GTR1999

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 for you some thoughts, just my opinion of course. Agree, disagree, whatever.

The best 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.

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 bursts through the gears, a 400-500 hp vette in street tires should handle it ok. Add some of the mods to these 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 shaft, 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 $5,000 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. 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.




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 just pulled on this one and here we have iron salad.




Notice the socket head caps I use did not fail,nor did the ring gear bolts. The posi case cross shaft did fail 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.

79VetOwner

Excellent post Gary. Thank you for the heads up.
A question tho.
What components within the diff should I upgrade if est hp/tq is around 500? I would rather not spend the $ for a full Toms unit (assuming they are really expensive) I would rather upgrade the internals on mine and keep looking stock.
What are the first things to upgrade? What are the cheapest things to upgrade?
Jon

norvalwilhelm

Thanks for taking the time. I always read your posts and pick up something.

schmegeggie

Gary,
Good stuff!
I ecspecially like your reasoning about abuse, it can get very expensive, very quickly with these cars.
I'll stick to just cruising...everybody wants to "race" you, but they're just wishing they were driving it....but remember....you are!

GTR1999

I drive a 325 hp 72 automatic with 336's. Dull, yeah probably but my days of trying to prove anything to some idiot next to me are over.

Can you improve the diff? Absolutley, look at those cap screws in the blown up diff- they still held tight and that's not even a steel cap on the left.

Now keep in mind this particular unit was kind of a test mule for me. I wanted to see how the case would hold up once it was hit by the pinion earlier in it's life. The cuts were fair but I blended them good.
Next I had my test driver, John Milner Jr, drive it for 3 years. Of course bald rear tires in 8k miles are clue this wasn't driven as I do my 72. Clutch may be next! Would it have lasted if driven in a regular manner, yes I have no doubt about it. Still getting up in the 500+ range does tax this setup a bit. I used to see this on the old 65-67 BB diff's for sale a swap meets.

steve J06

Gary
Thanks for the info. I really appreciate when those "in the know" take the time to share.

steve

OldKarz

Very good post. Thanks for the information!

Hadez

Wow!!! Just the explanation that I needed. I only have 1 more question. Is the Tom's unit a direct R&R in that I don't need to make any other modifications to make it work? If all goes good in the next 2 weeks I will be in a very good position to order a lot of new toys and have been seriously considering Tom's differential.

L-82kid

I don't know what you're talking about........

rexx78

Thanks for the post Gary. The rear end used to be something that intimidated me; your posts have changed that.

Gordonm

Thanks Gary. I have had mine out twice since the rebuild I did. So far it all looks good and no problems yet. I do drive the crap out of it and someday expect it to go. I'll keep checking on it every winter and see what it looks like. I'll be doing some drag races and autoX and just plain driving it hard this year. I'll keep you posted on its condition.

MidYearRoadster

The rear end has always worried me..and probably kept me from hammering the gears on a regular basis. So we took out the 4:11's and threw in 3:36's but every now and then......one does get the urge to drop the clutch ......In any case...a GREAT article and it is most appreciated. Keep them coming.

GTR1999

Gordon you push your car pretty good. I don't recall but I think when we talked you pretty much set it up like the one that is toasted here?
373's polished and tuned,no steel cap? If so the only difference was this one was hit, which could be a big difference. You now have more power as well with the 383.

corvette_maniac

GTR1999,

I'm getting very close to dropping the pumpkin. I needed to get my `79 Firebird 400 / 4 speed running first. It's good to go.

I've had the Vette for almost a year and have only driven it maybe 100 miles. The rear has got a sloppy yoke on one side and makes a clunking sound on turns. It also has a camber problem there as well. I will most likely need some advise once I open it up and swap those yokes.

Jamie

Gordonm

More power- definitly. Polished yes, not quite as good as you do but I did radius all the corners and smoothed them. No steel cap but I did put in socket head capscrews. If it survives the season this year it is coming back out and getting a thorough inspection this winter.

Ken73

Gary, any insight and/or pictures for the 80-82 differential?

MotorHead

Gary, you are a rear end man

Glensgages

Would like to hear that stuffing TOM's 12-bolt/spool into this hunk of aluminum is possible.....

gyopp

Great post Gary

JoeRags

I'm hungry for iron salad.