Hey guys, great site you have here. I just recently sold my ’91 Mustang GT and picked up an ’89 IROC. The car has unknown mileage but it has a rebuilt 700R4 from just 20k miles ago and has a Jasper stage 1 bottom end with another 30k miles on the warranty. I ran the VIN on this site and I couldn’t find any information on the original factory specs. It has the 4 wheel disks so I would assume it has the 3.27 rear gearing, right? Or did all IROCs in ’89 have 4 wheel disks? Also, I read that in ’89 the IROCs came with either a 230 or a 240 HP L98. Is this true? I also would like to know if the Jasper stage one bottom end would have the cam from the 230 or 240 HP engine.

I also would like to swap to a 3.73 rear end. Does this car have a 10 bolt rear? Remember I’ve never owned an F-body and I know nothing about these cars. Does anyone have an estimate as to what it would cost to have a set of 3.73s installed? Are there any good F-body shops in the Northern Va area? Any answers to any of these questions would be appreciated! Thanks, and have a nice day…

 

You didn't say if it is an L98 or an LB9. Does it have the G92 option? TPI?

engine hp torque intake gears
LO3 170@4400 255@2400 TBI 2.73
LB9 195@4000 295@2800 TPI 2.73
L98 230@4400 330@3200 TPI 2.77
L98 240@4400 345@3200 TPI 3.27

So you're looking at 170-240horsepower stock, depending on displacement, gearing, and intake. The 3.27 rearend is G92. Apparently that is the 10 hp difference. I think all IROCS came with 4 wheel disc brakes.

 

All 350 IROC's came with rear discs. They were optional on 305's. Also, all 350's got the 9 bolt rear, only a select few 305's got the 9 bolt. If you have 2 catalytic converters, you have the 240HP L98, with 3.27 gears. If only 1 cat, then you have only 230HP with 2.77 gears.

As far as the Jasper engine having the same cam, there was only one cam in the 350. The 10HP difference was in the higher flowing exhaust. Also, did you get the same cam as the factory? Call Jasper on that one. And good luck too, I had a real hard time trying to get the specs on mine.

Now, about the 3.73 gears - since you have the 9 bolt, you should forget about it. Gears for these rears are very rare and very expensive. I'm sure someone else here knows a little more than I do about this with the 9 bolt - its a common topic. Your best bet would be to swap in a 10 bolt. Parts are available and much cheaper, but the rear isnt as strong.



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1985 Z28, Jasper Class 1 350, Edelbrock Performer intake, Dual snorkel air cleaner, K&N, Edelbrock headers, Catco 3" cat, Hooker 3" cat-back, Richmond 3.73's, built 700r4 w/Darrell Young 2400 stall TC, JET Stage 2 chip, Hypertech coil, Lakewood LCA's, Powertrax locker, SSM subframes, Centerline Auto Drag's

 

Okay, thanks guys. It is a 350 TPI with two cats so it's a 240HP L98, huh? Now what's this thing about the 9-bolt rear? Some guy was telling me it had a rear end from Australia that they don't make gears for and I thought he was blowing smoke up my *** . What's up with this rear? I'm looking in Summit and there's no 9-bolt rear listed. There's 7.5", 8.2", and 8.5" 10-bolts...none of those are it? What can I do about getting gears for this thing or swapping a rear that I can get rears for? Thanks...

 

Hey Maartingale, I can't help you much here, but welcome to the group! Congrats on your car purchase, you made a good move.
Later
Mike

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(Gran Turismo Americano) "The Ultimate Firebird"
Smokin 1989 GTA- Bone stock except for exhaust. (no catalytic-stainless pipe into Flowmaster 2 chamber).

 

Leave the gears alone and swap to a better torque converter, or swap heads, cam, intake, etc. Stock TPI engines aren't high RPM beasts, and if you have the 9-bolt it makes sense to do other mods first until you move the powerband up where it will take more full advantage of a 3.73 rear. Not saying a gear change can't help, but it is not the biggest detriment to your setup. Just my $0.02.

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, Accel coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley AFPR, ported plenum,
Ruger P95DC, hot wife, new oil filter, thick rubber floormats, no cats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

I am not a rearend expert by no means but a friend of mine has the 9 bolt in his GTA and just decided to get a 4th gen rear instead. He was telling me that just getting a new rear for the car would be WAY cheaper then trying to put new gears in the 9 bolt. Almost all of the parts for the 9 bolt are discontinued so it would be not worth your time to swap gears. Congrats on the buy I would love to have a 240hp 350TPI that was one of my dream stock engines. Yea I know not much of a dream but hey I am a realist. And welcome aboard.

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"I'm not dumb, I just have an amazing command of thoroughly useless
information."
Bill Waterson creator of Calvin & Hobbes.

 

The 9-bolt rear was made in Australia by a company named Borg/Warnner. It has a 7.75 inch ring gear and the axles are held in without c-clips. This makes the rear very strong and can hold up very well beneith a 11 second car. Since you have 3.27 gears in the 9-bolt you have the series 3 carier, this is good!!! You CAN get gears for this rear from Drive Train Specialists, Randy's Ring and Pinion, SLP, TPIS, US Gear, and a few others. The only ratio available from these places is 3.70 which go for around $300 for the gears and $100 for the bearing/install kit, and after you figure in $200 for installiation the swap isnt too bad. Since you have the 700R4 auto trans the desired ratio for the street/strip with an L98 is 3.45. This ratio can only be found in the US new from GM, they go for around $600. If you add the $100 for the install kit and $200 for installation it gets pretty expensive. You can get gears from a junk yard used for very cheap but you realy dont know what you are getting and they are hard to find. You need to look for an '87 to '90 IROC or GTA with rear disks, 305 TPI, dual cats, and a 5 speed standard trans. To verify the ratio when you find this car you need to look on the axle tube for the stamped code or in the glove box for the RPO code and run them through the decoder on this site. Now there is a member on the Transmission/Drivetrain board that is trying to get the gears sent over from Australia. Since the rear is very common there, many ratios are available from 2.73 all the way to 4.10. There may also be some sort of price break since the gears are very cheap in Australia. However he has been on this for some time now and there is no telling when he will be completed.
Good Luck!!!

 

What blows my mind is nobody has suggested he simply crawls underneath the car and counts how many bolts are on the rear end cover before we decide he has a nine bolt for sure.

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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."

 

Thanks a lot for the replies and welcomes guys. Like a ROC, that's a lot of info there...thanks. From what I can understand from your response the optimal gear ratio with a 700R4 and an internally stock L98 is a 3.45 which will run about $900 installed. That's a little more than I'm willing to spend to swap from a 3.27 to a 3.45. I guess it would be cheaper to locate a 4th gen rear and have that installed...but then that wont be as strong, will it? I don't plan to build a high HP motor so I guess if I can take an LT1/LS1 it can take an L98. Oh, and is that really how you tell if your rear end is a 9-bolt? Count the bolts on the housing? I was under the impression that it was the bolts in the carrier or something. Some guy went under there and said it was an Australian rear so is the only Borg Warner rear the 9-bolt? Thanks...

 

On the B/W 9 bolt there is only 9 bolts holding the cover on. Easy way to identify it.

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1985 ASCD Ram Air Trans Am
Readers Ride at https://www.thirdgen.org/rides/index...ew&rideid=1067

-=ICON Motorsports=-

" I get drunk, I race cars and I fondle women. But... I have weekends off and I am my own boss"

 

Pick up a couple of issues of super chevy they do rear swaps all the time. They wanted 3.73 gears so they took the gears out of the 4x4 s-10 blazer in a junk yard and just reused thier posi. It is an idea if you need one.

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91 z-28 305 700R4
3.73 gears out back
T-Tops and Black all over.