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LSD buster

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Old 11-12-2009, 12:02 AM
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Smile LSD buster

Porsche makes great cars but there are couple flaws that get pass down from generation to generation much like a bad gene. We all know about rear main seal leak which remain an issue from 996 to 997. Then there is the limited slip differential (LSD) which I consider to be a major performance component that has failed to deliver for 5+ years!? I had no idea what LSD do until I came across a bad one in my RS. The rear of my car is super nervous under hard braking and I seem to have problem power out of turns - I am doing slow in and slow out instead of slow in fast out.

I did a simple LSD test by lifting one of my rear wheel off the ground with e-brake off and tranny in neutral and the other wheel on the ground. I could spin the lifted wheel without much resistance as in an open differential!? This is REALLY BAD! In an open differential, if one wheel has no contact with the ground (like my test above) the contacting wheel will remain stationary, and the non-contacting wheel will rotate freely. The torque transmitted in an open differential vehicle will be equal at both wheels, but will not exceed the threshold of torque needed to move the car. This means the car will remain stationary if one wheel is off the ground or slipping such as in a turn or wet situation. In everyday use on flat roads, an open differential can do the job by taking you from point A to B. For more demanding use, such as driving off road or on a race circuit, an open differential is a cruel and unusual punishment!

Ironically, LSD came about when in 1932, Ferdinand Porsche designed a Grand Prix racing car for the Auto Union company. The high power of the design caused one of the rear wheels to experience excessive wheel spin at any speed up to 100 mph. So, in 1935, Porsche commissioned the engineering firm ZF Friedrichshafen AG to design a limited slip differential that would perform better which is the father of modern day LSD. I know what Ferdinand Porsche would say if he finds out his modern sports car deploy defective LSD .... "entwickelt, die diese Scheiße Idiot!"

OK, LSD class is over. I do need to give credit to wikipedia for some of the above info. Time to roll up the sleeves and do Ferdinand Porsche right by fixing the LSD. First off, there are couple ways of doing this. there are several after market LSD to replace the OEM unit but this means you need to have the transmission removed and have a highly trained professional properly shim and set the pre-load on each side of the LSD as not to destroy the LSD or tranny with use. This means $$$$ as far as labor goes. The cheapest way is to remove the OEM unit and have it professionally rebuild to your spec. After it's rebuild, all you need to do is put it back since the pre-load on both ends of the LSD are already done at factory.

After much research, I decided to go with Matt at GUARD transmission because they make one mean GT2 LSD and super helpful with all my questions and concerns which is totally reassuring. The cost? $1600 (add $75 if you left the ring gear on) which is totally reasonable considering friction plates and ramps are replaced with beefy GUARD GT parts. GUARD or GT gear is one of our sponsor so you can contact Matt for more information or if you have other questions. He is the real expert when it comes to LSD.

Matt Monson
Guard Transmission LLC
303-530-1094
gtgears@yahoo.com


Here is a video of the LSD test and also driving impression after LSD rebuild

LSD buster


Now the fun part! Let's get that defective LSD off the car. First, raise the car up with your build in lift - or blue collar lift by putting couple wood blocks under the wheels lol. Take great care to make sure the car is secure when you work underneath as not to hurt yourself.

Click the image to open in full size.

Remove 4 screws securing the transmission cover then you can see the LSD cover on the driver side which is secured by 10+ bolts

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LSD cover plate on driver side

Click the image to open in full size.

Next drain the transmission oil by locating the drain plug which is under the transmission base and the fill plug which is on the driver side kind of hidden toward the front of the transmission. One thing about the transmission fluid - they really smell bad!! lol

Drain plug

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filler plug on driver side

Click the image to open in full size.

Both drain and filler plug uses 10 mm Allen. The space at the filler plug is very limited so it's hard to fit a socket/wrench. I used a long 10 mm Allen wrench and fit the end of the Allen wrench with a 10 mm socket and socket extension to provide enough leverage to break it loose. None of the plugs have washers

Click the image to open in full size.

The drain plug has a magnetic tip which help collect all the metal shavings. It's a good idea to clean it really well before putting it back. it's also a good idea to change the transmission fluid at least once a year if you track

Click the image to open in full size.

After it's all cleaned

Click the image to open in full size.

Now let's separate the drive axle from the drive shaft flange. It is secured by six T-55 bolts.

Click the image to open in full size.

You need to remove the drive shaft and flange first on both sides to remove the LSD. I protect the drive shaft with pipe insulation foam and zip tie the driver side shaft against the lower body frame for ease of access

Click the image to open in full size.

Now you can see the center bolt securing the drive shaft flange

Click the image to open in full size.

I used two of the T-55 bolts to help give leverage to loosen the center bolt. I was told not to use impact wrench to break the center bolt loose to prevent damaging the bolt or the "bell" that it's attach to

Click the image to open in full size.

The "bell" holding the center bolt of the drive shaft flange are pretty lame. There may be extra play where you can rock it in and out couple mm even before you loosen the center bolt. Also you may need to pull the drive shaft flange out as you loosen the center bolt or the bolt just spin without catching the thread. Matt at GUARD transmission told me this is very common on all OEM LSD and does not affect performance. He will try to heat the "bell" up to make it fit better during rebuild.

Now gently slide out the drive shaft flange on both side and remove the 10+ bolts on the LSD plate located on the drive side

Click the image to open in full size.

Don't remove the LSD cover if you have not remove the passenger side drive shaft flange. This will lead to excessive movement of the flange on the passenger side when you try to remove the bolts which can damage the rubber seal around the flange. Once both flanges are removed, gently tap on the bottom "lips" of the LSD plate with the back end of a hammer. Don't tap on the LSD cover with metal - it will damage the cover. I used the hard plastic back end of a hammer. Be patient, the LSD cover will slowly come loose and be ready to catch some residual transmission fluid

Click the image to open in full size.

Take care in handling the LSD cover as not to damage the inside or the contacting surface

Click the image to open in full size.

Now you can see that ugly defective LSD for the first time! it sure reminded me of Alien's head lol. It is super heavy and weigh 30+ lbs. Take great care not to bang it around while removing it. You need to work it around the rubber hoses and the drive shaft but it will come out. it reminded me of delivering a 12 lbs breech baby lol

Click the image to open in full size.

This is what the LSD case look like inside

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Old 11-12-2009, 12:03 AM
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shipping and rebuild

OK, take a good look at the OEM LSD and get it ready to ship to GUARD for rebuild.

Click the image to open in full size.

Side view

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It really weigh 30 lbs! lol

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Factory etching indicated 13.14 Nm for the LSD which is only 9.6 lb/ft when new!? - It turns out I only had 2 lb/ft left at time of rebuild. It's practically an open differential!!!

Click the image to open in full size.

I send the LSD to GUARD with ring gear intact because I did not want to mess it up. make sure you pack the crap out of the LSD and use box in box method to prevent damage to the LSD/ring gear. It's $4K + if you have to replace the ring gear or LSD

Click the image to open in full size.

My OEM LSD was rebuild to 50/80 spec which is the same as GUARD GT2 LSD.

Here are some footnote from Matt when he rebuild my LSD along with pictures of the replacement parts:

"As for your differential I didn't see anything unusual other than the fact that there was practically no wear on your parts. I have to say that this is the best condition I have seen one of these that was being reported to us as failing and worn out. You can see for yourself in the pictures that the brass friction discs appear like new. However, this really points out the mistake Porsche made in going with those discs. They replaced the previous carbon based discs with brass in an attempt to reduce particulate in the gearbox that could potentially cause harm. The problem is the result of two things. Number one is that the brass friction discs just don't have a high enough coefficient of friction. They just don't lock the way they are supposed to.

Problem number two is their attempt to use an LSD with a low preload setting on it. Even with a very small gap of 1.35mm in there, they don't use an aggressive enough belleville washer to set the preload and don't get the differential to actuate and lock the way it really should. again, you can see a side by side picture of the Porsche belleville (spring) washer and ours. The Porsche one is 1.2mm thick. The one I replaced it with is 1.8mm.

In addition to the preload and our higher friction discs, the other thing that will make this differential more aggressive than the stock configuration is the addition of our ramps. It you look at the pictures, you see ours versus the factory. It's actually a little counter-intuitive, but a more aggressive ramp has a lower angle on it. What this does is it allows the cross-shaft to exert more outward force on the bodies of the ramps, locking the friction discs. And with a strong pre-load, once there is no longer wheel spin, the cross-shafts are pushed back to the passive position in the center of the ramp and the differential returns to an open configuration.

I reviewed my notes, and we actually increased the gap on your LSD by .2mm, so the gap is now 1.55mm instead of 1.35mm. Even with that increase, your LSD now has a bench breakaway torque of 50 lb/ft versus the 2 lb.ft it had when it came to us. One other interesting note is that regardless of what the factory manual says, this LSD has etched on it's body 13.14Nm as it's original preload. That's only 9.6 lb/ft when new!!!"

OEM plain plate

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OEM friction disc

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OEM vs GT disc

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GT disc

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GT2 replacement parts

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OEM ramps

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GT2 ramps

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Finished product

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Last edited by mikymu; 11-12-2009 at 12:32 AM.
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Old 11-12-2009, 12:04 AM
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Install and driving impression

After a quick couple days of turn around I got my rebuild LSD back. I like the blue dots on the ring gear bolts which indicated 140 lb/ft of torque applied to each of these bolts!

Click the image to open in full size.

The back side is all clean up - thanks Matt

Click the image to open in full size.

Let's clear the LSD housing and get everything out of the way because we have to push this 30 lbs LSD baby back up the birth canal lol. make sure you had your Wheaties for the morning. This thing is freaking heavy. Carefully clean the surface of the LSD opening and cover and lightly oil the O-ring on the LSD cover before putting it back. Gently work the LSD back in the housing

Click the image to open in full size.

Once you work the LSD in, the ring gear should easily mesh with the final drive shaft from the transmission. You will be able to easily turn the LSD by hand with the transmission in neutral. I held on to the LSD on the passenger side through the drive shaft flange opening to do this test.

Click the image to open in full size.

Once the LSD is in and ring gear mesh with the final drive shaft from the transmission. carefully place the LSD cover back on. I gently tap the cover from all sides with a rubber mallet then install all the bolts. Gradually tighten the bolts by going back and fourth from opposite ends to make sure the LSD plate slowly seal evenly with the transmission case. Tighten the LSD plate bolts to 19 ft/lb or 25 Nm

Click the image to open in full size.

Place the drive shaft flange back on each side and tighten the center bolt to 33 ft/lb or 44 Nm. You may need to work the flange back in if it's a bit tight. Again no impact wrench

Click the image to open in full size.

Fill up the transmission fluid with Mobil Delvac 75W-90 until the fluid flow out of the fill plug opening. I got my transmission fluid from lufteknic. here is the link:

http://www.lufteknic.com/Merchant2/m...ode=lubricants

It takes about the whole gallon to fill the transmission - it was a major PITA with a hand pump lol. Install the filler plug and count yourself lucky that you don't have to smell this crap for another year

Click the image to open in full size.

Install the drive shaft back on the flange and tighten each bolt to 67 ft/lb. You will need to rotate the wheel to get to all the bolts.

It's a good idea to replace these bolts. They can be had from Suncoast Porsche

Bolts - 12 total both sides and $1.20 each: 999-073-449-01
washers - 6 total both sides and $ 2.90 each: 996-332-191-00

Click the image to open in full size.

Start the car up and enjoy your newly rebuild LSD!!!

I had no issue in rain and it does not make any noise on tight turn. The car was rock sold under hard braking on the track and my tail does not do the wiggle anymore - which is confidence inspiring. I can rocket out of turn and throttle steer through turn much like my M3. I was able to best my time at Thunderhill by 3 seconds. I am sure I will get faster with better brakes (replacing my PCCB with steel rotors and aggressive pads will be my next project).

I can not be happier with the result and I would recommend fixing defective stock LSD to anyone especially if you plan on tracking your GT3/RS on a regular basis.

Hope this write up is informative and give some insight on the defect and repair process of our stock LSD.

Here is another video at Thunderhill doing a fun chase with Donovan's Lotus Exige

RS Lotus
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Last edited by mikymu; 12-18-2009 at 10:54 PM.
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Old 11-12-2009, 01:29 AM
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Quote:
Originally Posted by GTDeux View Post
Well in case anyone had any doubts that GUARD is the best when it comes to diffs and trannys... just look at the quality of their components.

One question though, why didn't you want to go with the Guard billet housing? Was this because of increased labor cost?
I want to avoid the need to remove the transmission to properly shim and pre-load an after market LSD. By keeping the stock housing I was able to put the LSD back and good to go. Labor cost for transmission removal/install are in the thousands not counting the cost to replace LSD. I am just too poor now days. Any saving counts!
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Old 11-12-2009, 02:29 AM
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Fantastic write up... thanks for taking time to post... +rep for you... think I'm going with Matt also.
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Old 11-12-2009, 07:02 AM
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Superb write-up and respect earned for you to tackle this at home. Bravo my friend! From one home mechanic to another.

But...please, in the name of sweet baby cheeses, get some jackstands. I was in all kinds of mental anguish looking at the assembly of wooden blocks under the wheels

Reading about an enthusiast crushed under their car would be dreadful.

Awesome job and congrats on a sweet, tight, rear-end!
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Old 11-12-2009, 07:16 AM
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Mate, another fantastic write up. And certainly has me watching the behaviour of my RS now. Two more track days to year end and will be very mindful of possible changes in the handling of my car as a result of your awesome article.... will definitely look up Guard once mine shows excessive wear...
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Old 11-12-2009, 07:33 AM
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Great write up, how long did it take you to Remove and then Install?
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Old 11-12-2009, 11:00 AM
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Boy, this website is pretty slow today. I will keep my reply short and sweet

Quote:
Originally Posted by 996TT_STEVO View Post
Fantastic write up... thanks for taking time to post... +rep for you... think I'm going with Matt also.
You bet. Matt at GUARD transmission is an excellent choice for our LSD repair

Quote:
Originally Posted by sayboy View Post
Superb write-up and respect earned for you to tackle this at home. Bravo my friend! From one home mechanic to another.

But...please, in the name of sweet baby cheeses, get some jackstands. I was in all kinds of mental anguish looking at the assembly of wooden blocks under the wheels

Reading about an enthusiast crushed under their car would be dreadful.

Awesome job and congrats on a sweet, tight, rear-end!
lol!

I am in love with my RS now after getting our crappy LSD fix. It's rear end is rock sold with excellent acceleration response. lol on the wood blocks

I have jack stands but it's easier to use the wood blocks for some jobs. I am trying to convince wife that we need build in hydraulic lift

Quote:
Originally Posted by Harold View Post
Mate, another fantastic write up. And certainly has me watching the behaviour of my RS now. Two more track days to year end and will be very mindful of possible changes in the handling of my car as a result of your awesome article.... will definitely look up Guard once mine shows excessive wear...
Our LSD are dead once you track just once. Do the simple test i demonstrated above. You will like how it behave once the LSD is fixed

Quote:
Originally Posted by NorthVan View Post
Great write up, how long did it take you to Remove and then Install?
It took a long time - couple days because I had to figure out how to tackle the task ... and taking pictures/note. if I had to do it all over again, it will take maybe 3 hours to remove and 3 hours to install.
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Old 11-12-2009, 12:23 PM
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Great write up, thank you!
I am happy to hear the new owner of GUARD is as helpful as the old owner.
You may want to check into having to replace the half shaft bolts. I was once told they are one time stretch bolts. I had a half shaft completely separate from the transmission on the track because the bolts were not replaced.
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Old 11-12-2009, 12:40 PM
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Quote:
Originally Posted by Md11skipper View Post
Great write up, thank you!
I am happy to hear the new owner of GUARD is as helpful as the old owner.
You may want to check into having to replace the half shaft bolts. I was once told they are one time stretch bolts. I had a half shaft completely separate from the transmission on the track because the bolts were not replaced.
Good point. I can't believe your drive shaft came lose!? I will look into the drive shaft bolts and update once I find out more
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Old 11-12-2009, 01:50 PM
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Holy crap what a great write-up. Points coming your way for sure! I love seeing the GT cars get used for what they were intended and I love, even more, an owner who gets that involved with their $100k car. Well done, mikymu!

And, btw, the Big Wheel comparo had me on the floor! That's right. I LOL'd.

Last edited by medpilot105; 11-12-2009 at 02:03 PM.
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Old 11-12-2009, 02:13 PM
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Quote:
Originally Posted by medpilot105 View Post
Holy crap what a great write-up. Points coming your way for sure! I love seeing the GT cars get used for what they were intended and I love, even more, an owner who gets that involved with their $100k car. Well done, mikymu!

And, btw, the Big Wheel comparo had me on the floor! That's right. I LOL'd.
LOL I was wondering when someone is going to get my hot-wheels joke

Hey I see you are local and a doctor too? I am at Rocklin so we might meet at some of the local tracks. My next event will likely be at Infinion on 11/21 with trackmaster

I was pretty timid when I first got the RS - will not even change the windshield wiper. I got over that quickly once I start tracking the car - that's what it's mad for right?
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Old 11-12-2009, 02:28 PM
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Quote:
Originally Posted by Md11skipper View Post
Great write up, thank you!
I am happy to hear the new owner of GUARD is as helpful as the old owner.
You may want to check into having to replace the half shaft bolts. I was once told they are one time stretch bolts. I had a half shaft completely separate from the transmission on the track because the bolts were not replaced.
Hey Peter, I just spoke with Rick at Suncoast and also got some feedback from Matt at GUARD transmission. The conclusion is that it is a good idea to replace the bolts and also the washers that secure the drive shaft to the flange. One long washer per two bolts. I went ahead and order a set from Suncoast and here are the parts number and cost:

Bolts - 12 total both sides and $1.20 each: 999-073-449-01
washers - 6 total both sides and $ 2.90 each: 996-332-191-00

My local stealer was going to charge me $2.60 per bolt and $6.18 per washer!? What a bunch of loser lol
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Old 11-12-2009, 05:13 PM
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Hello,

Michael, thank you very much for the kind words. It's customers like you who make it worthwhile to get up in the morning and start answering the phones. Not everyone in this business shows the sort of gratitude you express here. Research says that customers are 10 times more like to talk about a bad customer experience than a good one, so when we see something like this write we really appreciate it.

Just a couple of technical comments to add to Mike's very thorough write up above. If people want to rebuild their factory LSD's installing new ramps in not a requirement. It was something Mike decided to do because he wanted something with more aggressive locking than what was offered from the factory. If your car sees less track time than Mike's, or if you just don't want to spend the extra money ($700 of that $1600 number) the factory ramps still are a good option.

And while I am on the subject I want to clarify and correct something I have been seeing on a couple of other boards. When we talk about the ramps, we are talking about locking percentages. We are not talking about ramp angles. In fact, if you look at the picture of our ramps, you will see that the locking percentage is inversely related to the ramp angles. The 80% ramp is a lower angle because it's designed to be more aggressive and activate more quickly. The way a ZF style LSD works is when there is wheel slip the center spider gears spin the crossshaft that they sit on up the side of the ramp and press the ramps apart, locking the plates and discs together and stopping the wheel spin, while simultaneously transfering the power to the other wheel.

Just to clarify further, when it says 80% locking that means it allows no more than a 20% differential in wheel speeds from side to side. Again, it's an inverse relationship. And once the LSD locks, it's locked. That seems to be another misconception I see floating around. An LSD at rest functions like an open differential. It doesn't matter if it's got zero pre-load on it or 50ft/lb or 80ft/lb. The preload on our differential is actually pressing the ramps together and holding them there while the differential functions like any other open differential. It's only when there's enough wheel slip to accelerate those crossshaft spiders up to the point that they push the ramps apart that the LSD locks and does it's job.

I think that's a very important point because the other thing I have been seeing recently said about how our LSD's (and Porsche's) work are that the preload in them causes the car to push and makes it hard to turn. This is one of the most ridiculous things I have ever heard. As Mike has indicated (as well as I have seen on other threads around here) when the factory POS LSD wears out the car gets super loose and wags it's tail all over the place. Being tail happy and oversteering has always been the achilles heel of the 911 and even though they've made great strides in recent years with respect to suspension and moving the heavy stuff more towards the center, with an open differential a 911 is still a car that has a tendency towards snap oversteer.

Saying that the differential's involvement in giving the car more push is a bad thing is like saying putting wider rubber on the car will reduce performance because it makes it harder to turn. It's a nessecary correlary to the addition of the performance part. And to suggest that the 50ft/lbs or even more like we use on the race LSD's is enough to make the car refuse to turn is really the most fantastic idea of them all. We're talking about cars making 400ft/lb of torque here. Even at 2000rpm just off of idle they're puting down enough power to overcome 50 or 80ft/lb of preload. And no doubt the kind of forces going through the rear end under heavy braking is far more than that. Furthermore, it's under braking that the LSD most aggressively locks and gives the rear end stability and the only pushing it causes is when you trail brake deep into the corner, which is unavoidable with an LSD but is just part of how it does what it does. In fact, in the transition when you quickly lift off the brake and move over to the throttle the LSD returns to it's open state and doesn't lock again until you've buried the throttle so deep that there's a 50% speed differential between the two wheels.This is where the majority of the turning occurs. Mike's experience after this rebuild corroborates that because as he said above it doesn't fight him at all on low speed corners. If that doesn't make sense, say something and I'll explain it in more detail.

Lastly, I want answer a question above about why not get the billet housing of the GT LSD at the same time. Beyond Mike's desire not to have to reset his ring and pinion and do this thing on his own, there's another practical reason. The reason is that it's not required. Sure, you read here and there about cracked LSD bellhousing on the stock cast LSD's. It does happen. But it doesn't happen as often as you might think. And 9 times out of 10 it happens on a dedicated track car that is driven at the limit for extended periods of time (like 4 or 6 or 12 hour races) and is subjected to stresses that most of us will never put our cars through. I tell you this because the way I do business is I am never going to sell you something you don't need. I will give you a benefits and liabilities analysis of why you may or may not want to consider a part, but I'm not going to push you into something unless I am 100% confident you are going to need it. Based on what Mike told me about himself and his car I didn't consider it a risk he really needed to worry about. For most of you, you really don't have to have that pretty billet differential bellhousing unless you really want it. If next year Mike decides he wants to go run the 25 hours of Thunderhill in his car, then maybe he'll want to seriously consider our GT2 Chromoly Billet housing, but for now he should be just fine. So, it's just something to consider for any of you who have factory LSD's in the car. Instead of spending all the money on a new LSD and throwing away a part that does have some merits, you might consider a rebuild. It will cost you less than the cheapest option in the aftermarket and I assure you it will perform better as well. Or as I offered to Mike, if you want one of our complete units, we are always open to a core exchange on a rebuildable factory LSD to help defray the cost of getting you into one of our motorsports units.

Kind Regards,

Matt

Last edited by GTgears; 11-12-2009 at 05:25 PM.
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Old 11-12-2009, 05:13 PM
 
 
 
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911, 928, 964, boxster, bulletin, guard, limited, lsd, plate, porsche, rebuild, service, slip, speed, transmission, whine


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