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Discussion Starter #1 (Edited)
I ran this yesterday on over on CF. Thought some here may like it.


There have been a lot of "how much will this diff handle with 400-700HP" threads here. That is like saying how far up is high?

I'll offer you some thoughts, just my opinion of course. Agree, disagree, whatever.

The best IRS diff for a drag vette and hard usage is a Tom's 12 bolt. They have been on drag cars for the past 35 years or so. There is a DANA that is near suited to fit and is pretty strong and also pretty expensive if you do find one.

When asked to build a differential I ask what the intended usage is going to be, street driving, occasional abuse, or just plain beating the car into the ground.

For "normal" street use and short brusts through the gears a 400-500 hp vette in street tires should handle it ok. Add some of the mods to these diff'd and they'll perform better then stock setups. Certainly can address the clutch hammering issue and wear.

But continual abuse is going to break parts no question.U-joints, 1/2 shafts , posi's, axles,etc Some guys here have even broken the 12 bolt setups if I recall correctly. I would suggest if you are going to push a car very, very, hard you might consider a different model chevy that is geared more to drag racing then a 40 year old sports car. They can be made strong but figure about $7,000 worth of parts for the diff and suspension. That's a lot of cash if you're only going to drive on the street.

Lately there have been a lot of questions on exactly what is in these mysterious blocks of iron.

So for your viewing pleasure here some pictures from the files, I'm sure you will recognize some of them from past threads.

Here is a stock unit, like you will find in the 63-79's.The 63 & 64's were a little different, they were junk imo. Look in any GM manual and this is what you're going to see for corvettes.The posi springs keep tension on the clutches and side gears all the time. Depending on how they were setup, the clutches will hammer as they get hot. The spring tension will prevent them from slipping smoothly enough to take a corner without chattering. Some times additive will help, other times not.




Now to eliminate the chattering they went to what I call "snowflakes" clutches in 1971. They didn't eliminate anything but the strength the old solids had. Many times I find them broken when opened up.





The rough castings and the 3/4" cross shaft on the Eaton posi are weak points that torque and HP will eat way at over time.
Here we have an early Eaton(65-68) that has begun to crack. Now join the 2 cracks together and you can see what will happen.







To help them live longer you can polish and tune them without the springs. If done correctly they will last a long time under "normal" driving styles. Here is one I did ready for a ring gear install.



You can also buy a new loaded posi.These are ok for mild driving styles, low HP and torque. They're going to hammer, look at the spring size. Also MADE IN MEXICO now using fiber clutches and weaker spiders.




Now we come to what happens over time with an abusive driving style, 150' burnouts, hard shifting, pushing the car hard. The cross shaft will start to load the case until it can't handle the stress any longer and give out. Polishing may have helped for a while but nothing is 100%.


The cover was pulled on this one and here we have iron salad.




Notice the socket head caps I use did not fail but the posi case cross shaft did and once the crack starts and load is constant.......



The side yokes are still good and the bearing caps as well but the posi and gears are scrap now. The engine is about 370 hp and this posi case was previously hit by the pinon. Under normal driving loads this would not have happened so you really have to think about what you plan to do with the car and all that power some of you are building.



Hope this helps answer some of the questions I have seen posted lately. Buying a used differential sight unseen is a gamble if you plan to just bolt it in. I always go through them first to be sure.
 

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DC Crew
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Nice post and writeup. :thumbsup:

I have always wanted to do a "how to" like this with the THM350 which is my fav three speed. Now, if I can just find some time!

Good information! :cheers:
 

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Discussion Starter #3
To follow up here are some more pictures of "iron salad"

This differential was just plain destroyed by continual abuse. Even the side yoke journal on one yoke was damaged. The housing does make a nice scatter shield. Also the socket head cap screws I install in my jobs held up no problem as did the aftermarket fully threaded ring gear bolts.














 

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wow, how'd I miss this the first time around!?! nice write up and pics, as usual Gary. Never seen the inside of a blown rear end. That sucker's toast!

Thanks for the post and the info. Hopefully I'll be doing this at some point for preventative maintenance rather than calling you to order a new one!
 

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Discussion Starter #5
Jason,
My son was the destroyer of this 75 diff in his L-82 coupe. We built the car 3 years ago and he's been driving it hard ever since!:crazy:
 

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Just a quick update...but there are strong rumors that a couple of companies are working on a "bolt in" (maybe some minor welding on some versions) conversion to the FAB 9 inch solid axle for the Vette. I would hold off on any major upgrades/conversions until these conversions are released. :thumbsup:
 

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Discussion Starter #7
Personally, I wouldn't consider a 9"for a vette. It gives up what the car is supposed to be, they were never intended on being a drag car. Rather then chop up the car, I'd rather just get a solid axle car to build.
 

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Personally, I wouldn't consider a 9"for a vette. It gives up what the car is supposed to be, they were never intended on being a drag car. Rather then chop up the car, I'd rather just get a solid axle car to build.
I beg to differ. A Vette is an excellent starting point for building a drag car. Other than the abundance of U-Joints, pretty much everything else on it is heavy duty and built to take a beating. Lose all the luxury dead weight like PS, PB, PW, AM-FM, AC, heater, headlights (heh-heh) and you have lost a LOT of weight. The more weight you lose, the faster it goes. There have been quite a few Vettes purpose-built for drag racing. Mine will NEVER see city streets again. I don't do "corners" and I definitely don't do street racing. Have you priced the "solid axle" cars??? Most of them are in the collector car catagory and cost 3 times what I paid for my '72.
 

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Discussion Starter #10
Hi Donnie,

Very true on the cost of the older cars, although growing up with them I sure don't see the high value in them today-but to each his own. I was thinking more of 80's cars to build.

Most anything can be built into what ever the owner would like, but I think the majority of vette owners ( at least the guys I know)are not going to invest the money required to beef the car up for drag only usage. They would like stronger but still keep the car streetable where it is going to spend most of it's time.

You are obviously set up to run at the track and it sounds as if that is your passion. I think that's great, how is your car setup?

Again everyone has different opinons on what they want. Me, I couldn't care less who is the fastest around as my enjoyment is a summer cruise through the country away from highway, lights, and traffic.

Good luck and enjoy the car.
 

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DC Crew
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Again, everyone has different opinons on what they want. Me, I couldn't care less: who is the fastest around as my enjoyment is a summer cruise through the country away from highway, lights, and traffic.
I agree with that 100% bro. I built drag strip only cars back in my youth (1960's) but really enjoy the power and handling these cars provide. Cruises through the mountains on the winding roads are just pure joy!



:cheers:
 

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gtr1999: Definitely to each his own :thumbsup: :thumbsup:
Right now my car is in storage while I get a 3-car garage built. You can't imagine how hard it is to NOT be able to get up in the morning and tinker with it :(
I've pretty much completed stripping off the heaviest stuff. May be more to go but without a garage to work in, everything is on hold. I did get a T&T steering column installed (I'm a geezer with a pot belly...T&T is my salvation!) and clutch pedal assembly (it was a Turbo 400).

rjent: Watch those mountains!!! I live in Silver City. I towed my Vette up here from Illinois over the Gila range. Scary stuff!!! :surprised
 

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Discussion Starter #13
I was looking through my old threads for something and came across this old thread. I got a chuckle out of it and figured some of the new guys might want to see what happens to these stock 10 bolts when you really hammer them.

Since that period the car was built faster, I built a 12 bolt IRS using all of Tom's parts and the kid is now a father himself.

The car caught air at the track last year but the continual abuse will continue to break parts. If you guys go that route you should setup a maintenance program to open up the diff in the off season and check it.

Enjoy!
 

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Great thread as usual! What do you think about a stock 77 rear end that sees primarily street use with maybe 7 or 8 autocrosses a year? Motor is basically stock, so no crazy power involved.
 

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Discussion Starter #15
Great thread as usual! What do you think about a stock 77 rear end that sees primarily street use with maybe 7 or 8 autocrosses a year? Motor is basically stock, so no crazy power involved.
Well a lot of the stock diff's were decent enough to hold up with much issue.

There was not perfect vette differential built between 1963 - 1982. The very early ones had posi and pinion gear issues. The 69 -79's had other issue from weak clutches to RG bolts backing out. The 80 -82 are my least favorite of the group.

1977 diff's are not too bad. Check the endplay in the side yokes if they are the original soft ones and drive it. If you have a issue you will know. If on the other hand you plan to remove it then you go further into it and decide if you need or want to upgrade it.

These, like any production built part, vary all over the place. We have beat up some stock diff's without issue over the years and then seen some come in with low mileage that were pretty sad inside. The 80-82 do make very good planters or door stops.
 

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WOW! A definite blast from the past. :laughing:

Okay...the original premise was that the stock rear swingarm stuff really wasn't gonna be able to take a high horsepower, probably stick shift, blast off. And that still holds true today. Just too many parts whirling around, even if you install heavy duty parts in them. A 12 bolt is okay if you aren't doing serious drag racing. At this time Tom's only offers a kit to put together a 12 bolt setup for the Vette. However, we also have the choice of a complete conversion of the rear end to a 9 inch Ford rear end. Why Ford? Because Ford is THE most heavy duty rear end you can use. It's what they use in NASCAR and many other competition organizations. Just like Ford Toploader transmissions are the ultimate 4 speed, the Ford rear end is the ultimate rear end. Here is the place where you can get a 9 inch rear end conversion kit:
http://autofabcart.net/1968-1982corvettesoildrearconversion4-linkkit.aspx

Install this kit and the only question will be should I be DRIVING something that makes enough horsepower to destroy it? :partyon:

BTW...thanks to Bird for providing the link to the solid rear axle conversion. Hi Bird!!! :D
 

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Discussion Starter #17
We're putting down about 400-4254hp at the wheels, the car is in the mid 11's right now. It is a stick car running the 1350 12 bolt setup. The car could make it into the high 10's on the right day but that's all my son's game now, I don't have the time or desire to drag race. So he goes to the track, tweaks his car and I do the machine work and unit building.

We may step up to 1480 axles next.

I have built 12 bolt vette conversions to work with up to 1000hp(engine on the dyno) and the IRS has held up to what I was told, BUT anything will break. Those that use the term bullet proof are only fooling themselves. We have sheared solid spicer joints clean off in one launch that were supposed to be bullet proof.
 

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One thing I really like about the Ford rear ends is the "drop out" carrier. VERY easy to remove, rebuild and replace. You can even have spare ones to bolt in if track conditions aren't ideal for the rear end ratio you have installed. Try doing that with the GM or Dana rear ends. :(
 

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Discussion Starter #19
ya, good point but for the most part a properly built vette diff shouldn't need to come out but it has to be built to the application.
 

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I'm not talking about the differential build. I am talking about custom-tailoring the rear axle ratio to the track where you are racing. Of course if you aren't racing for $$, or are just driving a street-strip car, it's not really an issue. A solid axle rear end also takes away all the worries of all those u-joints and axles that might come chain-sawing up into your butt and testes. No need for all those safety loops on a solid rear axle too. It's just a simpler, stronger way to go for racing use.
 
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