8.5 switch from 2:73 to 3:42 gears

73 Survivor
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8.5 switch from 2:73 to 3:42 gears

Postby 73 Survivor » Tue Jul 04, 2017 11:57 am

I own a 1973 Buick Century with the original factory open differential, with 2:73 gears. Car only has 26K original miles. Original turbo 350 hooked to a J code 4bbl. 350 Buick motor. I want more punch off the line, so looking at lower gearing, but not too short for hwy cruising, and considering something in the 3:25 up to 3:42 range. Not sure of my options for gears for this differential. It appears from my research that I will need a thicker ring gear or a spacer to change this over from 2:73's to 3's? What other components recommended be changed with the gears? Plan to stay with an open diff, as car is a cruiser and has a fairly stock motor, other than re-curved distributor and dual exhaust. so wheel spin won't be a big factor with the 245-60 TA's on it.

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monzaz
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Re: 8.5 switch from 2:73 to 3:42 gears

Postby monzaz » Tue Jul 04, 2017 12:44 pm

Well I think we should ID the rear to be sure that is what you have before getting all torqued up.
The LOW horse BOP FULL SIZE cars did come with 10 bolt 8.5 rears.... so it is very possible...

Send a few pictures and we can ID that.
NOW if it is a 8.5 10 bolt corporate rear- 2.73 IS a 3 series gear so NO you will not need a spacer or thick gear to change the ratio to a steeper 3.08 3.23 3.42 3.73 4.10 etc gear.

There are plenty of Limited Slip Posi units we can or others can provide for BOTH wheels gripping the road if you want.

PLEASE be careful how far you go with a 3 speed trans as you have to remember 3rd gear is 1 to 1 with your rear end gear ratio so 3.42 will be close to 3000 rpm on the freeway with 65 -70 MPH speeds.

Jim
J D Race
J D Race

73 Survivor
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Re: 8.5 switch from 2:73 to 3:42 gears

Postby 73 Survivor » Tue Jul 04, 2017 9:05 pm

I figured it was an 8.5 according to your diff ID photos, and the fact ID indicates Regal (same as Century), Cutlass, Grand Prix, Malibu, 73-77 use this rear end. Took photos with car jacked up, no hoist here. However, it matches exactly the photo of the tired looking open differential marked 10 bolt on the cover in white in the photo on your 8.5 ID page. The same dimples on the sides of the cover, and the two quarter size holes machined into the axle on each side of the dimples. Also the gasket exposed on the bottom is identical in shape as well. Compared my photo and yours side by side.

I'm not a techie (old guy with flip phone), so would need instruction on how to load pics to your site unless it's a simple email. In case I can't do this right away, there is also a large letter S stamped into the housing near the fill plug on the passenger side. Axle stamping on front of right axle is ACB 18 31, also number 165 above and in front of fill plug on pumpkin. Will try and send pics if this info isn't sufficient for an ID.

From what you have indicated, I would likely be better off with the 3:25 gears for occasional highway travel. Tires are 245 60 15 TA's.

73 Survivor
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Re: 8.5 switch from 2:73 to 3:42 gears

Postby 73 Survivor » Tue Jul 04, 2017 10:18 pm

Having more closely examined the photos, realize that what I thought was gasket is actually part of the housing itself below the cover. :roll:

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monzaz
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Re: 8.5 switch from 2:73 to 3:42 gears

Postby monzaz » Mon Jul 10, 2017 8:32 am

simple email jdrace3@gmail.com

From what you described so far it sure sounds like a 10 bolt 8.5...
J D Race


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