How much power can a stock iron case c3 differential and axles handle? I am assuming that the axles and u-joints would be the first weak link in the chain so if those were replaced with heavy-duty axles and u-joints what would be the next thing in the chain to break? The axle yokes? Something in the differential like the spider gears or the ring and pinion gears? How much power can the stock differential handle before the internals need to be upgraded? Thankfully the car has an automatic trans so the driveline won't be getting the kind of shock loads it would get with a manual trans. I'm asking because I put a supercharged 454 in the car and I'm worried about how much power I can put to the differential without blowing it to pieces. I have the boost turned down low (11% underdriven 8-71 blower) so that it doesn't make too much power but I would like to be able to increase the boost eventually without blowing up the rest of the driveline. If I were to upgrade everything (axles, u-joints, yokes, spider gears, and ring and pinion gears) how much power would it be able to handle? Any help is appreciated.

 

If you are running street tires, you may be fine for a long time. If you are running slicks, get ready for carnage.

Stock differential without any rebuild/modification is probably good for maybe 400hp on the high side. It can be rebuilt to be good to somewhere in the 500 to even 600 hp range, but it will take some significant modified internals (heat treat, etc). Beyond that you are likely to need a 12 bolt conversion.

But again, as long as you stick with street tires, you may be good as you are.

 

Only 400 horsepower? What did the high horsepower vettes (L-71, L-88, ZL-1, etc) with manual transmissions use for a diff? Those engines made a lot more than 400 horsepower and when coupled with a manual trans are more than capable of destroying a diff that could only handle 400 hp. Do those cars have a problem with blowing diffs every time they dump the clutch? I would think (and hope) that a stock Vette diff could handle a bit more than 400 hp.

 

Well you didn't mention what year vette you are working on, but assuming it is a C3, yeah, 400 hp is about it reliably. Maybe 500 if you push it, but then it is not going to be very reliable after 40 years of past service. Remember 400 hp was more than really any of these cars had from the factory. Even the L88 was only advertised at 435 HP. They were arguably in the low to mid 500 hp but this was not rear wheel HP either. In that case, the rear end was seeing maybe 450 HP in the very top end car of the time.

If you are running a blown BB, you will need to rebuild your rear end with all top end parts. If you are not dumping the clutch with slicks you should be able to live a long time with a nicely rebuilt rear end.

 

Upgraded rear parts were part of upgraded engines. As for strength, it depends a lot on the abuse. Manual transmissions and slicks will break stuff on launch. Street tires and automatics can handle more power because the impact is less. The street tires are sort of a safety valve in the system.

How fast do you want to go?

427Hotrod has a pretty stout setup and he is definitely fast. You might want to look at his setup.

 

Keep in mind that is rear wheel horsepower not horsepower measured at the crank and 73racevette is pretty close to correct if not spot on.
My research showed that the OEM rear IRS was designed to handle about 500 horsepower measured at the crank.
A 70 LS5 is about max what the rear ends were designed for.
Will they take more?
It all depends on your driving and use of your car.
If you want to run a 3.73 or 4.11 ring and pinion gears I would suggest going to a 12 bolt set up, but if you want to run anything from 3.08 to 3.55 gearing you might get by running a heavy duty 10 bolt.
There are several options available and people who can answer your questions better than I can, and who can build you a rear end that will take whatever you want to throw at it.
Speed/horsepower/torque=$$$$$

 

Don't put sticky tires on it and you'll be good for 500hp. I've had a 510hp zz502 in my '69 for over 10 years with no problems.

 

By the way, here is the car (it's a '69), I made a thread about it here on the forum when I first got it going:













Here is the link to the thread if you want to see it:

https://www.corvetteforum.com/forums...come-true.html

 

If you stick with the tires you have in the photo, you will be fine for a while. Just be careful about a situation where the tires could catch grip quickly, because something will break.

 

Life of the diff depends on several things

1- power
2 - Trans
3- Traction
4 -Driving style

Very nice looking vette, it should attract many at shows. Enjoy it,

You have enough power to break anything south of the flywheel- depending on how abusive you are.

We have experienced:
1-twisting a Tremic slip yoke
2- twisting drive shaft
3-Ripping off the frame to snubber bracket
4- Completely sheared both inner and outer Solid Spicer u-joints
5- Bent 1350 HD axles
6- Blew apart 10 bolt diff
7- (2) super T10 4 speeds= cases cracked like an egg

A 69 diff is not the best foundation to build on, the housings are good but the posi cases and spiders are not what I would use in a HP application. While better then the posi cases prior to 69 these cases are not as good as 72-79's. The spiders are 10-18 and I would not use them at all. You replaced the axles with HD, are they stock 17 spline "HD" if so you really didn't gain much. They are the same as the Std yokes only use straps and bolts to retain the u-joints. While I have seen plenty of broken axles and u-joints I never saw an axle yoke break or u-bolt break. Not saying it can't happen, just that HD axles are not an end all solution.

If you want to get a diff to last you have to tighten it, a real polished and tuned posi is required. A tuned posi will not have the springs in it, nor any lash and has to be balanced. A polished posi is more then spending 10 minutes cleaning up the windows. To those that say polishing a posi will weaken it, well that is total BS - unless the rebuilder is clueless which may be the case based on some things I have seen here. The good 10-17 heat treated spiders have been gone for years now and many use the current powder metal spiders. They are ok but not the best. Fiber coated clutches also should not be used, stay with new solid steels.

Use ARP ring gear bolts, don't cheap out here. The ARP bolts are the best and should be loctited in place using #271 or #272. I see pictures of so called custom work and see the kit grade RG bolts with plated lockwashers used that come in common master kits.

A LH steel cap is needed if you are going to launch the car hard. It is only needed on the LH side, the RH side does not need a steel cap unless you like to spend money. Under load the LH side of the posi wants to lift outward, the RH side digs in. Stock iron caps under enough load such as a hard launch with high power will stretch enough to cause RG to move and bind, resulting in breaking the diff. This is not new or unique to any individual, steel caps have been around for 50-60 years. A RH steel cap is not going to add anything to the build other then cost.

A solid sleeve should be fit in place of a stock crush sleeve- another area where corners are cut by some. Given enough abuse on launch the stock sleeve may compress. The solid sleeve will not. Again this is not a new part, they have been around for years. They have to be machine fit. Now you may hear some say they are not needed, Tom told me he didn't use a sleeve at all. He just Loctited the bearings on the pinion. I prefer to use a solid sleeve. If you need a steel cap you need a solid sleeve.

I also use a Billet pinion yoke since it is stronger then a stock cast yoke. While not as common to see a cast yoke break, when someone is looking to build a real deal strong diff it should be used.

A HD cover is also something I use for the thicker lower ears.

The above is based on beefing up a 10 bolt and if you can built that BB motor you probably can build a real custom diff for your application. You will need access to a mill, lathe and surface grinder. You can add cryo for added strength and REM but REM is not required. I just did a REM setup and while it looks pretty and will cut heat it isn't needed unless you are going road racing for extended time that creates a lot of heat in the diff.

If you want to maintain the IRS and want more strength a 12 bolt is pretty strong. I have one in back of a 900hp BB 72 coupe that is pushed. If you want strength and don't care about the IRS you can go solid. If you have money to spend you can go beyond to some of the newer IRS setups so it comes down to what you really want and need. In our case, I built a complete 1480 12 bolt, including 31 spline Tom's outers, upper link kit, custom steel spring, adjustable shocks and that resolved the issues. However, I never use the term bullet-proof as anything will break given enough power and abuse.

The last piece of advice, do your do diligence or you will be doing the diff over again.

Good luck, call Desiree at Tom's she will have all the parts you need.

 

Thanks for all the info, that's the kind of advice I'm looking for. By the way, you mentioned that the 72-79 case is better than the 68-71 case, can you tell me what makes them better? I want to start acquiring all of the stuff I'll need to build the best diff that I can.

 

The first vette Eaton posi cases were used from 65-68 and were parallel seam castings that were weaker then the later cases. The seam started at the rough surface of the large window and that often is the starting point for a crack. The other area is at the cross shaft holes. Once a crack starts it is a time bomb to implosion. It might take years or month but the crack will only get longer until it joins the cross shaft and window - then it is all done.

The 69-71 cases were revised, notably the small 1" window was enlarged to a tear-drop window. The casting seam design was carried over though and the window radius were tight. The 72 castings have a larger radius and a staggered tooth seam that is stronger. The castings are still rough though and really need to be correctly blended and polished to help prevent cracks from forming. Some of the BB posi's were shot peened which helps but the rough edges are still in place. Properly done, you will not remove much material and it will be smooth. REM polishing is an option but again not really needed if you know how to polish it. However it takes time to do it, I have done 100's and it still take me 2-3 hours to get them where I want. That might be why some so called polished posi's are just cleaned up a little at the windows. I have an unused rebuilt diff here now that was done by a well known rebuilder and you can shave on the edge of the windows.

The NOS GM/Eaton cases are better then the current new cases. The new cases aren't bad but if I put one next to one of the NOS cases I have you can see where they are thinner. I would not use one in a high power application. The best vette 10 bolt posi is correctly polished, pad milled, cross shaft fit correct, and cryro treated. FYI, you cryo after you complete all the posi work & tuning.

A tuned posi has no springs and has to be balanced which is a whole process in itself. There is much more to it then leaving the springs out. To those that say a tuned posi won't work or last, I know of some with over 100k miles on them now. The first time you try this it will take time and become frustrating. You may have to grind or have ground the shims. You can look at Tom's video on youtube. Tom built a machine as did I but they can be done by hand just adds more time and the machine fitment works better.

 

Great points and advice above. I don't have anywhere near the experience of Gary, but I can share my personal anecdotal experience. I have a '79 with a very healthy (500hp+) 388 stroker small block that I use for track days and hillclimbs. For the hillclimb events I do a small burnout to warm the tyres and then a launch, but it's not a sticky surface like a drag strip. Having said that, I do always load the drive train up before dropping the clutch - I don't needlessly abuse it. I run the car on semi slick track tyres - not as much stick as drag tyres but much more than any regular street tyre.

When I originally built the car I had the diff rebuilt by a local diff shop, and they tuned the posi and built it with solid steel clutches (information and parts provided by me) but no other changes. It held up perfectly fine for a few years and ~40,000 street and track miles, and earlier this year I pulled it down and rebuilt it myself because I had the rear suspension torn apart and just for my own peace of mind. I polished the case, fitted a solid pinion spacer, ARP ring gear bolts and fitted the bearing caps, as well as reshimming the clutches and just making sure everything was fine, which it was. I'm currently rebuilding a 3.7 center in the same way to try out as a different ratio from my factory 3.36.

Obviously my engine doesn't produce the same torque as a supercharged big block, nor is it abused on a drag strip, but it does see a much harder life than any street tyred sensibly driven street car. The diff with some minor upgrades has not been a problem for me, touch wood. I tend to think that a carefully rebuilt diff will handle a lot of power in a car on street tyres that isn't deliberately abused.

 

Hello, i am sorry that I allways ask, but is that HP estimate 400 FlyWheel or Rear Wheel HP? Thank You

 

In refrence to the L88 cars, I actually read that is why the AIR James Garner race Team switched to Lola's after some time. The rear diffs went out a lot under racing conditions and other parts. Not the engines, those were running strong... This is what I read... But of course this is racing conditions with Slicks on.

 

You will run into problems if you start shocking the drivetrain. That's where a lot of the parts breaking occurs, trying to go from 0-5000RPM in the snap of a finger breaks parts sooner or later. If you get it moving and then romp on it, you won't experience near the breakage. The quicker the holeshot, the more stress you put on all of the parts in the drivetrain.

 

100% correct.

 

Put a 4 link out back. Not only is it going to be unbreakable. But you can distribute the weight of the car across all 4 corners .

 

Even with street tires you are one wheel hop away from breaking something. I broke a brand new cryo treated axle with heavy offset trailing arms the first time at the track. Snapped it in half with street tires. I trailored it, got it home and installed 4-link 9"

 

Isn't there an aftermarket IRS kit out there built to handle the higher power apps?

Or can't a late model Mustang IRS be fabricated to fit a Vette?
I am sure there are plenty of rolled GT-350's in wrecking yards that the parts can be sourced from??

https://www.ebay.com/i/254278064692?...gaApiLEALw_wcB