915 Limited Slip Differential Options?

[frankobra]

I'm exploring options for upgrading to a limited slip differential on my 86 Carrera. In searching I didn't find much discussion on the topic in recent years, and there are a lot of options on the market. My main questions are recommendations on clutch-type vs gear-type diffs for my application and what brands are being widely used with success. My car is mainly used for spirited street driving and mountain rallies. I currently have a 3.2, but will be looking to upgrade to a "warm" 3.4 in the future. Any input would be appreciated.

[andino]

I’ve used wavetrac diffs in the past and would recommend them. That’s what I plan to put into my 915 when I get it out for a refresh.

[Bill Verburg]

Originally Posted by frankobra

I'm exploring options for upgrading to a limited slip differential on my 86 Carrera. In searching I didn't find much discussion on the topic in recent years, and there are a lot of options on the market. My main questions are recommendations on clutch-type vs gear-type diffs for my application and what brands are being widely used with success. My car is mainly used for spirited street driving and mountain rallies. I currently have a 3.2, but will be looking to upgrade to a "warm" 3.4 in the future. Any input would be appreciated.

2 main types
clutch type like ZF or GT


gear type like Torsen, Quife , Wavetrack, and I think Gt has a gear type too


clutch type works during braking and helps stabilize the car, gear type doesn't

asymmetric clutch type is even better, there are all kinds of brake downs, but 40/60 is common, 40% nominal on acceleration 60% nominal in braking Gt makes these

[Michael.]

How apropos... I have a GT LSD going in my '86 now.

Personally I prefer the feel of clutch based diffs vs the helical type.

[frankobra]

Bill thanks for breakdown. What would pros of the gear type diffs be other than the clutches not wearing?

[Bill Verburg]

Originally Posted by frankobra

Bill thanks for breakdown. What would pros of the gear type diffs be other than the clutches not wearing?

gear types generate a little less heat,the difference isn't worth worrying about


the big issue is not working under deceleration

[Cherry Bomb]

OS Giken LSD works beautifully in my Hot Street 911. Super quiet after break in and a world of difference in terms of handling safely. Swepco friendly too.

[stownsen914]

It depends greatly on what you intend to do with the car. For track or racing, most recommend a clutch type. You’ll see a variety of options available with a range of prices. Many consider Guard to be the best, and is at the higher end of the price range.

A clutch type can be a good option for the street or other applications too, though for street or autocross, some prefer a TBD or gear type. Quaife is a common brand. Guard had one, not sure if they still do? I believe Wavetrac is a sort of cross between LSD and TBD. There are lots of threads here discussing the pros and cons, it would be worth doing some searching/reading.

[Matt Monson]

Originally Posted by frankobra

Bill thanks for breakdown. What would pros of the gear type diffs be other than the clutches not wearing?

That clutch wearing thing is largely an obsolete myth the internet won’t let die. In the 15 years that I’ve owned GT, I’ve sold more than 5000 LSDs. I sell a couple hundred rebuild kits a year. For street driven cars and DE, it’s a do it once endeavor. Most PCA club racers get 3-5 race seasons. I’ve never seen a street car guy wear one out in under a decade.

Regards,

Matt Monson
Guard Transmission llc.

[kevingross]

I agree with Matt: the clutches last a good long time.

To answer the question regarding the difference between a clutch (aka true LSD, or Salisbury) and worm-gear (TBD, Torsen, etc.) differential: The clutch style provides locking (torque control) both under acceleration and deceleration. The biggest influence on bias in the clutch-type is the angle of the ramps in the LSD's pressure rings. Which means that by swapping ramp angles, you can tune the differential. It also true that that distinct accel vs decel ramps give you different lock on accel vs decel.

Torsen style differentials as a rule go open under braking: that is, they act just like an open differential. Exception: Wavetrac has a trick in their design by which the differential locks under braking. A disadvantage of the worm gear design is that it is pretty much impossible to tune the locking unless you substitute different worm gears. Which don't exist.

There is a lot of room for personal preference in choice of clutch vs worm gear, as well as locking rate. I have a customer who keeps winning at SCCA Nationals with his Wavetrac, and I am not going to win any arguments with him about the benefits of clutch over worm gear. Plus he has a PhD in mechanical engineering.

There are a lot of very poor quality differentials out there. I always recommend educating yourself as best you can and working with an installer who understands the engineering.

[stownsen914]

@Kevin - out of curiosity, is your customer running SCCA Nationals road racing or solo/autocross?

[kevingross]

Originally Posted by stownsen914

@Kevin - out of curiosity, is your customer running SCCA Nationals road racing or solo/autocross?

Road racing, unless I am mistaken. Cheers,

[dedyplay]

I prefer the feel of clutch based diffs ..