Corvette transaxle

Does anyone have any links to some comparative information between the C5-C6 & C7?

My searchfu is apparently weak.  I only get semi-retarded, no I mean semi-related info.

I am wanting to see if one is "daintier" than another, get some good physical dimensions of the transmission end, and find out gear & rear end ratios.

I have a need for a transaxle and while I would prefer the Audi/Porsche/Subaru configuration, cost, LSD availability, rebuild costs, and connectivity have made me look toward the Corvette as a possible alternative.

I will only be running 325-350 hp with a little under 300 torques so the Vette is a bit of overkill and I'm hoping there's not a huge weight penalty.  At this point I can extend the wheelbase a few inches (as long as it's only a few) and use the Vette if it's not too heavy or cumbersome.

I got my hands on a Boxster transmission yesterday and I see how I can convert it to a driveshaft connection with minimal fabrication but I haven't been able to find a 968 transmission, which already has a driveshaft connection, nor have I been able to reach out and touch a Subi trans.  I'm betting the same solution I have for the Boxster trans will work on the Subaru.  I also haven't touched a VW/Audi one yet.

The big issue with VW/Subie trans is the lack of LSDs and once I pay to add one I might as well buy the Porsche or the Vette.

I am wanting a fairly high rear end ratio (3.20ish to 3.40ish rather than 4.10ish) and some fairly high 1st and 2nd gear ratios so that when the car is autocrossed I have minimal shifting.  Also a first gear ratio of  3.5+ would make it useless in a lightweight Locost like car.

My ideal transmission would be a 5 speed (for physical size) with a 4th gear of 1.+ rather than 1:1 just because that could make 4th useful on the track.   I know I can get custom gearsets, but this IS a Locost and I don't need 2 overdrives like comes with most 6 speeds as this won't be used on the street except short cruises or drives to the tracks, but I'm betting I'll have to live with a 6 speed.

BTW does anyone know where to find the custom Porsche tool used to remove the bearing on the input shaft of the Boxster transmission?  The Race shop I was at yesterday has been looking for one for some time and apparently Porsche doesn't stock them any more and he's not been able to find one through the usual sources or on Ebay.  I could be a hero and possibly get some tuning for cheap or free if I can find one.

Have you looked at 951 transaxles?  

That will give you an incredibly long powertrain. Someone on here has a mid-engined racecar with this setup and it's loooong. I'd recommend a modded Subaru box, or just save up for the Porsche box.

the corvette transaxle has the axle outputs on the back of it, basically a diff shoved onto the back of a transmission. from the list of other trans you've listed it sounds like you are wanting something with the outputs up by the bellhousing.

I put out a thread looking for the car I was thinking of, and Jah29 found a similar one with the same powertrain layout, so this is the length you'd be looking at:

https://www.southbayriders.com/forums/threads/139451/

Update: Found the exact car I was looking for:

https://grassrootsmotorsports.com/reader-rides/3779/

We're all assuming that this would be for a mid engine setup, but the fact that the OP mentions using a driveshaft means he's not going to be bolting the engine directly to the transaxle. And that makes me think he might be planning a front engine design with the transaxle in the rear, which is exactly what the Vette transaxle is designed for. Pretty sure he said something about a transmission tunnel in a previous post on the subject too, which also indicates that the plan is to keep the engine up front.

Some clarification from the OP about his plans might help avoid confusion.

For a Locost, I'd honestly look at using the Porsche 915 based solution from the 79 924 or the 79-80 924 Turbo.  They will handle the power even in their small spline form.

The Audi box has that huge bellhousing piece that just takes up unnecessary room.  Plus the 915 setup cna be rebuilt or modified using parts used on the 911 or 914.  Limited slips are easier to install (pop the side case off, install, shim, measure, install side case) and are available from Porsche and other manufacturers.

Just me though.

For the special tool for removing the bearing on the Boxster transaxle, I'd look at what Audi does since its their gearbox originally.

Finally, I think you're over-thinking things with regards to the limited slip.  Yes it can help, but you'll have a ton of setup and tuning on the suspension to do before that becomes a huge need.  Often using stickier tires can be enough to overcome that issue.

Here's the nose of the 968 transaxle showing the spot where the pinchbolt for the coupler goes.  Pretty much the same as the 924 and 944 use:

Here's an interesting discussion about swapping the 944 Audi 016 5-speed for the VW Passat 01E 6-speed:

https://rennlist.com/forums/924-931-944-951-968-forum/773163-944-to-passat-01e-6-speed-gearbox-conversion.html

Here's a 924 Turbo 915 gearbox and torque tube:

https://www.ebay.com/itm/924-Turbo-Used-Transmission-931301901ph-warfl-/183061343273?vxp=mtr&hash=item2a9f4e6c29

https://www.ebay.com/itm/Porsche-924-Turbo-Drive-Shaft-Bearings-Snail-Box-with-Insulator/300598445940?hash=item45fd104774:g:srIAAMXQLbVRZ5--&vxp=mtr

FWIW, Audi Sport used to hack off the top 2-3 inches of bellhousing in order to make transmission swaps faster.

In reply to dean1484 :

The 951 hasn't come up on any conversations with the Porsche guys or transmission shops.   So now I'll look at it.  Thanks

Same as the 944 but it can handle the HP and Torques that you are talking.  A 944 box would work if it is a light car AND you do not do a clutch drop and never drop a tire in the dirt and then have it catch under power on the pavement. 

So ya don't use a 944 box  I have blown up a couple racing.  Swapped to a box out of a 951 and never had any further issues.  The problem is the R&P in the 944 box's are weak.  In the 951 they are hardened and can take the abuse.  If you want to get really nuts you need to find the box from a 924s and put the R&P from a 951 in it.  This gets you the better gearing of the 924s (only the 924s has this gear set) and the better R&P. 

carguy123 said:

In reply to dean1484 :

The 951 hasn't come up on any conversations with the Porsche guys or transmission shops.   So now I'll look at it.  Thanks

Its nothing special, its just a slightly stronger version of the 924S/944 Audi 016, slightly more likely to be found with a factory limited slip and/or an oil cooler, the gearing is different which may or may not help you.

The standard 944 016 or even a smaller spline 924NA 016 would work just fine in your lightweight Locost, there's no need to go hugely beefy unless you're running weights around the same as a 944.

Read this:

http://flussigmagazine.com/11/post/2013/12/g31-transaxles-016083-transaxles-and-01e-supplement.html

especially the gear ratio chart and use that to determine which box you want to use and add a limited slip to that.  You can buy them from breakers or new from Porsche or other vendors like Guards, etc.

What are the hp limits of the 924 box's? I know the early ones were 4 speeds and I think the later ones were 5 speeds.  Is the box from a 924 turbo different? 

@ OP don't confuse the 924 and the 924s.    

dean1484 said:

Same as the 944 but it can handle the HP and Torques that you are talking.  A 944 box would work if it is a light car AND you do not do a clutch drop and never drop a tire in the dirt and then have it catch under power on the pavement. 

So ya don't use a 944 box  I have blown up a couple racing.  Swapped to a box out of a 951 and never had any further issues.  The problem is the R&P in the 944 box's are weak.  In the 951 they are hardened and can take the abuse.  If you want to get really nuts you need to find the box from a 924s and put the R&P from a 951 in it.  This gets you the better gearing of the 924s (only the 924s has this gear set) and the better R&P. 

 

 

He's building a Lotus Seven clone.  He said so in the original post.  For a 12-1400lb car, you don't need a massive transaxle to handle that sort of power, nor do you need super tall gearing, aerodynamically you'll be pretty well limited, so I would go with the gearing that suits the engine and lower top speeds.

dean1484 said:

What are the hp limits of the 924 box's? I know the early ones were 4 speeds and I think the later ones were 5 speeds.  Is the box from a 924 turbo different? 

@ OP don't confuse the 924 and the 924s.    

Read the Flussig link.  They are all pretty strong, the larger splines of the later versions provide more overhead safety on the heavier cars and will of course handle more, but the input shaft rarely fails, its usually the final drive pinion and that is an issue for all of them and the fix is to shim it tight, use good fluid, add the bearing retainer plate that the GT40/Lambo guys use and keep the transaxle from twisting and flopping around.

For his needs, I can't imagine you'd need a full fledged AOR box, there are regular 924 NA's running 2-300hp on stock transaxles with superchargers or turbochargers strapped to their 2.0L power plants. 

Since you asked about Corvettes, here is what I know.  YMMV and its off memory so take it with a few grains of salt.

• C5 coupe/vert and Z06 have different ratios.  The C5 coupe/vert are better for long legged/high HP cars and the Z06 is set up better to run at the track where you want to stay 1-2-3.  The jump to 4th is large on a Z06 box and I noticed a several MPH increase in traps going from a Z06 box to a Coupe box in my supercharged 2002 coupe, 535 wheel horsepower on the Dynojet.  It would trap around 127 as memory serves with the coupe box and 122 or so with with the Z06 box.
• The Z06 box is supposedly stronger but I still broke it.
• Available C5 ratios are 2:73 (stock Auto coupe/vert) 3:06 (optional coupe/vert A/T) and 3:42 (manual trans coupe/vert/Z06)  The Z06 had shot peened gears and are supposedly stronger but of course I still broke it.  The gear ratios are identified by a tag on the diff, the shot peened gears have an "SP" suffix
• The stock limited slips are pretty decent but most in yards have broken belleville springs and act as open diffs when you find them.  C6Z06 springs and clutches are an upgrade and drop in.  I would tear down any diff and inspect these parts before putting one into service.  No special tools required.  You should also replace the output shaft seals and o-rings in the diff case as they all are leakers otherwise.  
• Stock half shafts in the diff are weak, C6 Z06 parts drop in.
• The cases are pretty flexy but you can get a brace kit from a few shops that will solve that problem.  Brace kits are around $300.  
• Service parts are tough to come by, at leas they were 3 years ago when I rebuilt a coupe trans.
• Stock C5 shifts through the bellhousing preventing you from running a standard clutch.  In theory you can change shift rails and block off the output of the shift rod in the bellhousing and shift a C5 trans from the traditional position.  
• C6 and Z06 08 and down are upgraded versions of C5 transmission.  C6 Z06 08+ got a TR6060 and a much bigger diff.
• You can machine the pinion of a C5 diff and use a Viper output shaft with larger splines, this was the hot setup before the TR6060 started showing up in yards.
• Its a long trans but pretty skinny/small diameter in the grand scheme of things.  No practical way to get the diff moved further forward, its mounted to the bellhousing.
• The stock rear diff has provisions for a single mount in the center similar to all other GM transmission mounts.  

Sorry guys that little thing called work got in my way.  I got 5 minutes earlier and so I posted about the 951 but then had to go back to work. When I finally got off work & looked through the rest of the thread I realized the 951 is the 924/944 box. I have looked into and California Motorsports swears it simply won't hold the torque.

I was told the 968 used a drive shaft but the picture someone posted it looked as if it used a torque tube too.  Which is it?

Yes the transmission will be divorced from the engine and the transmission will reside in the rear while the engine is upfront and connected to each other by a driveshaft.

LSD is an absolute must!  It's not something that I can skip and add later because when you have 300 hp and only weight 1300-1400# you can't tune the suspension when you can't control it.

I have quite a bit of experience with these vehicles and know that even running the S2000 motor with limited torque it is more than a handful.  Adding 50 more horsepower and 100 more of those cute little torque critters will make it more than you can handle with one wheel drive.

The Vette trans was suggested by a friend and I kept looking at text pages rather than searching through images because by that time I was tired of surfing so I kept getting variations of this image which makes the trans look shorter than it does when I finally began searching images and found hundreds of pics.  But still no dimensions.

I haven't had time to look at the thread posted about the Audi 5 speed swap on rennlist, but I will do that once I post this.

The 924 trans for sale looked good, but the auction has ended.  Interestingly the torque tube was about the same as the transmission.

A driveshaft seems to be a better option for me since I'm hooking up a dissimilar engine & trans.  An adapter for the driveshaft seems easier and smaller than a torque tube.

I will go put my hands on a Subaru trans tomorrow.  It's an automatic but since it has to hook to the same engine it should give an idea about the adapter.

Honestly the only thing really going for the Vette is that I know LSDs are common.  Also from the cost of Porsche transmission rebuilding and Subie addition of an LSD (plus I'm not really sure how robust the Subie trans is) I could simply replace the whole darn thing less expensively than working on the Porsce.  But it would take more than adding 4" to the wheelbase. 

In the morning I'll go out and see what that would do the chassis and wheelbase.  If it were to be a track car only the extra wheelbase might not hurt but for an autocross car it could make it uncompetitive.

I know I can put this together in time if I use the traditional transmission, but because of the added power I'd really like to put the trans in the rear to help with traction and balance.  Once I put it together I'm sure I'll never get around to tearing it up and rebuilding with a transaxle so if it's possible I'd like to get it right the first time.

 

968 uses a torque tube.

I can't think of any rear transmission cars that don't use a torque tube type arrangement.  You need that to keep the engine and transaxle inline.  It is very difficult to get a U joint to live for very long at engine speeds, and even then they will have speed oscillations that you really don't want to happen between the engine and trans.

Note that GM kind of got around the "inline" problem on the original Tempest (half a 389 up front, sorta kinda Corvair trans in back) by having a flexible driveshaft mounted on four bearings.  There were no U joints but the "rope drive" described an arc instead of a straight line.  This meant the floor could be lower.

Arg.  Now you have me thinking, and that's never a good thing.  What about mating a high RPM engine to an old 3 speed transmission with 1st and Reverse eliminated, and then run the driveshaft out of that to a transaxle in the back?  The gear reduction from running in "Second" in the front trans would cut the driveshaft revs down to saner levels...

I’ll put in a second suggestion for the 915

i used it in the frankefiat because it was cheaper than getting a vw bus box to handle the power.

however they do not like to be shifted fast

In reply to carguy123 :

Ok.  I disagree, but it’s your car.  You do you.

You'll probably have to shorten the torque tube and respline the driveline to fit the Locost sizing.  A driveshaft shop should be able to make that axle for you after you shorten the tube.  Luckily the tube is pretty thin steel.

STEFAN somehow I missed you post with the flussig link.  I'd left my browser open for a couple of hours and since the new forum didn't warn me there had been new posts like the old forum did I didn't realize several other people had posted.  Let me catch up on that.

All the transmission type people have said the 924S/944 simply won't handle the power.  I am going over to another performance transmission shop tomorrow so I'll ask again.  I was on the GT40 forum and 944 trans were their weak link.  I'm still reading to see if they've found a solution, but I'm hoping to find a stock type unit that only requires external modification to box.  This IS a LOCOST!

What I have been told by multiple people is that the torque tube isn't necessary and that a driveshaft will work better for the home builder because, while the torque tube shortening is fairly straight forward, the issue of harmonics on the drive shaft inside the torque tube would play havoc with the input shaft and it could take several failures to get it right.  That isn't a problem with a traditional driveshaft.  I have an idea for a support & bearing to take the driveshaft.

One of the functions of the torque tube is the handling of the torque.  If you don't have a torque tube then you need to have stronger mounts on the trans capable of handling the torque transfer. 

Here's a thread some of you may enjoy.  It's not about transaxles but the transmission is one of the issues to be solved.

http://www.ozclubbies.com.au/topic/17423-subaru-power-front-engine-clubman/

The 951 box is the only 944 variant that I know will handle it.  These cars are regularly boosted to the numbers you are looking at and with your car being much lighter it should not be a problem. The 944s2 also uses the better box and the 968 uses a different one all together so more research on those is needed.   The standard 944 box will not take. Are your sources differentiating between the 944 and the 951?  Do they know the difference between the two?  The down side the the 951 box is the gearing. It is a bit taller than the regular 944 box. The 924s got a 944 spec box but the gearing was even shorter than the 944. In the spec 944 series the 924s box with an updated R&P is the hot setup. As for the 924 box’s I am not up on them to give you any info. 

Alfa GTV6 has a rear transaxle.

In reply to RossD :

I guess I should add the 61-63 Pontiac Tempest LeMans to the list too, but that's pretty obscure and there's no good reason to search one out as a donor.