speed that would make sense.

Still if a single wheel spins at the rear then more power transfers correct, so limiting that with a lsd is beneficial imo and what I have experienced so far with my car.