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Discussion Starter #1
At what point do you upgrade the differential stub axles? I have a 3.70 rear and am expecting around 600 ft-lbs at the flywheel from an engine upgrade.

I have upgraded the driveshaft. The halfshafts are now 3". The tranny has the larger output flange. The diff has the large input flange also and the "big window" Eaton carrier. All solid u-joints.

Do I need to change the axles?
 

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If you have the axles out you can measure them to see if they're worn. If they are then replace them and set your endplay to 005-010" This would require rebuilding the posi.
It's good you have a large window posi(69-79) but unless you polish it then then they can be more prone to cracking and then it's a time bomb. New cases are also rough cast as all are and have their own issues.
A lot depends on how you're going to drive the car. I've seen posi's cracked in 1/2 with 350hp and 400 ft/lbs of torque. Shock loading the diff will break them with or without sticky tires.
There are several areas to address with more power as you've done. The solid spicers & 3" shafts are good, you're not going to gain any advantage installing new yokes unless they are worn, the rebuilts are not any stronger. The best would be 30 spline Tom's setup correctly but break out the check book because parts and labor to set them up add up fast.
Good luck
 

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The best would be 30 spline Tom's setup correctly but break out the check book because parts and labor to set them up add up fast.

NO ****! I am doing this right now.... $$$....:thud:
 

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Discussion Starter #4
If you have the axles out you can measure them to see if they're worn. If they are then replace them and set your endplay to 005-010" This would require rebuilding the posi.
It's good you have a large window posi(69-79) but unless you polish it then then they can be more prone to cracking and then it's a time bomb. New cases are also rough cast as all are and have their own issues.
A lot depends on how you're going to drive the car. I've seen posi's cracked in 1/2 with 350hp and 400 ft/lbs of torque. Shock loading the diff will break them with or without sticky tires.
There are several areas to address with more power as you've done. The solid spicers & 3" shafts are good, you're not going to gain any advantage installing new yokes unless they are worn, the rebuilts are not any stronger. The best would be 30 spline Tom's setup correctly but break out the check book because parts and labor to set them up add up fast.
Good luck
Gary,

The diff is in the car. I was just wondering what I could do to beef it up, before it breaks. I'm not a clutch-popper though, so maybe it will last. I didn't realize that it was that big a deal to change to stronger units. I'll take a look at TOM's site. Thanks.
 

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Fred,
You would have to drop the diff to replace the axles anyway. I usually use one of my Super 10 bolts when in your power range. It's one step below the full 12 bolt conversion.
Now if it's not really giving you a problem now then I'd wait on it. Slip the clutch a little if you launch it, otherwise normal driving should be ok.
If you have a 69 diff then it has the stronger solid steel clutches in it, they didn't go to the junk until 71. The spiders are the weaker 10-18 they went to the better 10-17 in 71 I believe as well, interesting one was an improvement the other made it weaker- all because of the posi set up procedures on a production line.:thumbsup:
 

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Hey Gary.... How much weaker is the 71' 10 bolt posi case then a 12 bolt posi carrier IF the 10 bolt case has the following:

Case deburred/polished
Machined for 12 bolt cross pin
30 spline spider gears (for 30 spline Toms inner axles)
Steel clutches setup tight with no preload springs etc etc...just a good tight measured setup...

Also how much weaker is a Tom's 10 bolt 8.2" ring and pinion then a 12 bolt 8 7/8" ring and pinion?

I am just wondering how much "strength" I am giving up by doing the modded 10 bolt (or "Super" 10 bolt as you put it) as opposed to the full 12 bolt conversion.....?

(I wanted to use 3.36s so no 12 bolt)
 

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The stock rear is good right up to the point you need a broom and a dustpan.
For some odd reason, the rear in my 69 is still hanging in there. But I know it's never been launched. I think it'll be ok until the tires stick. :spanked:
 

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Discussion Starter #9
The new engine isn't in the car yet, that's why I'm thinking about this stuff now. I'll be staying with my 275/50/15 non-sticky "relief-valve" Radial T/As. That should help. I usually roll-out and then get on it at about 1500-2000rpm. I am staying away from an on-off-switch torque cam on purpose. If the rear blow up, I'll be PM'ing Gary for a Super 10 bolt.
 

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Well as said a lot depends on how the car is used, although I haven't met anyone yet that is going to spent the cash to build 600+ hp and/or torque and not hammer it.
The T/A's will help unless again a lot of abuse and then they aren't going to do as much as you suspect.

How much stronger is a 12 bolt conversion then any 10 bolt, it's been said to be 4x as strong. Al, I know your builder and along with me, we're about the only ones doing this build,... correctly. The strongest super 10 I built also had a cold stabilized case.

I use 500-550hp as the range for a super 10, this may be under rated but once you're into the 600 + range the 12 is the way to go. Only 373,411 or 456 ratios are available at this time unless I grind and machine fit a 12 bolt set, the cost to do this is beyond most applications.

Look up Curby or Zim64 on CF they will fill you in. Some builders do attempt to fully polish the posi cases but the ones I've seen and even had shipped to me are not at the level I do them, primarily due to the time it takes to build them.

You probably seen these but this was a working differential. It was a nice blueprinted 373 setup, good to 400-450hp in most cases. However, the driving style was to burn bald new rear
tires in 8,000 miles. The car had about 375 hp, 4 speed and 225x70 T/A's. It would burn rubber in all 4 gears, rolling starts, lauches, etc. I wanted to see just how long a 10-bolt would last in this application so I put it in my son's car and told him to "test" it. Not exactly the proving grounds GM had but you get the point.





 

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DAMN so that posi case was already polished and clutches set up...? Did it have the 12 bolt cross pin? I see the 17 spline spiders so I assume a stock 10b cross pin... Still it sux that the 10b posi case is so freakin weak. Its crazy when you consider I had a corporate 8.5" 10 bolt in a 78 Trans am and with stock axles the rear end held 10.60s for years....Other then a basic rebuild and 3.90 gear set, nothing special and was still sounding/running perfect when my buddy sold the car 5yrs after I sold it to him...(after I had ran 11.80s for about 6-7 yrs on that same rear end)

Grand National guys run mid 10s on 10 bolt corporate rears all the time with only an axle swap......I guess it was just the technology improvement over the 18-20 yr gap in build time from C3s to GNs....

You still would have thought SOMEONE would have built a strong posi case for a C3 rear, after all, they were in production for 20 freakin years....

I can't do the 3.73 12 bolt thing because I actually wanna drive on the freeway faster then 60mph....:down:
 

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Discussion Starter #13
While I'm not looking forward to seeing that, I think my biggest concern is a stub axle failing and all the damage the halfshaft could do to the body. Going out of control isn't too thrilling either due to instantaneous rear wheel steering.
 

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Well Gary it looks like you can save you son some money on posi clutch plate washers as the one I see looks like it can be reused:laughing:
 

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Well Gary it looks like you can save you son some money on posi clutch plate washers as the one I see looks like it can be reused:laughing:
Heehhee, well that was something. The LH side yoke was ok.

Guys this was used in a pretty agreessive application, most of you will not be abusing the cars like this. If you do then you will break it, pretty simple. Even 12 bolts will break with enough abuse.

It really comes down to power,use and how the diff is rebuilt. You pretty know my feelings on the common rebuild found everywhere, one of those would not have lasted 8k miles in this application, maybe 800. I just did a 69 diff that was done by a local shop in the guys town. It was screaming with gear whine and the rebuilder refused to fix it. I found it setup pretty sloppy, wrecking a good gear set in the process and having about .125 endplay in the yokes.
 
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