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Discussion Starter #1
i got this car this past year, the guy i got it from didnt know much about it because it was owned by fam member who had passed. from what i can tell by marking driveshaft and spinning the rear wheel, it has a 4:11 gear. the car has a 3400 stall. i was wondering with a nice set of tires if the rear would hold up at the track. the car currently makes 225 at the rear with a mild cam and stock heads. i got a baseline done to see where it was. i have a pile of parts that should make it arrounf 3-325 is. will it hold? or what needs to be done? thanks in advance. this is my 1st chevy and im trying to learn about them. i have always been a ford guy.

my plan with the car is to drive for fun, and make several trips down the 1320.:cheers:
 

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i got this car this past year, the guy i got it from didnt know much about it because it was owned by fam member who had passed. from what i can tell by marking driveshaft and spinning the rear wheel, it has a 4:11 gear. the car has a 3400 stall. i was wondering with a nice set of tires if the rear would hold up at the track. the car currently makes 225 at the rear with a mild cam and stock heads. i got a baseline done to see where it was. i have a pile of parts that should make it arrounf 3-325 is. will it hold? or what needs to be done? thanks in advance. this is my 1st chevy and im trying to learn about them. i have always been a ford guy.

my plan with the car is to drive for fun, and make several trips down the 1320.:cheers:
The rear end should be fine with 325 HP. The differentials can handle quite a bit of HP. The driveshafts are another matter. As long as you run street tires you can put lots of HP to the rear. Once you start to run slicks you run the risk of destroying the halfshafts. I've seen pics of halfshafts twisted up like spiral pasta.

:cheers:

Rick B.
 

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You might want to ask forum member GTR1999 about how much HP he thinks the stock rear can support. He is one of the best Corvette differential builders in the country.

:cheers:

Rick B.
 

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With good street tires, your rear end should hold up just fine even up to 350 HP.

When you do the engine upgrade though, maybe look into a rear end service, making sure the fluid is clean before you change it out for new fluid and some GM rear end additive.
 

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Discussion Starter #7
thanks for the info... here is my old car, doin low 9's



here is the new one. not going to be as fast, but still fun!

 

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If you look over the link on the iron diff's you'll see some ways to beef them up.

The iron IRS came about in 63, those that had posi had issues with that design blowing up at the track and on the street. Then in 65 the Eaton posi came out, it was stronger but this design wasn't the best, it was very prone to cracking and then it broke. In 69 the 2nd design Eaton was used in the vette and it is the better design. In 71 they installed 10-17 spiders which are stronger then the 10-18 used thru '70.

Your 78 diff is pretty much the same as the earlier ones only they used what I coined years ago as "snowflake" clutches. These are pretty weak and will break in your application. Also if you still have the stock axles, ( side yokes), then those were pretty lousy too. Std or HD didn't matter, the 75-79 axles were soft and the face worn away, wrecking the diff in the process. If you don't have over 040 endplay in the yoke chances are they were replaced. If you have access to the history of the car maybe you'll know if it was worked on in the past. The ring gear bolts used in your diff are also not the best, they are flanged head bolts,no lock washers or loctite. Many backed out, sometimes in warranty. When a RG bolt backs out then it will gouge the inside of the housing, sometimes jam the spiders and break them or the Ring & Pinion.

So if you have a stock or typical rebuilt diff and you plan to use sticky tires and launch the car it will break. I've had diff's come in with blown posi's with stock cars. My son's first set up in his 75 was about 375 hp and he completely blew apart the posi with a LOT of abuse.

If you have it out I suggest you pull the cover and take a look after you read my thread above. You can call me if you have any questions.

Many times the question comes up what will my stock iron unit hold up to. It really depends on the HP, traction,and driving style coupled with the fact the stock posi's are production built. You'll see my posi are custom, no springs- they hold up better then anyone's but even so I wouldn't push a stock diff past 400-450hp. My super 10 is good to about 500-550hp after that I go to my 12 bolt conversion which are in 1000hp cars.

Good luck with your new vette:thumbsup:
 

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Discussion Starter #9
thanks! i am reading your other post now. there is a lot of good info there. i do everything on my car except for difs and short blocks. i never felt comfortable doing it. other then that i do everything else.
 

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Discussion Starter #10
i may change the gear anyway and have it re done. after the heads and everything are done, im going to do some shake down runs and see where the rpm's are at the end of the track. i dont want to kill the stock block too fast.
 
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