Mike Simard

Thanks Rob.
Could you please also see how that gear seats as it's slid over the splined shaft?
I've looked through the WSM but don't see that detail.
There must be a shoulder of a certain diameter bigger than the splines and the nut tightens against that.

Rob Edwards

Many hi-res pics will ensue.

slate blue

John the reason I wouldn't propose using the corvette diff is unless you have the ZR1 version you definitely have something weaker than the 928's.

http://forums.corvetteforum.com/c6-t...wheel-hop.html

I agree the problem is associated with launches but not necessarily drag racing, the failures are associated with wheel hop and the resultant loadings from that. The other reason I wouldn't stick with the Vette diff is the fitment. Unless it is possible to machine a new casing that would clear the crossmember it won't fit. I think you will find this is the reason Aspkiller went the direction he did.

The 928 rear diff housing has a unique shape to it that I don't think, well, I certainly haven't found on any other car. It may or may not be possible to get a custom case to fit the Corvette differential and then contour the 928's rear crossmember.

If you can't get the transaxle low enough, it will effect the geometry of the half shafts I would have thought.

I think once you have to go that far it is better sticking with the 928 diff especially if you are running the PSD and most of us take the speedo reading off the diff, I know I do.

Regarding the size of the 928 diff, I think the PCD of the bolt holes is 150 mm or roughly 5.9, that would put the outer part of the crown wheel around 6" so it is nothing like the size of the Corvette's but the proof of the pudding is in the eating and generally I think the 928 diff has stood up well and with new top quality after market parts it will be better again.

With a supplier like Albins you can also get aftermarket top quality drive shafts, so that we could do what the engineers did with the Corvette and have one side thicker than the other. If the Aussie dollar comes back to some normality it will be quite affordable to buy parts from here.

Mike if you would be so kind, can you have a go at answering the question regarding the machining of the Corvette out put shaft if it wasn't hardened in the area that needed work. Thanks.

Mike Simard

Machining already hardened steels is something I specialise in and I'm sure that would be simple for me to cut a transmission shaft.

However, I still wouldn't do that for this kind of project and would prefer to make a new shaft from scratch.

The hard 'skin' plays a part in the overall strength and cutting into that makes for alot of unknowns and risks. Maybe it would all work just fine but I wouldn't personally do it. I would rather just settle in and make a proper part, it's already a big project anyway.

slate blue

Fair enough Mike, thanks, I have heard that machining hardened parts can bring the carbon out in the steel and as you say involve unknowns. I suppose if this project ever went ahead regardless of who took it on, it could be sold then as a kit where you just fit up the parts in the kit versus making modifications to existing parts.

I also thought about the possibility of making a new shaft and tossed about getting somebody like Emco gears to make the shaft as they already would have all the programming for the internal part that is in the gearbox, that way they would only have to program the Porsche end. However that is for people with a fair bit of cash but certainly would be absolutely fantastic in a car like yours. Surely that would make you hard to beat!

For somebody like yourself Mike how hard is it to duplicate a shaft like the one in the corvette box that has all those spines on it? I would guess as an amateur non practicing machinist it would be pretty hard unless you had some pretty terrific measuring equipment?

Cheers

slate blue

Errka, the other option I investigated some years ago was the ferrari box in the 575 to 599. I was offered a 575 box for $2K but it had a noisy 5th gear. The same box with minimal changes is in the 599, the only real change is the diff ratio for the 599 as it is capable of higher revs. The box is really sturdy and all forward gears have triple cone synchronizers and we know that it shifts fast because they have the paddle shift option.

The Diff I had measured up and it is made by ZF like ours and the Porsche shafts would bolt straight up. The diff itself, that is crown wheel and pinion looks bigger than the 928 and the LSD has more clutches in it also. Although probably less than our PSD.

The casing doesn't suit the 928 at all, so the internals if they could be fitted into a new custom 928 case it would be very strong. However this would take a gearbox expert and that is why I left it alone. I also thought the option of running a carbon fibre torque tube and Corvette clutches was something that shouldn't be ignored as these cars, our 928s, though the various parties have really moved the game on and is effectively finding the next weak spot.

Mike Simard

It is a complex part but there's nothing wrong with that. Your typical local machine shop might not be able to make one but that doesn't mean it's impossible, it just takes appropriate effort.

You've planted a great idea and I'm working on it although not with a Vette box. I've always wanted a real racing box like the 4 speeds in Trans Am racers and stock cars.

Vilhuer

For things to remain as simple as possible I would build whole setup so that stock outer TT tube and 928 automatic diff could be used directly. Or at least use 3 or 4sp automatic TT outer tube and normal 928 clutch. If result is too far off from 928 this will leave large part of possible buyers uninterested. But if setup is such that as much 928 stock parts like PSD can be reserved even if it means use of automatic gearbox stuff and custom parts "bolt on" market for boxes is much larger than just few hardcore racers who are willing to swap and modify anything. Additional requirement is that there are 6 speed in the box and road version can be made which has same-ish 5th as stock GT and 6sp is overdrive for highway. If source gearbox has enough different ratio options it should be possible to make both racers and autobahn drivers happy. Triple syncros and such are also of course nice.

Its no big secret that locally we have been looking for years to make customs casing for BMW E39 540 and M5 & E46 M3 gearbox using stock 4sp automatic TT outer tube and 2.54:1 S4 diff. Based on my measurements BMW Getrag internals might fit into available space without having to do any modifications to body. Box has three piece casting and center piece could even be used as is. Problem areas are how to mate box output shaft into automatic pinion gear and keep box front end from going too far that body mods is needed. Gearing is ok for our purposes and 3rd, 4th and 5th is only slightly taller than normal GT plus 6th gives nice 200+mph overdrive. Box itself is fairly easily available in Germany but if is it strong enough for hardcore racers I don't know. There is also possibility of using SMG padlle shift mechanism from E46 M3 as box itself is virtually same on inside. Box has triple syncros for 1st and 2nd, doubles for 3rd and 4th and singles for 5th and 6th.

Already some years ago we went so far that we got one such box, opened it up and started making CAD model for custom castings. Whole thing got stuck there as guy doing it has been busy doing other stuff and avoiding the project at best he can. It would probably move forward if we payed someone else to model existing parts into CAD. Cardboard mockup I did says it at least fits into available space if input and output can be kept fairly short.

Early on we also looked into using 6sp Getrag from BMW E31 850 CSi, Toyota Supra MkiV and Nissan Skyline R34. Its definitelly strong but also at least 2-3x more expensive and much larger. Fiting it into available space can be a problem. Good thing in it is that Japanise tuners torture Supra's and Skylines so much that there has to be all kinds of go fast parts for it.

atb

A rock crusher in a 928? Better dust off my V-gate shifter.

Cheburator

It amazes me that people would spend $30k + on an engine, yet they would shy away from spending $15k on a proper transaxle from one of the reputed makers... Erkka, it was my understanding that the 850CSi box is the same as the V8/E46 M3 one. They are resonably strong - the problem is though, no parts are available from Getrag/ZF - BMW has an exclusive agreement and Getrag/ZF would only offer you an "exchange" box - don't ask me how I know...

Vilhuer

They are totally different. 850 setup is much larger.

That is true but there are many M3 which have seen side of the road way too close.

Cheburator

That is true, but as they get older/cheaper more and more people track them. As they track them, 2nd and 3d gears tend to go. The actual gears are very strong, but it seems to be the steel synchros that are giving up the ghost prematurely... Two people I know had to have theirs replaced under warranty.

As with regards to the SMG - it will be a very tall order to mate the SMG ECU with the LH/EZF of the 928. I would actually argue that is impossible - basic functions may work, but the Drivelogic bit would be very difficult to integrate with ECUs that are more analogue than digital... (figuratively speaking)

Vilhuer

Niklas is on it. Not really but we have talked about it and I think it could be done. I just can't imagine anyone would want to do it when normal 6sp is so good.

It doesn't matter where gearbox comes from. To us important thing is its usable in normal street car, has 6sp with proper gearing and kit is cheap enough. If this is impossible to achieve same time with full race setup which can take 800hp and 1000Nm then it is. Race only setup limits customer base so much than whatever is done will be series of one offs. With proper kit there would be much larger market of 4sp automatic to 6sp manual conversion to pay for development costs.

Mike Simard

I used to have an 850 with a 6 speed.
In casual driving I would use only 1-3-6. There are really more gears than are needed.
Something to appreciate about the 928 is that the ratios are very good, at least the non-2.20 ones are. It might be tricky to duplicate that.

Something funny just happened. A 928 friend just stopped by and said he wanted to have a G-Force 4 speed attatched to an auto diff. That was wierd since I've spent the last day researching doing just that and he wasn't aware of it and doesn't do the internet. He's building a twin turbo car.

Cheburator, what are these 15k transmissions you're talking about?

IcemanG17

this is a GREAT idea...since 928 ring-pinon and axles are very strong...change the trans to something strong & modifiy the torque tube and your done...

Keep in mind Jerico 4 speeds are around 80lbs.....the 928 5 speed is 158lbs....