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Discussion Starter · #1 · (Edited by Moderator)
Hey I'm Looking to change the rear end ratio in my 1994 firebird formula from the factory 3.42 to a 3.73 and I was wondering if anyone has any experience with an upgrade like this and how it changes the cars cruising ability's or performance and I was wondering what size rear end my car actually has and how to tell.

Thanks, Jake
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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I ran the T56 with a Strange 12-bolt w/ 3.73:1 gears (back when my "purchased new in 1994" still had the T56). Mathematically, it increases torque at the rear wheels off the line by 9%. You can feel the difference but it is not dramatic. Also increases your RPM in any gear by 9%. Makes 6th gear slightly more usable. For example, 6th gear at 70 MPH w/ the 3.42's is about 1,600 RPM. With the 3.73's it's 1,745 RPM. No problem, little if any effect on fuel economy.

That said, if I had it to do over again, I would have opted for 4.11's. The torque increase is 20% off the line. Most people find this a better choice with the T56. That 1,600 RPM at 70 MPH in 6th gear moves to 1,920 RPM. More noticeable, but the very steep T56 overdrive gears (5th = 0.74:1; 6th = 0.50:1) give you a lot of flexibility even with the 4.11's.

The "size" is a GM 10-bolt 7.5/7.625" rear axle assembly. Your 3.42 gears have a 3-series carrier. Or, are you asking how to verify your car still has 3.42:1 gears?
 

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Discussion Starter · #3 ·
Thanks so much for the response that helped a ton and yes I was asking what size rear end the car has for example 10-bolt or 12-bolt so thanks for the help

One question about the 4.11's is how "dayilyable" is the car after this because it is currently my daily driver and someone else I talked to told me that with the 4.11's 1st gear is pretty much a joke so I'd be curious to hear what you thought about this.

Thanks, Jake
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Assuming you are shifting at 5,800 RPM, shift point in MPH:

GEAR - 3.42 - 4.11

1 - - 48 mph - 40 mph

2 - - 72 mph - 60 mph

3 - - 99 mph - 83 mph

4 - 129 mph - 107 mph

Works for me, maybe not for someone else. Note that calculation is based on the optional Z-rated 245/50-16 (25.65") tire. If you are trapping the 1/4-mile over 107 mph, the 4.11's would force you to shift to 5th. But if you are trapping over 107 mph, you most likely have an aftermarket cam and are shifting at higher RPM. Shifting at 6,200 would allow you to cross the line at 115 mph.
 

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Discussion Starter · #5 ·
Assuming you are shifting at 5,800 RPM, shift point in MPH:

GEAR - 3.42 - 4.11

1 - - 48 mph - 40 mph

2 - - 72 mph - 60 mph

3 - - 99 mph - 83 mph

4 - 129 mph - 107 mph

Works for me, maybe not for someone else. Note that calculation is based on the optional Z-rated 245/50-16 (25.65") tire. If you are trapping the 1/4-mile over 107 mph, the 4.11's would force you to shift to 5th. But if you are trapping over 107 mph, you most likely have an aftermarket cam and are shifting at higher RPM. Shifting at 6,200 would allow you to cross the line at 115 mph.
Ok sweet thanks that's actually not that bad I was also wondering if I would need a new case for the 4.11 gears or if the stock differential case would work with these. Also is there a certain kit or manufacturer you would recommend
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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You will need:

- 4.11 gear set for GM 7.5/7.625 10-bolt, 3-series carrier. I like Motive.

- a compatible install kit - bearings, shims, bolts, cover gasket, crush sleeve, seals, pinion nut.

- given age of car add axle bearings and seals to install kit

- 2 quarts 80W-90 GL5 gear lube

- 4 ounces of GM Limited Slip Differential Additive

All that installs in your stock rear axle housing, using your stock limited slip ring gear carrier.

Unless you have done an install before, I would suggest you find a highly qualified shop to do the work. It requires specialized tools, and if the setup isn't perfect you can end up with howling gears that will drive you crazy.

You will need to have the PCM (Powertrain Control Module = computer) tuned for the new rear axle ratio. That is required to maintain the accuracy of the speedometer/odometer. Commonly done by a "mail order" tuner. While the tuner is at it, he can also tweak the stock program to produce 10 to 15 more HP at wide open throttle. I can recommend a source. Reliable LT1 tuners are getting hard to find.
 

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Discussion Starter · #7 ·
Yeah if you know of any tuners that are good with LT1 cars that would be great and if you knew somewhere that rebuilt differentials or somewhere that you liked to use that'd be awesome.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Only reliable LT1 mail-order tuner I am aware of:

Moe Bailey

(254) 644-2656

[email protected]

Just to be clear, you are not "rebuilding (the) differential". All you are doing is installing a new set of gears (ring & pinion). The actual limited slip differential (photo below) is a complex system of gears, clutches (or cones) and springs that can be rebuilt, but only needed if the limited slip function is not working. Do a real hard launch, spinning the tires. You should see black stripes from BOTH rear tires. That means the limited slip (aka "Posi-Traction") is working. If you only see one black stripe, from a single tire, the limited slip is not working, and your car is more likely to sit there, spinning one tire, and going nowhere.

The factory limited slip differential in most 4th Gens is an Auburn cone type. In theory, these can be rebuilt, but seldom are. There are alternatives like the Eaton clutch type (photo below) and the Torsen gear-type.

As far as a good shop, you need to find one in your area. I use a shop in PA for all my work, but that wouldn't be useful, since they need the car on a lift to do the gear swap.
 

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Discussion Starter · #9 ·
Only reliable LT1 mail-order tuner I am aware of:

Moe Bailey

(254) 644-2656

[email protected]

Just to be clear, you are not "rebuilding (the) differential". All you are doing is installing a new set of gears (ring & pinion). The actual limited slip differential (photo below) is a complex system of gears, clutches (or cones) and springs that can be rebuilt, but only needed if the limited slip function is not working. Do a real hard launch, spinning the tires. You should see black stripes from BOTH rear tires. That means the limited slip (aka "Posi-Traction") is working. If you only see one black stripe, from a single tire, the limited slip is not working, and your car is more likely to sit there, spinning one tire, and going nowhere.

The factory limited slip differential in most 4th Gens is an Auburn cone type. In theory, these can be rebuilt, but seldom are. There are alternatives like the Eaton clutch type (photo below) and the Torsen gear-type.

As far as a good shop, you need to find one in your area. I use a shop in PA for all my work, but that wouldn't be useful, since they need the car on a lift to do the gear swap.
You will need:

- 4.11 gear set for GM 7.5/7.625 10-bolt, 3-series carrier. I like Motive.

- a compatible install kit - bearings, shims, bolts, cover gasket, crush sleeve, seals, pinion nut.

- given age of car add axle bearings and seals to install kit

- 2 quarts 80W-90 GL5 gear lube

- 4 ounces of GM Limited Slip Differential Additive

All that installs in your stock rear axle housing, using your stock limited slip ring gear carrier.

Unless you have done an install before, I would suggest you find a highly qualified shop to do the work. It requires specialized tools, and if the setup isn't perfect you can end up with howling gears that will drive you crazy.

You will need to have the PCM (Powertrain Control Module = computer) tuned for the new rear axle ratio. That is required to maintain the accuracy of the speedometer/odometer. Commonly done by a "mail order" tuner. While the tuner is at it, he can also tweak the stock program to produce 10 to 15 more HP at wide open throttle. I can recommend a source. Reliable LT1 tuners are getting hard to find.
Just Wanted to add to this thread for anyone else looking. As far as the PCM tuning goes I found a way around this that will solve the speedometer problem regardless of the rear-end gear and or wheel and tire size. There is a website The Yellow box speedometer corrector that sells a device that corrects the signal from the PM generator sensor that the factory T56 uses based on the input you give it. There is more info on their website about how it works but it is a fairly cheap and easy way around using a mail out tuner just to fix the speedo.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Not clear - are you inserting this between the VSS in the trans and the PCM? Or between the PCM and the instrument cluster. The first one works, the second one bypasses the PCM and will throw off things like deceleration fuel cutoff (DFCO), cruise control, and diagnostic routines that are initiated based on MPH. Might also affect ABS/TCS.

There is a similar product made in the US by Dakota Digital. I've had their earlier SGI-5 unit for about 18 years, to interface a 40 tooth auto trans sensor with a PCM programmed for an 17 tooth manual trans. But the only things my stock PCM controls are speedometer, factory tach, and fan on/off temps. Everything else is controlled by a MoTeC M48Pro ECU.

 
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