Author Topic: Help with differential 78 T/A  (Read 175 times)

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Offline Juventini

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Help with differential 78 T/A
« on: June 30, 2022, 08:31:43 PM »
Hello all, I am currently working on my 78 Automatic T/A, I replaced the 400 with a Len Williams 455. The transmission its still the original th350, but I will replace it within the coming months as it has been giving me issues and the cost of rebuilding it will be really close to getting a new one. Now, my question is the following; should I be looking into getting a new differential / rear axle? Since the new engine its quite more powerful than the previous one.

Thanks in advance and I hope all of you are doing well.

Offline Wallington

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Re: Help with differential 78 T/A
« Reply #1 on: June 30, 2022, 10:27:53 PM »
Unless you like spending money, I wouldn't bother until it starts to play up. Or wait until you decide what gear ratio suits it, if need to change. No point doing it several times especially if you also plan suspension work. The standard 10-bolt can take some high powered abuse.  I found it easiest to build a second axle with all the good parts and ratio, then swap over during a spring swap. So simple.
« Last Edit: June 30, 2022, 10:56:32 PM by Wallington »

Offline Juventini

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Re: Help with differential 78 T/A
« Reply #2 on: July 01, 2022, 02:30:12 AM »
Thanks for the reply, will leave it as is then. Its not like im racing it anyways, just trying to be safe as I dont want any mechanical failures mid highway.

And yeah, suspension is on my to-do list as well.

Again, thanks for the reply <3

Offline Rockford

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Re: Help with differential 78 T/A
« Reply #3 on: July 01, 2022, 07:41:50 AM »
I'm with Wallington, no need to change it, unless you're planning to repeatedly take it on the strip.  If you so wish, for peace of mind, your gear oil can easily be changed and you can also buy an additive from GM that you can pour in to help with (what I presume in your case) your posi/limited slip diff.  A long time ago I noticed a little bit of grinding when turning tight corners, and changing the gear oil and adding the additive took care of it.

Also, if you "were" planning on placing better gearing, make sure your transfer case supports the gears you're swapping over to.  I may be wrong about this, but if you have 2:73 gears, it won't support a set of 3:23's or 3:55's and above.  Maybe someone can confirm this with me.  In any case, if it's more highway driving that you'll be doing, 2:73's - 3:08's are fine.

Offline Wallington

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Re: Help with differential 78 T/A
« Reply #4 on: July 01, 2022, 09:53:45 AM »
2.73 is actually a 3-series carrier gear, so the carrier housing does allow for the 3.xx ratios also. 2.41 and 2.56 are the ugly ducklings of 2-series. Swapping to overdrive or a different box changes everything again. Don't get in too deep, too soon, or it'll sit for years and never be driven or completed.

Offline Rockford

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Re: Help with differential 78 T/A
« Reply #5 on: July 01, 2022, 10:30:07 AM »
Thanks for the clarification on the 2:73's, as I wasn't quite certain myself.