Hey again.

So I've decided on the trans I'm getting and whilst looking up the speedo gear I'll need, I ran back into the problem of my rear end ratio. It's currently 2.56 which is good for top speeds but not acceleration so I want to swap the gears, keeping the same axle. I've been reading through the FAQs and I've read that 2.56 to 3.28 has been done and no more is possible, but also that 2.56 to 2.78 is all that will be possible...

Has anyone done / knows what my options are? Aftermarket or otherwise... I'm hoping for a 3.36 but would be happy with anything other than my 2.56.

The gear ratio cut-offs for the differnent carriers depend upon whether the carrier is a posi unit or not. I'm searching in the archives and FGF's for the splits.

For quite a while, only Richmond gear has offered new gear sets for our FGF's but only from 3.55 and up (to 4.33 I think). I saw some discussion recently over on Performance Years about somebody who was looking at making lower ratios for the BOP 8.2 and testing the waters to see what kind of interest there was.

Here's what I found so far in the archives:

"The center section of the differential housing will accept all gear ratios. It's the carrier that is inside that needs to corrspond to the gear ratio desired. For example, the 2 series carrier accepts 2.56 to 2.93, 3 series accepts 3.08 to 3.55 ratios, 4 series accepts 3.73 to 4.33 ratios."

Don't know if that is for a posi rear or not.

Ah - found it on page 4B-1 in the '67 FB Service Manual Supplement:

Standard:
1 series 2.56
2 Series 2.78, 2.93, 3.08
3 series 3.23, 3.36, 3.55

Posi
1 series 2.56, 2.78
2 series 2.93, 3.08, 3.23
3 series 3.36, 3.55, 3.90

Thanks for all that info.

So... perhaps I can't afford to do this right now:

Carrier ($419.95) - http://www.summitracing.com/parts/STR-R542060/
Ring and Pinion Gears ($258.69) - http://www.summitracing.com/parts/MGR-BP882355/
Ring and Pinion Install Kit ($87.99) - http://www.summitracing.com/parts/RAT-311K/

Are those prices reasonable?! Presumably I'm also going to need a shop do it, or get some special tools? Any other options? I wouldn't mind taking bits from a scrap car, but that's not really an option over here.

Is this going to be possible to do with the axle under the car, i.e. at a later date?

I can't comment on those prices but they do not look out of line from what info I have seen here in the site.

The act of correctly lining up the ring and pinion gears so that they mesh properly (provide the correct contact pattern) is a skill in itself. If you have Service Manual, reading about it might scare you off as much as rear end "how-to's" did me.

This is one of those times where I went straight to an expert.

It is too bad that you do not have the supply of donor vehicles handy so you could just swap out and entire axle. Yes, this is something that you could tackle down the road.

I'm not sure about the 8.2 carriers but I know the 8.5's you can get a ring spacer that will move the gear further over so you can run steeper gears with your original carrier.

I see that you show different carrier breaks for posi and standard. Now I have been building 10 bolt 8.2 B O P and chevy and 12 bolt olds and chevy 12 bolts for years...I have NEVER seen any carrier break difference from any of these from standard(open / one wheel peel) to Posi (limited slip diff units)

From size and make rear like buick early to buick mid to buick 8.5 yes...But never from open to posi.

2.78 ratio will always fit a open or a posi...There is NO posi gear and or standard gear. Period.

As ratios for your Firebirds we DO NOW have these options available...3.36 3.55 3.73 3.90 4.10 ALL NEW and only 219.00 here through your site.

I also can get you posi unit 399.00 NOT much better but still money is money..

rebuild kit 79.00 again bearings are just expensive no matter what you do.

AXLE bearing kit will depend on whether you have sealed early bearings OR later internally oiled taper bearing. usually 69.00 to do both side with seals included.

As far as setting up the rear...It takes patients and does take understanding in how and what you are trying to do then you will be able to do what you need to. The only special tools you will need are a Magnetic base dial indicator And set-up bearing which can be made fairly easy with some power tools. You will need access to a 12 ton press for the pinion bearing and axle bearings for install and removal.

I am always here to help through the questions you might have about this sketchy subject of rear diff set-up. I do not expect people to have all the shop tools so I can help with back yard set-up pretty well.

I would go with an Eaton differential instead of a cone type Auburn.

There are a few reputable sources on Ebay that sell Eaton units, gear, races, bearings etc. I remember a few years back $675 or so.

Auburn gear sets

http://cgi.ebay.com/ebaymotors/EATON-POS...sQ5fAccessories

http://cgi.ebay.com/ebaymotors/EATON-POS...sQ5fAccessories

I would earmark another $200-300 to have the rear end set up if you do not have a gauge to set pinion depth. Includes free shipping

the 2.56 is also good for gas mileage....lol

its what I use....most of my friends have 3 and 4 versions...on cruises I get there as fast if not faster than they do....its not only the first mile that counts ,unless you race.

I am been interested in trading for a gear ratio that provides better gas mileage and easier cruising. I understand that the ratio can be determined from counting the number of teeth on the ring and on the pinion and dividing them.

I have also been told that you can raise the back wheels and mark the drive shaft with chalk and determine the ratio by counting the revolutions of the drive shaft for each revolution of the wheel. This is for car that does not have positraction. Is that true?

get an overdrive tranny. and 2.56 rear gears...or just a little more

Ya but on my 4 speed the 2.56's are tough on the clutch in stop n go....great on the road tho...

With an OD trans, you can go 3.73 or 4.11 and be fine with a .60 final drive.

With a 2.56 rear and a .60 OD, the car couldn't get out of it's own way.

clutch?? whats that...?

you are correct! I wasnt thinking....not the first time..lol

Jet lag?

Too many visits to the "bars" in Amsterdam.

For you that's what your wife does when you open it up on a corner....

yep, and screams!

no, not really, maybe just still thinking "metric"...lol
no bars in Amsterdam either , only had 1 hr 40 minutes between terminals!

OK, so I have been reading this and I have a question. I want to go from my 3.08 to a 3.55 on my THM. Will this work or do I need to go smaller? The build sheet says the "performance" gear for the 400 with THM is 3.23. Is this is big as I should/can go? The 3.55 was available, but maybe for the manual transmissions, both 3 & 4 speeds I believe. Anyhow, I am looking to get a better 'launch" out of my car but I do not want it to run at too high RPM on the highway. I do VERY LIMITED highway driving. Will the 3.55 work? Will it be a big difference on the launch?