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 Rear Axle Upgrade? Little info?

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81X11

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PostSubject: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 11:52 am

I'm working on that 92 Roady Sedan, and it's coming out GREAT, but I hate...I mean HATE, that 2.55 rear axle. The car is totally gutless, and to make it go you have to kick it down out of overdrive....something that hardly happens on my LT1 wagon or even my tow-pack '92 Custom Cruiser.

SO..if I were to go salvage yarding for a better rear...what are my options? It's my understanding the 94-96 Caprice and Impala rear is wider, and the wagons are just different enough not to bolt up correctly in a Roady sedan. Is my best bet to locate ac 92-96 Roady Sedan with a tow-pack? Will a Fleetwood rear bolt up?

My next question is, other than the swapping the axle, what exactly do I need for the transmission. Being a '92, does something need to be re-programmed, or would I just need to change something for the speedo to read correctly.

Budget build here, so I can't go spent $1000 to have the rear changed out, but I can go grab a used rear for $100 at the salvage yard, change the seals and oil, and use that.

Anyone want to tell me exactly what to look for any what's needed. BIG THANKS!

-Mike
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RedandBlack

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 12:20 pm

Impala rear is wider than a Roady sedan? I wonder how true that is and if it is, why. They're the same frames...

If you're looking for a drop-in solution, that's probably your best bet. Some sort of tow pack Roady with 3.23s. A Fleetwood rear would make you lose ABS, I'm pretty sure, because of their 4 channel setup the other B-cars don't have.

If it were me, I'd just squirrel away a few more bucks and do a whole gear swap though.
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jayoldschool

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 12:55 pm

Is it the 8.5 or small 7.5 rear? That is the first thing to figure out.
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81X11

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 1:06 pm

I've read that when they went to the open rear wheel-wells in 1994 on the Caprice/Impala, that the axle was made wider as well.

Thanks for the info on the Fleetwood...didn't know they were different.

-Mike
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Andebe

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 1:16 pm

I second what Roo said. Save up for the gear swap. One of the best seat of the pants upgrades out there. What are they paying for blood and plasma in your neck of the woods? cheers
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JaySS
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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 2:48 pm


AFAIK the 7.5 was not offered in a RMS, or behind the L05. Everything in the online parts catalogs is showing the 8.5” ring gear for the GM8 code.

http://www.impalassforum.com/vBulletin/5-drivetrain/237620-rearend-widths.html

2.56’s use a different carrier than the rest of the ratios – add the expense of a carrier and it’s going to get cost prohibitive pretty quickly.

You’ll need to change the driven gear at the transmission to match the new ratio. The service manual will tell you what color you need and they are readily available on eBay or rebuilder websites:

https://transmissioncenter.net/shop/700r4-driven-gear/

You shouldn’t have to change the drive gear if your new selection isn’t too far from what is there, but they have those as well:

https://transmissioncenter.net/shop/speedometer-drive-gear/

OBTW:

http://gmlongroof.4umer.com/t13668-kn-fipk-lt1-salvage-yard-score

Might be a good time to verify what this really is, a 9C1 rear with G80 would be a nice score.

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81X11

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 3:18 pm

Thanks All!
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RedandBlack

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 3:18 pm

Yeah I'm 99.999% sure the only way you got the 7.5 rear was with the L99 4.3.

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jayoldschool

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PostSubject: Re: Rear Axle Upgrade? Little info?   Wed Sep 20, 2017 7:55 pm

They came behind 5.0s as well.
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Fred Kiehl

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PostSubject: Re: Rear Axle Upgrade? Little info?   Thu Sep 21, 2017 12:04 pm

The RMS has an 8.5 as the only option. The 2.56 housing will accept all ratios from all other 8.5s. The gear carrier is different from all other carriers. You can probably use any 91-92 Caprice 8.5 rear (round housing). The 91-92 9C1 came with assorted ratios all the way to 3.42. The 8.5s from the 91-92 Caprices will all bolt up, and have the same brakes.

Check the G codes on the SPID for GU5 or GU6. GU5 is 3.23, and GU6 is 3.42. Of course G80 is a posi. There will also be a tag on the 4:00 or 5:00 bolt for the cover to designate a posi.

Check the right axle for wear. If it is worn, get a left axle from another rear (or your old one) and put new bearings in the housing. The left axles tend to wear minimally, and the right axles tend to wear excessively.

The e-brake cables can be removed from the backing plate, and inserted into the donor rear plates. Push the spring retainers in, and work the cable shroud out. No adjustment needed.

The trans needs no modifications. You will need different VSS gearing to correct for ratio and tire size (if the tires are a different size). If you find a rear that you like, grab the VSS and VSS gears out of the trans in that car, so the gears will fit, because there are two different VSSs for 40+ tooth gears, and 39- tooth gears. If you have the same color VSS drive gear, you can just install the entire VSS and gear from the donor car. The 2.56 ratio has the 39- tooth VSS. The drive gear has a clip, and is a light press fit on the shaft. If the speedo is off, one tooth of driven gear will change the speedo by about 3-6 mph. An inch of tire diameter will also change the speedo about the same amount.

Check here for a VSS ratio calculator: http://www.transmissioncenter.net/. The gears are difficult to find, and are the ones without the molded on shaft. You must specify the ones without the shaft, or they will send them with it, and the gear will not fit. Check ebay for gears as well. I also have some hard to find gears if you get stuck.
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81X11

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PostSubject: Re: Rear Axle Upgrade? Little info?   Thu Sep 21, 2017 12:48 pm

Great info Fred! Thanks!
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