I'm getting a rebuild kit and probably gears for my differential, but I need to know what size are in there now. It's an 89 IROC-Z with the 305 TPI and 700R4. I'm going to try to build the hell out of this diff to see if it will hold the hp and torque of my 383.

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Riccioli Performance Motorworks

--RPM Racing--

-Currently building first ever engine. 383 with Super Ram plenum and runners, Edelbrock lower intake, Bosch 24# injectors, AFR 190 heads, MSD ignition.
-Will be installing Baer Brakes
-Will be installing full Spohn suspension in rear
-Art Carr 700R4 trans??

 

10 bolt or 9 bolt?

If its a 10 bolt with G92, you have 3.23's with a 3 series carrier. If its a 10 bolt without G92, you have 2.73's with a 2 series carrier.

If its a 9 bolt with G92, you have 3.27's. If its a 9 bolt without G92, you have 2.77's.

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-Jason M. 1991 Camaro Z28

 

Most likely you have 2.73's, if it's a 10-bolt, but most anything is possible via special order.

Look at the front of the passenger side axle tube. There is a three-character code stamped there which identifies the type and ratio from the factory (something like "3KG"). Look that code up in a Haynes manual to find the ratio.

Or, remove the rear cover, the number of teeth is stamped on the ring and pinion gears. Divide the larger by the smaller, that's your ratio (rounded to two decimal places).

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam and intake, World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back).
57 Bel Air, my 1st car. 0.030 over 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlock header mufflers, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, LT MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Best 15.1/95.5 @ 5800' Bandimere.

 

Most IROCs have the 9 bolt rear, and with an automatic trans, I am guessing you have the 3.27s. A few cars (like mine unfortunetly) have the 2.77 gears. In that case, its cheaper to buy a used 10 bolt and set it up yourself.

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission
Borg Warner 7.75" 9 Bolt Rear End with 2.77:1 Gears.

Current Mods: LT4 HOT Cam, Comp Cams 1.52:1 Roller Rocker Arms, Edelbrock TES 1 5/8" Headers, Catco High-Flow Catalytic Converter, Hooker 3" Aerochamber Cat-Back System, Transgo Shift Kit, Performance Resource Chip, Accel Ignition, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET (w/o LT4 cam): 14.32 @ 97.7mph
(corrected for elevation)

Soon to Come: 7.625" 10 Bolt with 3.42s and Accel TPI Intake Base

 

The 89 305 automatic TPI came with a 10-bolt 2.73 posi drum brake rear end. Its 2-series.

Only the 350's, and 305 5-speeds with G92 came with 9-bolts in 85-89.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.23 @ 107.62 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

What's G92??

 

G92 is the performance package that includes the better rear end ratio, dual cats (89-92), oil cooler, etc.