Is a limited slip differential something that I would really need in the trans, and what are the benefits of having one?

Depends...

LSD really makes a difference on the track. TBD on an autocross course. If neither are your thing, your fine with an open diff....

Don't do either.

Will it make a noticeable difference in slinging around corners, my favorite thing to do, if I don't have it?

Only one wheel spins with open diff. Not a problem unless you have the HP to break that tire loose. Not a big deal unless your racing.

For normal street use, a limited-slip differential improves traction in rain, snow, or on roads with ice, sand, dirt, mud or gravel.

A torque-biasing differential (TBD), like a Quaife or Guard, is a cheaper alternative and probably all you need for street driving.

Jon B.
Vista, CA

Is that what would normally be in a trans, the TBD version?

No, it would have a "standard" or "open" differential as original, unless ordered new with an LSD or replaced in the past 40 years.

Jon B.

So a TBD is a cheaper version of a LSD?

Not much cheaper... Part alone is about $1200

So is this supposed to work with the renegade conversion you are thinking about in another thread? If so tire spin comes into play... Good advice above, but you should mention your goals and intentions, otherwise everyone responding is shooting in the dark...

A new TBD is less expensive than a new LSD.
Guard price list from '08 listed 914 TBD at $1635, LSD at $2395, to give you some idea of difference.

An LSD for a Porsche uses clutches and ramps to effect an adjustable percentage of lock. Modern versions can be adjusted for percentage of lock on power-vs-decel situations as well.

A TBD is a torque-sensitive differential that uses a series of spiral gears, not clutches, to distribute power to wheels. TBD's are more energy-efficient, but not adjustable and have no effect on decel, only power.

Jon B.

For renegade conversions, you need to check torque rating of any differential you might want to install.

Jon B.

Renegade LS conversion.

On the mountain roads, an open diff is safer.

I can pitch the car sideways just fine with an open diff on power in my v8. Its not nearly as good as an LSD or locked diff but it surely is nice.

On the track where you want to be at 10/10ths all the time a LSD is the best.

On the mountain roads where you just want to go fast, turn well and have enough fun, but be safe enough to get to your next destination, an open diff is better..

Unless your a really good driver... Then ya.. LSD (But you wouldnt be asking if you were at that level, you would know...)

Interesting as I would disagree, but that may be based on driving style and amount of horsepower. If you get sideways, an LSD saves your bacon by keeping the car better in control than an open diff. Additionally, depending on the type of LSD, you gain better control under decel.

In my experience, a LSD sucks in all of those scenarios. One of my cars is an old Supra with limited slip. It's nice for straight line take-offs but down right scary when turning on wet roads. I wouldn't let the new driver in our family use that car in the rain. Certainly not on ice.

If you get an LSD or a TB diff, you might as well get a 915 or 930 with the LS motor. You are mentioning wanting to throw the car around and the gears in a 901 are not going to like that for very long. Its like when, not if you break something in the gear box.

The reason an LSD cost is not just the purchase but the maintenance. The TB diff does not need periodic rebuild, the clutch type LSD does. You wear the clutches out and they stop working at the rated bias. They are just like the clutch disk in your clutch from the engine to the tranz, but less robust as they are small and there are a few of them to make up for the surface area needed to do what they are designed to do. If you race, depending on your skill, power, and schedule, you can see a rebuild of the clutch package ever race or every other race. In a road driven car, TTO is more variable, but still inevitable. There are many driving around with LSDs in their cars that dont even know that the clutches are shot and it is basically an open diff at this point.

I dont think this is a good option for what you are trying to do unless you do it correctly with a different transmission.

a v8 car... well you have a car that has a propensity for throttle lift over steer, now if you also install a device that will also give it a power on over steer you will be on a knife edge on loss of control.

doesn't seem like a good idea for a street car and might make the car something you wont really enjoy driving after a while.

Exactly. In situations where the back end could come out, LSD will put the back end out faster and further, giving you more chance to spin out. In an open diff, instead you get one tire spinning out moving the back end out while the other tire fights to try and regain control.

Again its not the FASTEST way around, but it certainly is the safest. and with a v8 there is pleny of power to still have fun.

I'm sorry, this is completely wrong.
Increased rear traction lessens oversteer, induces understeer.
This applies to both power and decel situations.

Jon B.

The are situTions where the LSD is not the ideal diff for track work. The LSD is
Superior on a track 911 but with the more balanced chassis of a mid engine car there are times that a mid engine car with less hp is going to be faster with a tbd

And for a high hp car the spool is also at times superior to a lsd

Currently have a 935 transmission here to rebuild, photo below.
Should have titanium spool, haven't opened yet. Big Ti axle flanges.
Car understeers tremendously on slow courses, so have to steer with throttle.

Increased rear traction induces understeer.

Click to view attachment

Jon B.
Vista, CA

Within frictional limits, you are absolutely correct. Outside of that, both rear tires lose traction which induces oversteer. You won't see any drift cars out there with an open diff.

Yes, with enough power to exceed tire adhesion you can induce oversteer, as I implied with the 935. But any Porsche with an open diff will oversteer more, or sooner, than one with an LSD.

I'm not involved in the drift car world, very different, but a Porsche "drift car" with an open diff would drift or oversteer sooner, be going slower, and have less control of handling with throttle.

Jon B.

I raced w2w with a locked diff in a 914 for multiple years. The driver does have to adapt a different driving style. But it was still faster than an open diff

I currently do w2w with a tbd
Paul guard and my race shop oth supported that a 914 is not the same as a 911 and driven correctly could benefit from a tbd over an LSD

is the op asking what would be good for a street car or a race car??????

all this "race car" advice is useless for a high powered street car that will deal with adverse road conditions both in wet a dry streets.

going straight down the road is all good with a tbd/ld/spool but pulling out in traffic, accelerating while turning and meeting an unexpected compromised driving surface will make the car much worse as far as road manners.

iv'e been btdt with a 400hp 2800lb car as a kid. I would single stripe all day long now over going to traction aid on a street car.


as far as straight line acceleration Andrew might be able to comment with to back this up , but with mid engine irs car like the 914 the traction available is pretty damn good and both tires will bite hard because there is not the torque reaction to the chassis like in a live axle car.

Sorry, not true.
Race cars also drive in rain, adverse road conditions, all things you mention above.
More traction = more control.
However, car and driver might need appropriate adjustments.

Jon B.

In some cases this is true.
BTW for OP, TBD's usually have lower torque ratings than LSD's.

Jon B.

Well,

excellent advice - you have better run over to France and only tell a few race team so they can win next years 24

oh that's good... than we should all be driving on slicks then too? more traction = more control... we will of course all need to adjust to driving on slicks in the wet and dirty road surfaces.

Uh, no, we should not all be driving on slicks, even if DOT legal which they're not.
Poor water dispersion in rain = less traction = less control.

Jon B.

dont waste your money. the car will always handle better with an open diff. If youre lighting up the inside tire on corners then you need something. But iv heard even with a v8 914s dont have much of a traction problem

lets let the people who have direct experience with the v8 cars make the valid comments here....

Low speed corners the v8 will most certainly spin some sticky street tires. I have 285's in the back and I can spin them on just about any corner under full throttle. But... THats a TON of torque and thats just not how you drive them.. since they have so much power you need to modulate the throttle out of the corner, Same as you would a LSD.

Going straight with small tires you can spin them pretty easy, larger ones and ~ 350hp/tq not so easy.

I can tell you this. I wouldnt take back country roads in my v8 with an LSD, no matter how much i've drifted it.

Also, I've been sideways in LSD equipped 9148's (two separate ones) at over 100mph... One of them was Chris Julians old white super widebody with 550hp and slicks... His slicks were cold and he punched it in 1st through 4th going sideways the while time.... The other one was a 400hp v8 with street tires, Coming round a pretty hot corner he got on it and it snap oversteered. Had a tank slapper until he could wrestle it down.

Downright scary when they let go. Its like an on off switch.

Follow-up photo, in case anyone was interested.
Titanium spool from the 935 transmission...

Click to view attachment

Jon B.
Vista, CA

From Direct Experience...

My 914-6 had an open diff in it. It would oversteer badly under power, because I was losing 50% of my traction on the spinning wheel. Putting the Torque biasing diff in it pushed it from oversteer to understeer. I took out the stiffer front torsion bars and put stiffer springs in the rear. That brought real close to neutral, with a slight understeer.

Up until I changed my rear tires due to an off track excursion, I could make the back end step out into power oversteer as needed. Pivoting for a corner in an AX was easy because I could control it with the throttle.

Now I have a mismatched compound front to rear, so the harder compound fronts make the car understeer badly.



For street and AX use, you will see improvements with the Torque biasing diff. It will also help with wet weather traction and overall control.



~~~from Guard Web site~~~~~~~~~~
Generally speaking, this is what most customers opt for:

(a) Street / autocross - Torque-biasing

(b) Street / track - Torque-biasing if light-duty track use, with stock suspension. 40% limited-slip if car has stiff suspension or power upgrades.

© Track only - 80% limited-slip differential or spool.

Limited-slip differentials provide lock-up on both acceleration and deceleration. The amount of lock-up on accel and decel can be adjusted by selection of the internal plate sequence. Lock-up on deceleration allows aggressive entry into a turn and late braking, reasons why all Pro race teams (that we are aware of) utilize LSDs, rather than TBDs.

Torque-biasing differentials provide lock-up on acceleration only. The amount of lock-up (5-80%) increases as the amount of torque increases. On deceleration, lock-up is negligible, making the TBD the ideal diff for the slower speed turns of autocross. (In the same slower-speed turns, an 80% LSD would most certainly cause understeer.)

A torque-biasing differential performs like an open diff whenever one of the two drive wheels lifts off the ground. Lock-up and traction are lost until both drive wheels are again planted firmly on the ground.

There seem to be two very distinct groups, one favoring the use of LSD, and the other favoring the use of TBDs. We at Guard Transmission try to avoid making the decision for the customer. We would rather our customer confer with the race shop or transmission builder performing the installation, in order to make an informed choice.
~~~~~~~~~~~~~~~~

Excellent summary.

Jon B.