Here's a write-up for those looking to install a C5 diff into your 05 C6. The reason why I decided to go with a C5 diff instead of a 06up C6 diff is simple...PRICE. That's right. A C5 diff is much cheaper to replace or build than a C6.

This modification consists of welding a aluminum plate to the rear cradle, grinding some cooling fins off the C5 diff and fabricating a custom diff mounting bracket, which is basically a wide trans mount. The 05up diffs are mounted via two mounts, one on each side. The C5 diffs use one mount which bolts to the bottom of the diff and onto the cradle.

First, I'd like to give a big thanks to Joe for all the info he supplied me with. Here's to you Joe...

Ok, on with the write-up...
I first started out by removing the C6 diff which involves dropping the entire rear suspension, cradle and mufflers. For more info on doing this check out http://www.vetteweb.com/tech/vemp_08...wap/index.html.

Once the cradle is removed this is where the plate needs to be welded to....


Here is a picture of one of the plates I cut to weld onto the cradle. The other plate looks exactly the same and each one is 1/8" 6061 aluminum.


As you can see in this pic the goal here is to cover up this triangle hole so we can create a mounting surface for the diff mount.


After doing a little grinding on the cradle to clean up the surface and a little welding, this is the finished cradle. Each 1/8" plate was welded one at a time and one on top of the other to create a 1/4" plate. My welder can only weld up to 1/8" so two plates were necessary. Later on two holes will need to be drilled through this plate so that the bolts that hold the bottom of the diff mount can pass through.


Next step was to modify the C5 diff which consisted of shaving about 1/8"-3/16" off of the bottom cooling fins. A grinder will get this job done in 10mins. In this pic the left fins were already shaved to give you an idea of how much material to remove.


The last modification is to fabricate a mounting bracket to be able to bolt the diff to the cradle. There are a few different ways you can go about this.
1. Fabricate an entire bracket from scratch with a rubber spacer.
2. Modify a C5 diff mounting bracket.
3. Buy a trans mount and fabricate around it to be able to use it for this application.
I used a Energy Suspension trans mount that I bought from AutoZone. It's about 1-1/2" tall X 2-1/2 to 3" wide and stronger than factory rubber mounts. Here's a pic of it.


As for the other part of the bracket, basically what I did here was cut a piece of 2" angle iron about 8" long. I then trimmed off about 3/4" off one side. As you can see in this pic I have the angle iron already marked for my cut.


Since the angle iron was only 1/8" thick I added an additional 1/8 flat stock to one of the sides for added strength. Here you can see the angle iron was already trimmed and had the additional 1/8" flat stock welded to it.


The next step was to cut a 2-1/2" wide section (or however side the rubber mount is) in the center of the angle iron that was beefed up with the additional 1/8" flat stock (remember one side was trimmed while the other was beefed up). This creates a slot so that the rubber part of the Energy Suspension mount can drop right in. Once this is done it's ust a matter of drilling a pair of holes and bolting the poly mount to the angle iron. The finshed product should look like this...



Next you need to drill two holes on each side of this bracket to be able to bolt it up to the bottom of the diff. To figure where to drill these holes I just measured the length from each hole on the diff. I measured from the center of each hole to the next. I think it was 7.xx"...not sure. Then I just centered that measurement on the top of the new diff mount, marked the drilled. Here's a pic with the holes circled in red.


Now bolt the bracket to the diff and reinstall the cradle. Once you bolt the cradle in place you'll be able to tell where you need to drill the holes through the aluminum plate. Once the holes are drilled you can now bolt the bottom of the bracket to the cradle.

Now it's just a matter of reinstalling everything.

NOTE: One thing I would have done different was use a wider poly mount and make a cooler looking bracket. If doing this mod yourself, be sure to measure from the top of the torque tube up to the bottom of the car. Use this measurement to now exactly how high or low the diff needs to be when you're fabricating your own bracket. You don't want the diff sitting to high or to low from the cradle.

Hopefully this will give anyone looking to do this mod a better understanding of what's involved. I'm going to be working on another bracket with a slight different design for a buddys C6. This bracket will be a complete bolt-in kit requiring no welding, however the welded plate looks cleaner and stronger IMO. I have plenty of material left over so I'll be making a few more if anyone needs one.

Sorry for not supplying better pics. It was late sunday night and after 2days I was beat to hell and frustrated.

Thanks for looking.

 

Wow that is some intense labor. Good job. I hope it holds up good. Keep us posted on the performance.

 

I'll definately keep you guys posted. I'll be doing two or three track days this month. In total I should have around 30 passes at least on the car by the end of this month.

I'll see if I can make a more better looking bracket. I love working on my car.

 

Way to go Sparo! I love the how to threads and hats off for knocking the job out yourself.

Don't forget to get some heat cycles and break in miles in your diff...but hey, you made your goal of Halloween! Good job!

 

nice work thanks for the write up

 

One of the problems that is often overlooked with the C5's is the RH cover stretching. This was solved on the C6 with the new covers. If you are racing the car and this is why you went to the C5 diff, this is going to be problem over time. Most C5 race cars are looking for a way to move to the C6 diff to keep this issue from happening. What happens is the RH cover stretches because it is the "main cap" and before you know it it breaks or blows the diff. The price on the diff between C5 and C6 is the same to build them, you are looking at the cost of the upgrade I am sure. You can start by just buying a used diff if that would help. The adapter for the trans is less than $400. It is just personal preference, but GM rarely goes backwards with engineering also, they always improve on the previous. If this does not work for you, then call me, I can help you figure out how to get into a 06 for about the same amount of money. There are always more than one way to skin a cat.
Justin

 

I totally agree, but would you agree that the rear in a manual 2005 Corvette might not take quite as much power/abuse as the rear in a manual 2004 Corvette?

 

Good write up! I've been jonesing to know what is required for the conversion. Up until now it's all been very double secret. I'd been looking at the PFADT C5 transmission mount as an option to mount it to a modified cradle as well.

Justin, I'd be interested in seeing a breakdown of how close an '06+ diff conversion could come to a C5 diff swap. Can't say I've seen too many '06+ diffs for sale used but I'm sure they come along. I guess if the tailhousing is ~$400 that would offset the cost of the cradle modifications and mount so if a used '06+ diff came along for a similar price as a C5 diff then it would make it worthwhile. I'd want to put upgraded output shafts in either diff.

 

When you crack your 05 case to smithereens like I did you don't have a core. That's when the C5 makes sense.

I picked up a newly built C5 rear with 4:10 gears and hardened left shaft, new clutches, etc. for $1300. Those deals are out there.

A similar C6 rear was twice that without a core.

Plus I've seen MANY of my buds race the heck out of a C5 rear - they'll crack after hundreds of runs, like any rear will. But when they go, and once again you don't have a core...then the C5 is much cheaper again.

I got 301 6000k launches and 3 years of racing out of my 05 rear. That's a good life. I don't think any Vette rear is going to hold up to much more than that.

 

I think GM went backwards in 05. That 05 diff is more flimsy than the rear in the 4 gen FBody cars. I blew my 05 diff on street tires and i'm sure I am not the only one. I've seen plenty of 9 sec C5 running the C5 diff with no problems so I'm going to give it a shot. Worst case I end up learning the hard way and going to a C6Z diff and i'm only out what I paid for the C5 diff and bracket.

Only one way to find out.

 

Thanks Joe, I probably wouldn't have made my deadline if it wasn't for you. Now that I know exactly what it takes to do this mod I can probably knock it out in one whole day.

I'll be ordering my next rear from HorsePowerSales. Currently I have a used diff in there with the stock 3.42 rears. I didn't want to experiment with a freshly built rear. I'll try and make it out to the track tomorrow and again on Halloween for a few 1/4 mile runs. Then i'll be swapping out the cam and getting a rear with 3.90s.

I'll try and get some videos of the runs too.

 

Nice job!

Many shall benefit

Thanks,Matt

 

I don't think they went backwards on the 05. I believe there was an underlying problem that did not surface until the mount in the rear was removed. The only difference in the housing on a C5 vs a 05 C6 is the lack of a rear mount so you have less material. The sides of the diff are much stronger. I am not arguing with what the OP did here, I think he was thinking and using the best resources he had. My only question is for one, is the 1/8 aluminum acceptable for the mount ie, will it be strong enough? The cast material is not much thicker but it is heat treated and it is beefed up one the C5 crossmember vs the C6 one. There are allot of alternatives on the C6 diffs, you could have bought and 06C6 auto diff that someone pulled out and use that as a core to build. I have bought those of a couple of hundred dollars. It takes some work, but you can change those into manual diffs. Like I said, I like the fact you are thinking about how to make something better cheaper, anyone can throw money at something and still not be much further ahead, but GM spent lots of money on all of the driveling to make sure they do not break.
Justin

 

I have nearly 800rwhp and MT Et street tires as my street tire. I did this mod 10k miles ago and the number of 700rwhp+ C5's out there with faster track times than any C6 is the proof that its better and holds up just fine. I dont see any C5 guys complaining about the mounting surface size or stretching side covers for which there are billet after market pieces. I think the c5 set-up is far better and rocks from a center point to absorb driveline shocks unlike the wishbone break action you get from the C6. Yes the Z06 unit is better but it weighs a ton more and you cant regear it.

I have sen 06 and 07 diffs break too so they arent the be-all.

A C5 core is 700 bucks new and a C6 core is nearly 3 times the price. If I break one, I dont have time to try to find a bargain c6 core. I havent broken one and if I didnt, its pretty strong. Mine has even survived the old wheel hop days with nitrous and its still here. As far as comparing prices, a C5 conversion will be 1/2 the cost of a 2006+ fix. Yes you can get a cheap C6 core if you look but the same can be said about a C5 core. I've seen them for 300 bucks.

The C6 set-up isnt as good because its too rigid. The center mount absorbs a lot and the counter wieght makes it run smoother by not transfering vibrations direct to the frame like the C6 unit did. I had both and wouldnt take a 2006+ conversion even if it were free.

As far as the 2005 differential itself, the mounting surface wasnt the issue. many of the early breaks i saw posted on were case fractures right up the middle where the metal was thinner after the 2005 diff had much thinner metal. The 2004 diff has all that extra metal where the center mount was previously located and this thinner metal was where the fractures were located. I even recall seeing pictures where the diff was intact but a hairline crack formed on the case dead center where the center mount on the 2004 case is located. The rest of the case failure was then incidental to this.

 

You said based on what you see. Well again GM does not go backwards, they do make changes that cause other things to change also. As far as the side cover I can tell you in every C5 diff that is built for racing or dragracing that it is an issue. So you mention the billet cover, ok how much is that? Cheaper than the C6 diff now? I have seen every failure there is with the C5 and C6's. Of course there are plenty of C6 failures, when you push the cars this hard parts are going to break plan and simple.

"As far as the 2005 differential itself, the mounting surface wasnt the issue. many of the early breaks i saw posted on were case fractures right up the middle where the metal was thinner after the 2005 diff had much thinner metal. The 2004 diff has all that extra metal where the center mount was previously located and this thinner metal was where the fractures were located. I even recall seeing pictures where the diff was intact but a hairline crack formed on the case dead center where the center mount on the 2004 case is located. The rest of the case failure was then incidental to this."

The 05 housing is the same except for the boss which the mount was on for the C5, yes that is where they break, however they no longer stretch the side cover. In 06 they made the pinion support stronger, just like I said, they do not go backwards, or at least they try not to.
I am not trying to make this into a he said she said and who's is better.
My question was why he did this, and is the 1/8 aluminum strong enough and is the cradle strong enough to hold the center mount diff that it was not designed to. I still think he did a great job thinking about how to upgrade and performing it, but that does not always mean it is better. There are always several side, this is mine, it is obviously not yours, and you believe yours is better. I have seen many C5 diffs blow up on the race track or were not getting much life out of them before having to be rebuilt which we eliminated both issues when the C6 diff came out. Our 05 diff lived the 24 hours of Daytona with no issues at all. The same comparison to a C5 diff had to be replaced due to side cover stretch after a couple of hours of being tortured out there.

 

Hey Justin,
You have a few good points. I first looked at doing the 06up conversion but after doing some research and getting some "first hand" opinions from C5 owners, I decided to go the C5 diff route. It's much cheaper than upgrading to the 06up diff and much cheaper to replace. Besides, I like modifying my car. Nothing like saying you fabricated or built something yourself.

As far as the mounting plate goes I think it's pretty strong. If you read closer you'll notice I used two 1/8" 6061 plates and double stacked them, so what I ended up with in the end was a 1/4" mounting plate which should be more than enough. I did this by first welding one 1/8" plate then after some grinding, welded the 2nd 1/8" plate on top of that.

 

That does change some things, a 1/4 plate I would think would be strong enough. My only other concern would be the actual strength of the crossmember after is is welded. One of the World Challenge cars actually reinforced his by welding in aluminum, but they boxed in the entire thing for added strength. I would keep an eye on it over time, if it did break I don't think it would cause an accident or anything horrible, but I would keep an eye on it.
Justin

 

Billet side covers are available at last check on the Dynotech site for a race built diff if you need it. Mine is fine.

Thanks for sharing the info. I'm not all knowing but I have what I have and have seen my fair share of C6 diff failures. People have to see all the sides to a debate and at least we took the time to share our views unlike those who care less when it doesnt earn a penny.

 

we need to change the term of c6 diffs to c6-05 diffs, or c6-06 diffs. im confused.

 

Spin,
I am not looking at this as trying to make money. For one, I don't think I need dynotech to build us anything, we do just fine. I am merily stating the issues with C5 diffs. Do all of them fail, no. Do all of the C6 diffs fail, no they don't. I believe I complimented several times on doing this on his own to save money. Just because we have a company and I am commenting on a thread does not mean it is only to gain to earn his penny. I just happen to a little about diffs and was asking some questions and pointing some things out.