Changing to a possi drive differential

Looking at changing to a possi drive on my 1971 GS 350 as I want to increase my street performance, after reading some threads in v8 Buick and some help/information from Larry, the wizard I've almost made a disission on a possi carrying unit maybe Eaton etc
I think I've got 2.73 gearing and I'm wanting to know changing to 3.42 or any other gear ratio will make the change I want in street performance.
Has any one made the gearing change & what's the best way to do it ? I know going to a possi drive is a great start but might as well do the lot , do it once do it right !

 

First your carrier has to be changed,as you will need a posi carrier,so buy a posi carrier with 342 gears and have somebody locally that knows how to do it,as this is no job for beginners.Anyway,the explanation is,your carrier cannot hold the bigger [numerically]gears anyway.So if you want some members to do the job for you,ask Monzas or Brian Trick here,they are very good at it and they are reasonable in price.Bruno.

 

My 70 GS455 came with the standard 2:9?? rear. It was a great cruiser!
Still had plenty of pep from a dig but I wanted more. I have had numerous BBB cars with 3:42 and that's a great all around rear ratio. When I changed my 70 rear I bought a complete 12 bolt rear, drum to drum from Currie or Moser or whoever had the best sale price. Went with a 3:73 and a little taller tire and it made a night and day difference!
If you have a 2:73 and move up to a 3:42 or 3:73 you will be thrilled with the way it wakes the car up!! Any higher than that and it could begin to get on your nerves when cruising at 65-70mph.
A 3:55 out of a Pontiac is also a good choice, or you could go to a 3:64 like a non a/c Stage 1 in 1970.

Good Luck!!
Philip T.

 

Done multiple gear changes on both 350 and 455 Skylark/GS's. both engines really do well on the street with 3.42 gears. and changing from a 2.56 or 2.73 to a 3.42 or mid 3 gear is really noticeable in the seat of the pants when you put your foot in the tank. My gs with a 3.42 is a serious street screamer. Night and day from the 2.73

 

Thanks Bruno for that, so are you saying bigger gears like the 3.42 won't fit in my factory
Differential housing, is that why guys change the complete differential from drum to drum . Thanks for your time

 

My 72 skylark had A 2.56 gear when I bought it.I installed A Eaton posi and 3.42 gears,it was like night and day.

 

Thanks Dwanyne, did your Eaton possi come with those gears or did you buy them separately and fit them to the Eaton possi carrier. Thanks

 

I brought the Eaton 3 series carrier and Motive performance 3.42 ring and pinion and set up kit from summit racing.

 

Your 1971 1972 Buick skylark should have a 8.5 corperate rear MEANING the carrier break is 2.14 - 2.56 is a 2 series FACTORY ONLY carrier. 2.73 - 5.57 ratios are all on the 3 series which came factory or aftermarket.
SO 2.73 in your rear is a 3 series gear ratio alone with 2.93 3.08 3.23 3.42 3.73 3.90 4.10 4.30 4.56 4.88 5.14 5.38 5.57 are ALL ratios you can use .
I had a ad up in the classifieds here and still selling the 8.5 Yukon Eaton Dura grip units and ring and pinions and rebuild kits. Which was a pretty good deal for quality Yukon performance parts. I was able to negotiate a better deal so I past the savings on also.

So Fi you wanted to just change the gear you CAN use the open NON Posi carrier you have in the rear and add Limited slip posi unit later...BUT as you said ...if your in there and you have the funds get it all done.

Jim
JD Race

 

You can’t go wrong with a 3:42. I’m actually building 3 of those rears this weekend. It’s a nice,all-around gear,for performance,and still be able to cruise down the highway. You could creep up to a 3:55,but there is only one company that makes the 3:55 gearset,so they are a little more expensive.
As far as the differential goes,an Eaton clutch style,or Detroit Tru Trac helical gear style will work great for your application.

 

I was thinking 308-323 for some lower revs at freeway speeds- what is your experience with 342 engine revs at like 70 mph? just curious...thanks!

 

I can see why an overdrive or 4 speed auto would help with that gearing; thank you Larry for laying out the math for us to see. I guess its finding the right compromise with what we can live with.