Garth S

Jim (Jadz),
The question may be stupid -only if it isn't asked:
The basic concept proposed seems to be design/manufacture a new casting, and install Eaton shelf item internals.
For the stupid part... there must be available a reasonable supply of (used) open differential carriers, as they represented the majority of 928 production: Could these castings be refitted with shelf item Eaton internals? - or is there a fundamental casting difference (that would not accept Eaton components)?
Recognizing that a new design would have a manufacture warrenty etc., and the latter is only a refit: however, if feasible, the time line/cost should be signifigantly improved?

Jim bailey - 928 International

Whoa there Jim ! changing the drive ratio means a complete ring and pinion change . The pinion shaft for the 5 speed goes completely through the transmission it is a complex and specialized machined part onto which all the drive gears and bearings fit . It is also different for different years no one size fits all . Changing final drive ratios on a 5speed means rebuilding the gear box . Scott Mohr who has a 3.09 final drive in his stroker GTS has indicated that at "normal " cruise speed he is turning fairly high RPM . The ring and pinion for an automatic is much simpler since the pinion gear is bolted to the out put of the "Mercedes" automatic .

Jadz928

I'm still learning about the internals of the 928 diff. , but I would say no. I'm sure there are fundamental internal housing differences. This is based on my experience with mating externals from a given vehicle to the different size internals we had available. It was one of my job functions.

Garth, I will make note of your suggestion. Thanks!

I used the term "shelf item internals" loosely, meaning Eaton may be able to use an internal pinion forging from another product, then machine it to match 928 specs. It will still be a custom job, but the potential is the to save money by avoiding extra tooling cost (forging).

If there is potential for a short production run, the lead time may be reasonable. When I worked there, we were putting out diffs from drawing board to storeroom in 6-8weeks.

Sueden

Jadz,
The 3.09 gears that Dave is refering to were actually made about a 100 or so miles from us in St. Thomas or London Ontario,Canada. I used to have his name and number but can not find it right now.Any how you had to send him your ring and pinion gear and he would built what ever ratio you wanted for $2500. You got a new gear set and your old one back also. It had to be installed to his specs. Sure would like to now how these gear sets held up!

-----------
Dennis
82 wiessach

Jadz928

Jim B, Thanks for the info. I'm mostly interested in an LSD diff. into.

Could you explain the similarities or differences btw a 928 LSD and a classic 12-bolt? I'm curious about the half-shaft interface to the diff. internal pinion gears. Also the interface of the main pinion gear to the diff.

jheis

Jim:

Add me to the list of those who are potentially interested -- depending upon cost of course.

mark kibort

a different rear end will not buy you anything. your transmission spacing is all you should care about. the only reason that the 2.75 box rear end is better than a 2.2:1, is that the gear spacing is all the same (roughly .75/1.33) the 2.2:1 boxes are also about .75/1.33, but the 1:1 fith gear brings that change in ratio to 1.50. so the rear end is not the real advantage as the gears in the box are different.
All you do by going to a 3.09:1 rear end is shift ALL the gears down. it will make 1st usless and 5th more usable, but will not effect the performance one bit, unless you make the test customised for these ratios. all tracks are different so it is impossible to say what rear end is best without knowing the cars capabilities. Right now, if mark anderson is getting to the top of 3rd 4th or 5th gear at all the main straights, a shift down of gear ratios by use of a higher numerical ratio diff, may hurt much more than help, besides lowering his top speed potential in 5th gear in a GTS box.

changing ratios do not make more hp.

Mk

at Road america

Jim bailey - 928 International

Jim Doerr , What is a 12 bolt ?? actualy what you are asking is more like how does a positraction work , "detroit locker " , Quaife ........ simply stated Porsche uses clutches and springs the early ones can be stacked to get up to 80% lock up , later units 40 % (fewer clutches) . The output flanges on the sides which hook to the axle are splined on the end and stick into each end if the differential housing ( the part which contains the spider gears . Much the same as the axles on a chevy 12 bolt live axle stick into the differential housing ...........l

Steve_C

I believe Mark K is exactly right considering a racing 5-speed manual.

1. However, there are a lot like me with an automatic 4-speed transmission.
Having both an 82 AT 3-speed which is nearly as fast as my 86.5 at "street speeds" with way less HP, I am very sure that the final ratio is very important to us.

2. There was an article in Excellence magazine about JK Crumb's new ring and pinion set purchased at GT Transmissions some 3-4 years ago. I have been considering this non-engine change because would like to improve Street Power (torque) by sacrificing 160 mph top speed.

3. I believe the LSD unit (or standard) bolts to the ring gear making ratio a separate issue from slip.

4. Jim b (here is a perfect opportunity to sell) having searched the Intl. site for "slip" have found used split 78-82 @ $650 (new and 83-89 @ $1000.)
New: 78-82 @ $3,521, 83-89 $2,436 and 90-95 $1,495. Will the 90-95 not work with older, where is the 40/80 split. what is the torque capacity of each? May I use the $1495 unit in my 86.5 S3?

5. If R&P are separate from slip what ratios are available and what combinations fit various years?

worf928

I know that I'm going to regret this...

MK, I quote from the Bosch Automotive Handbook:

"The higher the engine torque and overall transmission ratio between engine and driven wheels, and the lower the power-transmission losses, the higher is the motive force available at the drive wheels."

Non-traction-limited acceleration is directly related to available torque and how you are multiplying or dividing that torque via the gearbox. A higher overall ratio will allow for faster acceleration at a given speed in the same gear versus a lower ratio.

A higher final drive might not 'buy' you faster lap times at every track due exactly to the reasons you describe. But, it will generate more power at the wheels at any speed in any gear. For a high-power car like Mark's and assuming that he's using all of fifth gear - (Hey? Mark, do you see rev-limited top speed in 5th gear (170mph?) on any straight?) - a higher final drive may not be the appropriate place to sink dollars.

However, for the horsepower challenged, it might be.

The was a Pano article written by a Howard Jacobs in around 1999 about his switch to the 3.09. I looked briefly for it but couldn't find it.

heinrich

Jim Doerr, I am an customer for updated LSD. I have 2.20 rear and LSD 5spd '87 S4 and I am up for it anyday on a group purchase. I think the GT rear end with the S4 cams would tear a new one into about anyone else out there. Torque and gearing ... a match made in Heaven ..... I am rearing WRX's up now, and RX7's late-models too ... I am DROOLING on the gearing to rip up some of the models to come .... HAHAHAH no kidding I am very serious, I am in on a group purchase.
Heinrich

SharkSkin

Maybe just a bit off-topic, but I think that a Torsen would really help. That linked page gets more technical than some people care to absorb, but I think most will find it interesting. I first saw these many, many years ago, and the first application was for big snowplows used at airports. I think I saw it Popular Mechanics in the late 70's. They found that with a Torsen they could zip around on the ice like it was nothing... far superior to other technologies. Currently they live in Quattros and Hummers, probably others.

Just a pipe dream maybe, but a good one...

Jadz928

Eaton Posi Information

I explained my objective, but want to add: I like the idea of having a lower cost alternative to the Porsche price for a NEW LSD. I do understand that one can purchase a used LSD for $500, but would it be nice to get a new one for that price? Plus receive the benefits of newer technology, such as the carbon fabric clutch pack (which is virtually indestructable).

I believe that all 928s should've came with LSD factory standard. So, if we can make NEW units more available, and for a good cost, maybe more owners will decide to drop one in their 928.

bcdavis

Makes sense.

I have one already in my euro, but if it didn't, and it was only $500, I'd buy one.

I might *still* get one, if the drive ratio really has some noticable acceleration improvements.

I think what people would want to see, is a test one built and installed,
so people can see what it looks like, how it installs, and what the change is
to acceleration, traction, etc...

hacker-pschorr

Time for another stupid question.

On a 1981 928 5-speed, what is the easiest way to tell if I have an LSD? If I don't, I'm interested in the upgrade for around the $500 mark.