[Pierre1967]

Hi,

I need to rebuild my car's open differential and your advices would help me a lot. Here are the questions:

Car --> Firebird 67
Engine --> 400 engine (xe274 cam, KRE heads)
Carb --> Cliff R rebuild
Trans --> 700r4 (stall 2300)
Actual gear --> Open 3.23
Rear tires --> 255-60-15
Use --> Street only, 50/50 city, highway

1. For overall fun, performance and fuel "economy", what would be the best, 3.23 or 3.55 ? I like the 3.23 but not sure about overall 3.55 benefits in my case ..

2. If I stay with the 3.23 option, do I need to change something else to make it limited slip ?

3. Should I keep the original OEM ring and pinion or the risk of eventual failure is high ?

4. Wich limited slip diff. brand/type do you recommend to me (or complete rebuild kit) ? 3.23 seems not available, only for 3 series, 3.55 and higher (Yukon, Eaton, etc..)


Thanks in advance, it's really appreciated ... there is not much Pontiac enthusiastic car guys in the east Canada great north ... Pierre

[SRR]

Since you have OD, I would go with the 3.55.

[napster]

I have a '78 TA with a 406 and the same cam, but with 6X#4 heads. Just had the stock 3.23's changed to 3.73's. Also, have a 700R4 like you. You won't regret it. Makes a difference coming out of the hole.

[Radman]

I have a 700R4 with a Ford 9 inch 3:50 posi gears. Rear tire is a 28X9.5x15 Hoosier Street pro. At 70 MPH my RPM is 2000. And first gear has tons of grunt.

[amcmike]

Definitely do the 3.55s

[mgarblik]

The 700 transmission with a 3.73 ratio would be my choice. You will feel a significant difference and have a big smile on your face with that combo. Since you already have a 3.23 and would need to buy a 3.55, I would tend to stay with the 3.23 if that was my only choice. The move from a 3.23 to a 3.55 ratio, I doubt you will feel much difference. Probably .10-.15 sec. in the quarter mile. JMO.

[Pierre1967]

mgarblik ,

For me 3.73 sounds a Nova 68-74 differential ? Is it a lot of work going that way ?

[napster]

The 3.73 is no more work then going from 3.23 to 3.55. You don't have to change the carrier for the 3.73 and besides, the 3.73 is a better match to the cam.

You won't feel a difference with the 3.23 to 3.55. Going from 3.23 to 3.73 is a big change.

[gtospieg]

I think it depends on what your goals are. Do you want the car to be quicker or do you want better mileage? It seems to me that you already have the best of both ends of that equation. Plenty of 1st gear(3.06x 3.23=9.88) with the 700 trans., and a really high final drive(.7x 3.23=2.26). If it's a dollars thing, you can buy a LSD and reuse the gears you already have. You would only need to buy the unit and some bearings, and reset the backlash.

[Gary H]

I absolutely would not use a 3:23 gear with an overdrive transmission. I've done a lot of overdrive conversions here at the shop and we generally use 3:73 gears with an occasional 3:55 or 3:42. IMO, a 3:23 , and 255-60-15 tires, coupled with a OD trans and only a 400 is going to lug the engine in high gear a little

[mgarblik]

Originally Posted by Pierre1967

mgarblik ,

For me 3.73 sounds a Nova 68-74 differential ? Is it a lot of work going that way ?

I am not sure of fitment and availability of that ratio in your housing. I am sure other people on here can advise you better than me. I just like the ratio. Good luck with it.

[Formulabruce]

Originally Posted by Pierre1967

mgarblik ,

For me 3.73 sounds a Nova 68-74 differential ? Is it a lot of work going that way ?

Putting in a Nova rear will get you to a 8.5 rear end, its a upgrade over the stock 8.2 in '67.
This should be a direct swap as both rears have same width and perch mounts. The 68 and up cars used staggered shocks too, which is good.
Pontiacs have a lot of torque already, how deep is your fuel budget?
When Pontiac started putting 4 speed OD trans in Firebirds the Formulas and TA got a 3.42 gear. 26 tall tire.
A good combo that is easily available for the 8.5 Nova/ Ventura rear end.

[Pierre1967]

Apart from Powertrax, is there any other LSD brand that is compatible with 3.23 ? If not, is remanufactured safe-t-track carriers available somewhere ?

[locomotivebreath]

http://jdrace.com/ Try this guy , he goes by Monzaz on this forum .