Why don't most Porsches have Limited Slip Differentials?

[Default]

I can never understand this. Cars like the Corvette and the Honda S2000 come standard with an LSD, while Porsches like the 911 Turbo, the new Carrera/Carrera S, and most Porsches in general don't have one. I'd expect to have an LSD if I was paying upwards of $75,000 for a car, does anybody know why only top-notch models like the GT2 and GT3 have an LSD?

 

[MrElussive]

I thought they do have a Limited Slip Differential? It doesn't matter so much on the 911 Turbo because of the AWD, but I was under the impression that RWD Porsche 911's come with the LSD.

 

[Default]

Nope, they don't. The only ones I see are the Cayenne, the GT2/3, and the GT. I don't understand it. Maybe it's because they have disgusting amounts of grip?? Still though, I'd want an LSD for that much money, that's why I don't think money is the issue it must be something else. Porsche should easily be able to put in an LSD, the only thing I've heard is that they have compatibility problems with PASM (Porsche's stability management).

 

[Average Jae]

do they need it? this is the first time i heard about handling issues associated /w porsche, cept for having an exceptionally heavy ass which kills usually handling charas.. even improved w/ 997, having 61% bias in the rear would take some getting used to imo.

so my point is, if the car has no handling issues, why should it have lsd? not too long ago, most cars survived w/out having electro gadgets telling you and the car how it should drive, and drivers usually got by w/out too many issues. iirc.

 

[codex57]

that's why? The 3 series doesn't have one either right? Haven't checked, but it sounds like cars with electronic traction control (least the sophisticated ones) don't have LSD. Aren't the purpose of both pretty much the same thing? To get traction from both wheels?

 

[epj3]

The 911 turbo is all wheel drive... which is basically 3 limited slip differentials. Not quire sure what you expect, the traction control acts LIKE a limited slip by applying brake to the slipping wheel so the other wheel will have a little torque.

 

[jrt67ss350]

Eric's right. Modern traction control emulates a LSD but with more control. A Porsche with 61% rear weight bias and an LSD would be nearly impossible to drive in the rain or snow - the back end would be coming around on you constantly.

LSDs are best for straight-line performance. They can be a detriment in the twisties, especially if the conditions are slippery. That's the beauty of modern traction control - you have the easy driveability of an open differential when you want it, but the system "creates" a tuneable LSD when you need it. A physical LSD is not tuneable or changeable - its always working, even when you don't want it to.

As a bonus to the manufacturers, it's cheaper to add a few electronics to utilize the existing ABS system and engine management to alter power flow than to build an LSD.

 

[Chesty Bonds]

that's why? The 3 series doesn't have one either right? Haven't checked, but it sounds like cars with electronic traction control (least the sophisticated ones) don't have LSD. Aren't the purpose of both pretty much the same thing? To get traction from both wheels?

Traction control doesn't really operate in the same way as a slippy diff. Traction control and stability control systems will prevent slides by curtailing power and braking individual wheels whereas an LSD will partially lock the diff so as to gain extra traction out of turns and to hold a more predictable slide.

It's this last point that really separates an LSD from traction and stability control systems. I don't know if you guys have kicked the tail out in an open diff car and a car with LSD but the difference is amazing. The open diff means that during a power-slide situation it will send all the power to the inside rear wheel so that all you get is a shitload of tyre smoke and a very snappy recovery from the slide.

The LSD makes the slide far more progressive and predictable due to the fact that all the power is being split between the two driven wheels, as opposed to frying the inside one.

I dunno if i've explained this too well but basically i'm saying that stability/traction controls and all the associated driver-inhibiting technology are NO substitute for an LSD. This is why the M3 has an LSD as well as all the stability gadgets that we've come to expect from cars these days.

 

[jrt67ss350]

Traction control doesn't really operate in the same way as a slippy diff. Traction control and stability control systems will prevent slides by curtailing power and braking individual wheels whereas an LSD will partially lock the diff so as to gain extra traction out of turns and to hold a more predictable slide.

It's this last point that really separates an LSD from traction and stability control systems. I don't know if you guys have kicked the tail out in an open diff car and a car with LSD but the difference is amazing. The open diff means that during a power-slide situation it will send all the power to the inside rear wheel so that all you get is a shitload of tyre smoke and a very snappy recovery from the slide.

The LSD makes the slide far more progressive and predictable due to the fact that all the power is being split between the two driven wheels, as opposed to frying the inside one.

I dunno if i've explained this too well but basically i'm saying that stability/traction controls and all the associated driver-inhibiting technology are NO substitute for an LSD. This is why the M3 has an LSD as well as all the stability gadgets that we've come to expect from cars these days.

You are right about the differences between hanging the tail out on a car that has an LSD and one that has an open diff. However, the LSD was not originally developed so that we could hang the tail out predictably. It was designed as a form of traction control long before engines were computer controlled and we had the electronics and ABS systems to provide electronic traction control and stability control.

Modern stability control systems are replacing the need for LSDs - like I said, they are cheaper and more tuneable.

 

[Chesty Bonds]

I can see what you're saying Justin but I'm of the opinion that there is a difference between traction control and a mechanical system (ie: an LSD) that seeks to increase a cars ability to find traction. Traction control seems to work aginst the driver IMO primarily because they're far too intrusive. They have an uncanny tendency to nanny the driver to excess. Again, this is all my personal opinion.

An LSD, on the other hand, seeks to increase the driver's ability to move quickly over roads due to the fact that it gives more grip from standing starts (like you said) and when powering hard out of bends. The traction and stability systems generally slow the drivers time over a particular stretch of road (don't hit me with the fact that the F1 systems make the drivers quicker, that's different) because they're so intrusive.

I guess what I'm trying to say is that I think the slippy diff and the traction controls come from ideologically opposed origins.

 

[epj3]

I can see what you're saying Justin but I'm of the opinion that there is a difference between traction control and a mechanical system (ie: an LSD) that seeks to increase a cars ability to find traction. Traction control seems to work aginst the driver IMO primarily because they're far too intrusive. They have an uncanny tendency to nanny the driver to excess. Again, this is all my personal opinion.

An LSD, on the other hand, seeks to increase the driver's ability to move quickly over roads due to the fact that it gives more grip from standing starts (like you said) and when powering hard out of bends. The traction and stability systems generally slow the drivers time over a particular stretch of road (don't hit me with the fact that the F1 systems make the drivers quicker, that's different) because they're so intrusive.

I guess what I'm trying to say is that I think the slippy diff and the traction controls come from ideologically opposed origins.

My friends E36 with an open diff and traction control was just as fast as my E30 with a limited slip, and he's got a beat suspension... I really don't think its as intrusive as you're making it out to be. Traction control on my parents cars, I believe, just limited the engine speed, but smoe other manfacturers do that AND control the brake system - such as BMW.

 

[Chesty Bonds]

My friends E36 with an open diff and traction control was just as fast as my E30 with a limited slip, and he's got a beat suspension... I really don't think its as intrusive as you're making it out to be. Traction control on my parents cars, I believe, just limited the engine speed, but smoe other manfacturers do that AND control the brake system - such as BMW.

An E36 is newer than an E30 so of course it will be faster.

I really believe that the traction control is too intrusive. For example, my mum's car is a 325i manual and sometimes when i drive over intersections the traccy light flickers madly on the dash. When I do a quick start in it with the traccy on it just kills the engine power and so my start gets bogged down. When it's off i can get the optimum amount of wheelspin so allow me to have a quick takeoff.

And if it had a slippy diff it'd be even quicker.

 

[BMW-FW26]

I bet the porsche uses an electronic LSD instead of a mechanical one. My buddy's friend has a 02 911 CS and he can smoke both tires. It wouldn't make sense for porsche to make such an expensive car without some type of Limited slip style transaxle. Even if it doesnt have it on standard, I bet everyone has the option for it.

 

[MrElussive]

Actually, in a performance car, an LSD is still something important. Traction control cuts off power and applies brake and all that BS that slows you down. An LSD is mechanical and helps put the power to the ground (to both wheels evenly). It also helps out with turning and accelerating, but that's also where it can work against the car. If you're going through a turn in a RWD car, you'll be in a lower gear and accelerating through the turn. If you apply too much throttle, the tail will swing out on you, putting you into a power-slide or a spin...just like what happened to me!

 

[Default]

I think that's what PASM is, the simulated LSD thing. Can this actually work as well as a real LSD?

 

[epj3]

I think that's what PASM is, the simulated LSD thing. Can this actually work as well as a real LSD?

I highly doubt it - traction control slows ONE of the moving wheels - where as a limited slip will just transfer lost power to the other wheel - so all power possible goes through the wheels. I'm sure the traction control looses a ton of power.

 

[Chesty Bonds]

Actually, in a performance car, an LSD is still something important. Traction control cuts off power and applies brake and all that BS that slows you down. An LSD is mechanical and helps put the power to the ground (to both wheels evenly).

That's exactly what I was saying. You can't substitute an LSD for traction control, that's why most performance cars still have a slippy diff.

 

[Chesty Bonds]

I highly doubt it - traction control slows ONE of the moving wheels - where as a limited slip will just transfer lost power to the other wheel - so all power possible goes through the wheels. I'm sure the traction control looses a ton of power.

The Porsche Stability Management (PSM) can monitor all four wheels at once. PSM is different from traction control; it's similar to BMW's DSC because it can cut engine power and brake individual wheels before a slide gets out of hand (at least, when the computer thinks it's out of hand!).

 

[Average Jae]

after reading above posts, i remembered something. lsds are usually found in awd cars right? to provide the wheel w/ best traction all the power to get out of a situation. w/ a rwd, or fwd cars, is that as necessary?

the reason why i bring that up is back when last gen preludes came out, their SH model had what they call ATTS, active torque trasn system or something.. (dun know the acro, or if that' was even the name of the system.) and it was at first one of thsose sweet, you have to have it, features. it transfers all the power to the wheel w/ most traction. anyway when i read the review on it, basically the only time you can really benefit from the system is when the car is really pushed to the limits at the track, and in everyday driving, it didn't really do much to help. in fact i remember reading that it had some negative aspects to it also.

honestly, i don't know too much about it. what i know is, whether porsches have it or not, it still performs exceptionally well. so maybe they have something else in place, or it could be like the V-tec: honda promoted it, saying how great it is then showed it. now everyone has their variant, but it's not a huge marketing point anymore. (mainly because everyone has it.) so maybe porsche just isn't saying much about it?

and you guys are right. lsds trasnfers power, traction control limits the power, by engine management and controlling individual brakes.

 

[epj3]

The Porsche Stability Management (PSM) can monitor all four wheels at once. PSM is different from traction control; it's similar to BMW's DSC because it can cut engine power and brake individual wheels before a slide gets out of hand (at least, when the computer thinks it's out of hand!).

Thanks, i've been on their websites.