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Corvette C5/C6 transaxle in a high horsepower GTV6

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31K views 99 replies 26 participants last post by  Gabor K. 
#1 ·
This is an entirely hypothetical question. I've heard that the stock Alfa transaxle starts to blow up over about 350-400 horsepower and I know there are more than a few GTV6s making that. I believe there was a thread on here awhile ago that featured a 600 hp GTV6 with a Volvo transmission and aftermarket rear end mated together and mounted in the back of the car. I was thinking about other modern, transaxle cars and the C5 and later Corvettes came to mind.

Has anyone put a Corvette C5 or C6 transaxle in one of these cars to cope with the power? How about just the differential mounted to some other favorable transmission? The diff appears to be a clutch type LSD, which is not ideal but still a good unit. Better than the open diff. The whole thing is aluminum so it should only be marginally heavier.

Thoughts?
 
#2 ·
Nope, nobody I know of has swapped in a Corvette transaxle. At a glance it doesn't look possible without some major surgery. It's probably quite a bit heavier, the Alfa unit is actually pretty light for a transmission/differential, clutch unit, and flywheel, at least one that can handle that much power.

There are very few 350+ horsepower transaxle cars, of those a high percentage of them use front mounted gearboxes.

Greg,
Silicone Hose Kits
OKINJECTORS--Gasoline and Diesel Fuel Injector Cleaning Services + Remanufacturing in Oklahoma

P.S. a 300hp GTV6 is a very fast car.
 
#89 ·
hmmm



So, I guess from the very beginning,, Greg's post to this thread stand to be the "bench mark" so to speak?
 
#3 ·
Why? When u could use a front mount getrag , rb25 or tremec t56 gearbox that will hold 500 + horses for less work and less money / purchase cost?
 
#4 ·
another interesting thought would be the porsche 928 transaxle, although im not sure if the 5 speed boxes had the clutch next to the engine or in the back.. the 3 speed Auto boxes had the torque converter in the back with the transaxle.
 
#23 ·
Manual C5s and C6s have the clutch up front, just like manual 928s. So the prospective transferee would need to come up with a new plan for the clutch and rear brakes, both of which are missing from the Corvette transmission/differential assembly.

But hey, in for a penny... it sounds like a worthy project to me.
 
#5 ·
Brad, mostly hypothetical but purchase cost and horsepower capabilities are considerations. Also, I'm sure many of us would rather keep the car a rear transaxle for a variety of reasons.

Smotheredsteak, do you know what the difference between the 924/944 and the 928 transaxle is? Because those would also be an option.
 
#6 · (Edited)
i do. the 944 transaxle is based off of the FWD Audi gearbox, meaning the gearset is BEHIND the differential, whereas on the 928 the gearsets are in front of the diff., like the alfa gearboxes.

i was both a FWD audi 5000 and a 944 owner at one point. there was a problem with blowing pinion gears while doing a hard 2nd gear shift with these boxes.
 
#8 ·
I imagine the 928 box can hold more, but I really don't know.

The problem I have is this. To get over 400hp in a GTV6 you will probably need another engine, like a 3.0 from a Milano as a minimum, probably something even newer, along with another transaxle, driveshaft, clutch, guibos or u-joints. All this means changing the rear axles and brakes. When considering all of this, it seems to make more sense to me to start with a different car if you want 500hp.

Greg,
Silicone Hose Kits
OKINJECTORS--Gasoline and Diesel Fuel Injector Cleaning Services + Remanufacturing in Oklahoma
 
#9 ·
you must realize an alfa is an alfa, and if you want 500 hp you should buy a corvette. i can sleep peacefully everynight by living this rule.. however i will NEVER buy a corvette..

the mid 80's to early 90's with the 32 valve engines, the 928 was doing 400 hp easily. some with 6 speeds too i believe.
 
#10 ·
Even for a reasonably powerful track car that didn't necessarily have 350hp or more, a Corvette transaxle would be a much better choice. Much better shift quality, availability of well priced close ratio/dog ring engagement gearsets and just the shear abusable nature of having a transaxle that can take so much torque! Bouncing off curbs and over rises where the wheels get unloaded and spin a bit all shock loading the transaxle..........
Nice and reassuring to know it's all a walk in the park for the transaxle!



And why wouldn't/shouldn't the OP have a 500hp transaxle Alfa Romeo?
Just because you don't want 1?
 
#12 ·
Well, the OP (me) isn't necessarily planning on doing any of this. The OP is just an engineer who can't stop himself for looking for the best possible solution to a problem, regardless of how uncommon that problem might be. Thanks for the support Duk, on that note I like your signature.
 
#16 · (Edited)
Hey Garth, my transaxle is sitting on some make shift stands and while sorting out another projects clutch requirements, I happened to take the clutch disc from a very mundane car and slide it onto the input shaft of the Alfa's TA. I was not impressed with what I found :eek:

Just for pure comparison, the clutch disc was from a 1.8litre Corolla engine. An engine with 115hp and 115lb/ft of torque in a car that weighs a lot less than my 75 V6 does. The Corolla's input shaft is bigger than the nearly 200hp V6 Alfa's input shaft.

With big power, I wouldn't even bother trying yo get an Alfa transaxle to live, I'd just go either for a Corvette TA or a (not really nice to say it) 'cobbled together' TA set up of compact gearbox and IRS style diff held together in a frame of some sorts.
 
#14 ·
porsche 928 wheelbase: 98.4
alfa gtv6 wheelbase: 98.8

i suppose the whole gearbox and torque tube would fit quite good in a gtv6. if you can find a way to mate an engine to the 928's front bell housing you might be on to something here! id bet even the shift linkage lines up! do the corvette's have a torque tube setup, or a traditional drive/prop shaft with dampeners?
 
#18 · (Edited)
Duk, that had been my initial thought. The C5 differential is ~60lbs according to some forum I read. Adapting it to another transmission would totally depend on the tranny but it could be incredibly easy or incredibly difficult.

The issue is that I think that diff is a clutch pack differential, which is NOT my preferred type. I would prefer a Salisbury, but I found this article about the C6 diff that talks about an aftermarket LSD. That's the exploded view, 4th picture down. It only costs $2300! So to have a diff as good as a Milano LSD you would need to spend that kind of money.

If you're making enough power to want to go to a bigger T/A you definitely want an LSD for anything but drag racing. That brings us back to square one on cost, advantage is still a stronger transmission and more gear ratios. Maybe even a six speed with the Vette diff.

Edit:
Giken also makes an ATB torque biasing diff, if that's your cup of tea. Also about half the price.
http://www.horsepowerfreaks.com/partdetails/Quaife/Drivetrain/Differentials/Limited_Slip_ATB/16679
 
#19 ·
Im surprised nobody has mentioned a Colotti gearset at all. They're only 8000 euro, which may seem like a lot, but not really when you take into account how much a corvette conversion will set you back. And plus, if youre building a 500 hp Alfa engine, another 8000 euro is bugger all in comparison to the cost of building the rest of the car. Well, if youre doing it properly of course...
 
#20 ·
At the end of the day, the casing, ring gear/pinion and input shaft are all pretty small. These things were designed to be powered by smallish 4 cylinder engines and that is really shown by how small the input shaft is.

A Corvette transaxle is designed for 650Nm+ supercharged 6.2 litre engines.
Even with a Collotti gear set (there are others out there), you just can't make the Alfa TA that strong. Yes it will be stronger, but it's not 650Nm+ strong.
And that torque capacity will give it a massive resilience to shock loads.
 
#21 ·
Alfa Transaxles

I wouldn`t think of changing the Alfa TA to a Corvette unit.

The Alfa unit is plenty strong unless you are doing very dumb things to the unit (drag racing, or burn out starts).

Shaft size isn`t indicative of strength. It is alloy composition. None of the Supercharged or Turbo cars break the input shaft. They destroy the LSD clutch disk inner splines. For that reason 4 disk setups are the solution. 2 disk just don`t have the material to transfer that much torque.
 
#24 ·
I dunno guys, even though I started this little brainstorming session I'm starting to balk at the Corvette thing. It just seems too heavy and too much work. I'm leaning a little more towards MoRu's method, mating a performance trans to a modified or custom diff casing. (his thread: http://www.alfabb.com/bb/forums/alfetta-gtv6-1972-1986/197678-gtv6-3-2l-24v-600whp.html#post1154289) If I'm going to be practically ground up engineering anyway I might as well go all the way and build something very light and bulletproof. I was looking to the C5/C6 for a potential cheap, drop in kind of a solution.
 
#25 · (Edited)
Yes, agree, the MoRu choice with the Drenth box seems to be a much wiser and lighter choice than the bulky Corvette box. :



MPG | Drenth Motorsport Gearboxes

Reason for this arrangement, in contrast with with for instance the South African solution to move the box up front in high power solutions, is that in our racing series the gearbox must be in original place(transaxle). So the position has to be the same even if anything else than the original box is used!

G.K.
 
#26 ·
This Drenth gearbox certainly looks like the answer. It physically fits, it can handle the power, and it's already been put into a GTV6. Judging from the one picture, the fabrication required to mount it, isn't trivial, but a least it's standard machine shop type work.

Anyone know how much those cost?

Greg
 
#28 ·
It's always 'catch 22' when it comes to modifying cars, especially ones where there isn't much aftermarket support, so you end up going it alone.
The end results is always a high financial cost.
 
#29 · (Edited)
We've now started the conversion and the initial findings look VERY promising!

Some (not so quick) history: When I converted my 3.7 Milano to a forward-mounted 6-speed Getrag transmission with a conventional rear-mounted differential arrangement from a Sedan (Alfa Sei), it was less about the STRENGTH of the box, or the strength of the internals, and much more about the QUALITY of the shifts!

After going through many-many standard gearboxes and having everything break from the LSD, to the side covers, to the clutch housing cover, to the stupid Italian-snot GUIBOS, I decided that living with the weak synchros was to be the final insult! Up until that point we had lightened the gears by cross-drilling and back-cutting, we tried lightened front flywheels, lightened rear flywheels, small-diameter clutch kits, shimmed-up LSD units; we tried EVERYTHING that you can on these Alfa boxes! We got them to shift better, but not perfect.

You get in my car now with that 6-speed Getrag (unit crosses over to the E90 / E92 / E93 V8 M3 box and several other European models), and you just shift it. You don't even think about it. Shift up. Shift down. Slow. Fast. Smile. Don't even think about it. Heel and toe. Double-clutch it - or don't. Slam it home, or go smooth. It really doesn't care, it just does its job like it should!

Yes, it was the effen syncros and the weak 1-2 up-shifts and 3-2 / 2-1 down-shifts that finally did it for me. That and serviceability. I can live with many things, but pulling transaxle Alfa boxes in and out is a soul-sucking job. Call it what it is. Add that to the weak peripheral design of components OTHER than the actual gears themselves (the LSD, the weak syncros, the case, the dumb 2-piece driveshaft, the rubber donuts etc.) and that's what it took for me to just say "no more"... :smile2:

I love my conversion. It was expensive and difficult, but I have never looked back. (Hell, all of the money spent on Alfa transaxles was just as expensive - not to mention the frustration!) To mention the turbo cars (with EXPENSIVE Colotti gear-stacks) as a comparison is ludicrous! Failing to mention works cars (without the context of legions of mechanics swapping boxes during and between races) is misleading. No one else in the US has ever run 400+ horsepower through one of these transaxles, so to reference a fix that will only work MARGINALLY on a modified standard bore 12 valve power-plant is not realistic.

The OP asked about a Corvette transaxle solution for a "high horsepower" GTV6, and that's what we should be talking about! We have started the conversion on a GTV6... Here are some pictures of the initial disassembly and eyeball work! We'll first address the big question of weight!
 

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#30 · (Edited)
Enter my friend Mr. Ron who agrees and wants a better solution for himself, but who also wants to keep the transaxle layout of his GTV6. I was very concerned about the weight. We talked about it a-LOT and then he decided to just go for it. He acquired a complete C5 Z06 rear sub-frame, suspension, transaxle, brakes and torque-tube setup.

107 pounds! That's it folks! Only 48 kilos - with 24 of those kilos (53 pounds) of it, being nicely distributed for you along the tunnel of the car - BETWEEN the front and rear axles, along the center of the car, and at the lowest point of gravity that you can have!

The other measly 54 extra pounds going to the rear of the car (but still INSDE the axles! Keep in mind that the gearbox of the Corvette transaxle (as with the Alfa boxe it will replace), sits in front of the differential! (It's not like the Porsches where the gear-stacks over-hangs the rear of the car - BEHIND the differential...) This remaining bit of weight is again very low in the car, at the center of the car and INSIDE of the front and rear axles!

This bit of weight being the only penalty - against the backdrop of HUGE gains made in structural rigidity / engine to transmission connectivity), gear ratio options, final drive ratio options, strength, ease of removal and re-installation for service, etc.
 

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#31 ·
I was amazed by two initial findings:

1) How light that entire Corvette rear setup was - and
2) How close the initial measurements are for the length of the setup!

I was really worried about shortening the drive-shaft inside of the torque-tube - in addition to shortening the tube itself - in addition to all of the other variables! It looked SO long! Well, just from the initial eyeball that we have on it, we may not even have to touch the length!

The Corvette rear stance is WIDE though and we'll have to section the rear sub-frame to narrow it considerably! That - and then shorter side-shafts and a shorter anti-roll bar obviously. But that's it in terms of modifying the rear unit itself. Then there's the conversion to the bell-housing at the front and the front clutch arrangement, which is another story.
 

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#32 ·
The Corvette transaxle stops right behind the bulkhead - let's say right behind a rear passenger's heels. The torque-tube makes a perfect distance run from there right to the rear of the stock Alfa V6 engine! I mean, I couldn't have planned it better myself - it's like it was made to go there. The Corvette just looks SO long to me though! (The nose is long and it houses the extra two cylinders - must all be forward of the torque-tube!) The Vette engine must sit pretty far back in the chassis, I suppose...

Stated differently, the distance in the Vette from the rear of it's V8 engine to the center of the rear axles / side-shafts, is virtually identical to that same distance in the GTV6!

The biggest changes are going to be -
a) The amount of sub-floor that we'll have to cut away to make room for the Vette transaxle. (It's much taller than the Alfa transaxle. The Alfa box is actually quite flat on top! The Vette box is tall and round.) And -
b) The clutch arrangement. The Vette's clutch sits up front. (Ron opted for a Mantic clutch from Australia and had I a custom flywheel made that excepts it, but still has the correct forward and aft spacing and the starter ring-gear in the correct location!)

I'm still blown away by the paltry weight differences...
 

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