I know a little about cars, but have never dealt with the rear end. I have a few questions. I have an 88 IROC 5.7TPI with aluminum driveshaft and 2.77 rear end gearing, plus positraction. I autocross this car and run it quite hard all of the time. I have two concurrent issues that I was wondering about. First, I think my posi unit is not working. Second, I am considering swapping out the 2.77 rear end gear with the 3.27 gear that was optional for this model year (this will keep me in the stock class for autocrossing).

Questions:

1) Is the posi unit independent of the rear end gearing or is it an integral part? I suppose what I am asking is that do you have to replace the rear end gear when your posi fails?

2) Is there any way to test the posi unit? I did a burnout in my driveway on Saturday and I noticed that the passenger's side tire mark was considerably darker than the drivers side. This brings up a related question - does to posi completely fail or does it sometimes partially work? My driveway is slightly off-camber and could have put additional weight on the passenger side, causing the darker tire mark.

3) What all do I need to buy to replace the rear end gear ratio? Approximately how much should this cost? Is this hard or not too much effort?

Any help would be greatly appreciated. Thanks.

[This message has been edited by KeithO (edited November 19, 2001).]

 

Anyone?

 

all you need to replace is the ring and pinion, the posi is seperate, yes posi engages both wheels at a differant rate and does allow for some slipage, it often fails slowely. not sure if the 9 bolts can be rebuilt with new clutches or not.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

The posi unit is separate from the gears themselves. You can repalce either one separately from the other.

Posis can be weak or worn and result in poor performance. One way to check if it is totally broken or not is to jack both rear tires off the ground and you and a buddy try to spin both rear wheels in the same direction. Notice if only one person spins one tire, the other tire goes in the opposite direction.

As for price, I need to replace my broken 3.73 with a 4.10, the gears themselves will be about $200. The posi i want is an eaton unit from Drivetrain Specialists. This runs $329. Do not try to install the gears by yourself w/o proper tools, they need to be set up by someone who knows what they're doing.

These prices are not for stock replacements like it sounds like you want. I do not recommend a stock replacement posi unit from a 7.5" 10 bolt, they suck.

Hope this helps.

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84 Z28 L69. 5 speed with 3.73 posi (broken)Ported heads, intake, 10:1 comp. 230/230 .480 lift cam @.050" w/108 LSA

Triumph Spitfire SCCA roadracer. 1.1 liter 4cyl w/ 14:1 comp.
Outcorners anything!

 

1. The posi is independent of the gearing, within certain limits. Specifically, as the rear end ratio changes, the relative diameters of the ring and pinion change too, along with the number of teeth. The ring gear diameter is a constant for a given model of rear end. So as the ratio increases, the pinion gear diameter must decrease. The ring gear therefore must become thicker. At some point in the progression of increasing ratios, the factory changes to a carrier with the ring gear flange closer to the pinion, and starts over with a thin ring gear. In the 9-bolt I don't know where this occurs; but it's usually somewhere near 3.00. So it would be worth researching to find out if your 2.77 carrier can accomodate 3.27 gears, because there's a possibility that it won't.

2. A posi unit consists of a set of clutches forcing the axles to rotate with the carrier such that a certain amount of torque is required to make the axle turn at any RPM other than the same as the carrier. The only way to test a posi unit is to hold one wheel still and measure how much torque it takes to turn the other one. Around 50 ft-lbs is typical for most models of rears, I don't know exactly what it would be for a 9-bolt, but I would expect it to be in that ballpark. But, since you have a motor that can put out a couple of hundred ft-lbs, it's easy for the motor to overcome the clutches, especially if one tire has better traction than the other. In other words, posi doesn't guarantee that the wheels will turn at the same speed, it just establishes a minimum set of circumstances that have to exist before they won't.

3. You'll need gears (duh), maybe a carrier, some shims for the pinion to locate its depth correctly in the housing, and some shime for the carrier to locate the ring gear the correct distance from the center of the pinion. Miles (88 427 Camaro) has a source for parts for 9-bolts, e-mail him for details. No doubt he'll be able to help you out. I can guarantee it won't be cheap. I'd expect it to cost about $500 for parts, plus shop charges in labor, which would probably be in the $300 neighborhood in most places. If your carrier won't accomodate the other gear, or you have to buy one for any other reason, that would be extra too.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Thanks for the replies. They are very helpful. I think I first need to do some targeted "homework" with the information above, then research the exact part #'s and costs... When/should I do something, I will post the results, including part numbers, prices and labor time so that others may use this information.

Thanks again.

 

Here is what I know. 3.2x to 4.10 ratios will work with the series 3 carrier.

I am currently offering new 3.2x, 3.45, 3.73 and 4.10 gear ratios for the Borg Warner 9 bolt. I am also offering new series 3 posi carriers. Other 9 bolt items will be available soon.

WARNING: There are several companies claiming to sell a 3.73 ratio for the 9 bolt. They are actually selling the weaker 10 tooth 3.70 ratio, not the stronger 11 tooth 3.73 ratio!

Everyone complains about the cost of 9 bolt stuff, but remember these things:

- It was the high performance option for 3rd gens. 9 bolt posi carrier prices are inline with hi perf 10 bolt prices.
- The 9 bolt posi carriers has 4 spider gears instead of 2 (10 bolts)
- A complete refurbed 9 bolt is ~ 1/2 the cost of a 9 inch or 12 bolt (and weigh much less), and will handle most street cars.

There are ways to make 10 bolts and 9 bolts last under more power. The reason rears (10 or 9 bolt) grendade is from clutch dumps or trans brakes launches on slicks. Also, an incorrectly setup or a badly worn posi (or gear set) will grenade under much less power (ie stock power levels).

I have been running my 9 bolt for 3 years now (first with 3.27 gears and now 3.73 gears) with slicks and 1.72-1.75 60 ft times and it's all stock stuff. I haven't even put the good stuff in it yet! Then comes the bigger converter and trans brake


Prices? Here is what I offer:

Gear set (your choice) ~295

Install kit (with pinion & carrier bearings, shims, ring gear bolts, gear marking compound, pinion seal, rear cover gasket, install instructions) ~165

Gear set with install kit ~445

Series 3 posi carrier ~445

Carrier with install kit ~595

Carrier with gear set ~695

Carrier, gear set & install kit ~795

A group purchase (at least 10) could knock 10-15% off these prices if there is enough interest.

Miles

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88 427 Big Block Camaro
(Best 1/4) 12.18 @ 113mph 1.75 60ft
(Best 1/8) 7.76 @ 94mph 1.72 60ft

Currently installing 3.73's into my 9 bolt (YES, it is a 9 bolt & YES they are 3.73's, not 3.70. I currently have 4.10 gear sets and new series 3 posi carriers for sale for 9 bolts )
www.koolmeister.com

 

Miles,

Is this a limited supply, or or do you plan on having them around for a while?
I just laid out over $2k for a T56 conversion, and I am going to be short on cash for the next few months. I know I need to go to at least 3.73's instead of my 3.27's
I have now, but it is going to have to wait until April or so....



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My 89 Formula
13.98 @92 mph 2.06 60 ft
1989 Firebird Formula, WS6 Package, ZZ4 Crate engine.
MSD 8.5mm Super wires, SLP 58 mm throttlebody, Edelbrock TES headers, SLP 3" SS exhaust,
Random Tech 3" hi flow cat, K&N filter, 700R4 trans w/ vette 2/4 shift servo, worked valve body, kevlar band,
2200 rpm stall converter, Borg Warner 9 bolt 327 rear (full poly mounts) Hotchiss panhard rod, Lakewood LCA's,
Spohn subframe connectors, custom chip, Walbro hi-flo fuel pump.
Accel stuff: 24 pound injectors, Superram intake, Large tube runners, billet distributor, supercoil.
In the works: 1997 Trans Am T56 Trans swap, Spohn crossmember/torque arm/driveshaft loop

 

They should be around for awhile.

Miles