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I ran this yesterday on the CF, some of you may be able to use 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 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 brusts 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.Teh 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.
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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.
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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 joint the 2 cracks together and you can see what will happen.
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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 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.
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 brusts 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.Teh 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 joint 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 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.
