Limited Slip Differential (LSD)

Which drivetrain to choose, so many choices, 4-cyl? 6-cyl? NA? Turbo?
Midlana1
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Re: Limited Slip Differential (LSD)

Postby Midlana1 » 26 Sep 2013, 17:36

Hope you also kept the wire harness!

bgkast
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Re: Limited Slip Differential (LSD)

Postby bgkast » 26 Sep 2013, 20:14

Nope. :mrgreen: I think I got $300 for the computer and wiring harness. I guess Neon folks need them to swap SRT4 engines into 1st gen cars. For my Midlana I got a megasquirt harness from DIYAutotune and plugs that match the engine sensors and injectors for free from the junkyard.

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Re: Limited Slip Differential (LSD)

Postby AndreasMergner » 27 Sep 2013, 03:34

The Racelogic TC has more than one different setup so you can have one for race, one for safe every day driving, one for when you let others drive it. ;) It has a bunch of fine tuning available so you can really tune it to how you want it. It looks like a really good system from the reviews I've read. You can also have it keep RPMs at a certain level for shifts when the clutch is depressed. That way you can floor it, push the clutch in, then shift and release the clutch without ever needing to release on the throttle. Oh, and there is a launch control too so you have RPM control + TC. The only downside I see is that it is $2k! :shock: I'll see what I can do to put it in my own DIY ECU.

Bryan, what parts did you get $2500 for? Sounds like the body might not have been too good if hit from front and rear.

Copart in NY requires a dealer license so you have to pay $200 dealer fees...plus the car needs to be towed adding at least another $100 since my class I trailer won't work. So there is at least $700 of fees on top of a bid.

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Re: Limited Slip Differential (LSD)

Postby roy928tt » 28 Sep 2013, 05:18

If I may add this observation. My race car, which had the drivetrain from a 3.5 litre Mitsubishi Diamente at the rear. Had no LSD and on the dirt circuits that I raced on, had fabulous drive traction. I didn't believe an LSD or locked diff was required and felt certain that it would induce understeer. Indeed even conventional layout vehicles with locked diffs suffer from understeer induced by the diff. A good chassis/set up reduces the requirement for an LSD.

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Re: Limited Slip Differential (LSD)

Postby bgkast » 28 Sep 2013, 11:05

AndreasMergner wrote:Bryan, what parts did you get $2500 for? Sounds like the body might not have been too good if hit from front and rear.


Major parts (over $100) that I sold were wheels/tires ($280), seats ($320), dash board ($100), rear disk brake setup ($175), suspension parts ($179), side skirts ($240), scrap ($166), computer and wiring (guess I only got $200 for that). There was also numerous lesser items that I sold. I can send you the spreadsheet if you want. Factory "tuner specials" usually will have some special parts beyond the power train that can be sold for higher prices than parts from standard models. All of the parts listed except for the scrap and the dash are SRT4 specific parts.

Now back to your regularly scheduled LSD discussion. :D

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Re: Limited Slip Differential (LSD)

Postby WickedOne » 28 Sep 2013, 11:28

bgkast wrote:
AndreasMergner wrote:
Now back to your regularly scheduled LSD discussion. :D


I haven't had a regularly scheduled LSD discussion since the 70's. :shock: Did I just say that out loud?

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Re: Limited Slip Differential (LSD)

Postby Midlana1 » 07 Feb 2018, 13:45

Noticed this old thread as just today I ordered parts for a new transmission. One of the changes is going from a WaveTrac (a type of Torsen gear differential) to an OS Giken clutch-type limited-slip differential. Virtually everything I read claims that they greatly improve lap time, and probably make you taller and more handsome to boot, which is a good thing because it certainly makes you poorer. I'll update this after I've built some time on it, but the consensus appears to be that they're about as good as it gets on-track. That said, for the street, an open diff is absolutely fine as long as you don't have much power.

Over the years I've heard persistent rumblings that a clutch-type LSD is faster (by "some" amount) on-track than a gear-type differential. As part of researching the new transmission build, I searched car forums for data and it seemed to back up the claim. In terms of production cars, probably the closest there is to Midlana in terms of weight distribution is the Porsche 911. Once that was realized, I trolled through their forums and came up with some informative posts.

First off, For rear weight-biased cars, a generic clutch-type differential isn't the best because it's typically tuned for front-engine cars. In the case of FWD cars, traction decreased under power. With rear-wheel drive, it increases. With a mid/rear engine placement, traction increases with power. For this case, the differential needs to be configured backwards from "normal" in what's typically called "reverse 1.5-way", where it locks up more in braking than under power. The latter part of that is because since under power, weight transfers to the driven tires, increasing grip and lessening the amount of required lockup. The biggest claimed improvement over a gear-type differential is its gradual lock-up under acceleration, and nearly as important, it maintaining a partial lock under braking, something the TBD doesn't do. The tail-heavy Porsche guys say that's a really big deal and said it greatly helped stability in corner entry braking and partial throttle mid-turn (I'm still trying to understand why that is, but okay). They specifically say how getting off the gas mid-turn is a bad thing with a

In these posts below, "TBD" stands for Torque Biasing Differential, which include Torsen, Quaife, and WaveTrac. In no particular order:

Only thing I would add to what others have said is that the performance of a particular differential in a chassis other than a 911 pretty much never crossses over to how it will perform in a 911. We need to remember that this is pretty much the only car in the world with a rear engine, RWD configuration. That makes for very unique chassis behaviours and requires differential set up specific to that configuration. Put a LSD that is tuned for a Front Engine RWD car into a 911 and it's not going to drive correctly. FE RWD vehicles historically tune their LSDs exactly opposite of how we tune 911 LSDs.

BMWs, Mustangs, Camaros, Mazda RX7's,8's and MX5's all go with more lock on throttle than on braking. They might run a 60/40 configuration. 911s prefer it the other way around, 40/60, with less lock on throttle because the rear engine design does offer such phenominal on throttle traction. The weakness of the 911 chassis is all that weight out behind the rear axle makes for a certain pendulum effect and a rear end that feels like it wants to walk and step out on you. A lot of braking lock up remedies that and settles it down.

However, when you do that, you make the car want to push on corner entry. This is why the auto-x 911 racers prefer the TBD. It has no lock under braking. In fact, good autoxers use that rear engine weight to toss the car coming in and get the car rotated way early, like a rally car, so that when they put the loud pedal down, the car is already straight and going right where they want it to go. I prefer a TBD for auto-x, though I can, and will tune an LSD for people that would prefer to have one. That's usually the guy who does both auto-x and DEs. The dedicated auto-x guys almost exclusively run TBDs.


1) - OS Gicken makes diffs for the 915. I've got one in my 2.8 liter track car, and one in my stock engine 3.0 liter track car. Had them for a couple of years, and they work fine, but obviously some have had other experiences. Perhaps OSG made some improvements since they started? No flies on the Guard stuff, though.

2) I ran a Quaife for years on my street/DE 911, largely because I thought I was spinning a wheel on the exit of just one track out of the five or so we could drive in our region back when. Probably was my imagination. But for sure it is transparent. But the fact is neither of my cars are really powerful enough to spin rear tires unless you are silly enough, perhaps, to dump the clutch starting out, or there is some sand or water on the track. I'd sure vote for that type of diff for a street car which sees the autocross.

I also ran a Quaife for years in my track only car with 220 rear wheel HP and a dry weight around 2,000 lbs. Can't say as I ever experienced any issues with it, but I succumbed to the siren song of the clutch type and its deceleration bias potential to allow deeper braking with the rear end still in line. I didn't have issues that I was aware of in that regard, but I may not have been braking as hard as the tires would stand, or otherwise was oblivious (that, sometimes, I know to be the case).

So I'd say a Torsen type would be the way to go for anything other than a car set up to optimize its track performance. Perhaps with newer, higher HP, 911s, and the new mid engine cars, this doesn't hold. Don't know. But for the cars which came with the 915 transmission, that's my experience.

Did someone say they had driven a clutch type LSD and had no push? Two ways to look at that - one is that when the clutches and things wear, the locking gets reduced. The other is that if you set the diff up with a high bias toward deceleration, because your car isn't apt to spin its wheels, then you aren't going to feel much push on acceleration. I'm not an optimized autocrosser (in several dimensions), but the decal biased LSD I have doesn't seem to be a problem. And for both cars when I track them, I can turn in to a tight corner, lift, rear end starts coming out, and put the right foot down when the nose is pointed in what will be a nice arc clipping the apex and kissing the track our rumbles. Or at least I can do that when I get things just right.

So I'd say an LSD set up with a high bias for decal ought to be good for the track, and work OK in autocrosses. It is easy to push in an autocross no matter what you do anyway, with slowish speeds and tight turns, and no safety reason not to try to make a perfect skid pad radius for most turns.

I'd still get the Torsen type for a dual Autox/street car.


I'm lucky enough to have two Carreras, an '87 and '88. I race the '87, and it has an LSD, the '88 I drive daily, but I have tracked it many times, without LSD. There is definitely a difference. The LSD creates a much more stable car under high speed braking at turn in, and tends to fight oversteer once you're back on the throttle, resulting in more confident cornering.


In a lower HP car like a 3.2 Carrera it might help most for braking. This allows the front corner weighs to be equalized which usually means the rear might not be. This helps the front work harder with out one wheel lock up and the LSD keeps the lesser balanced rears from one wheel lock up.

A strong LSD can also take some of the throttle braking over-steer out making the car more stable with changes in throttle input.

The last thing a LSD can do it help with wheel spin out of tight corners. On a low hp car this becomes more important in AutoX than most track conditions.

For the track a clutch type LSD is preferred. Think the hot set up is something like a LSD set up for something like 60% lock up on de-accel and 40% on accel but not sure.


Keep in mind that there's always the risk of me having a confirmation bias: a tendency to only pass on that which backs up a preconceived conclusion.


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