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View Full Version : What make and models use 3.55 open 8.8 diff IRS?



grey500se
06-30-2012, 11:10 AM
Anyone one know this, I know the SC came with 2.73 manual and 3.27 for the auto but what are the options for the 3.55's? Its not for an SC, so I don't care about the fitment, as long as they have stub axles, also need the spline count?

RalphP
06-30-2012, 11:26 AM
Unless it was special ordered, none of the MN12s/FN10s came with 3.55s from the factory.

The options now are to pick up an open diff and swap the gears out.

New gears aren't terribly much - FRPP gears can be bought for $150 from reputable dealers out of MM&FF magazine (Tommy Vaughn has a nice listing in every issue I've gotten over the past year.)

Some of these places will also have the rebuild bearing / seal kits.

RwP

grey500se
06-30-2012, 11:40 AM
Unless it was special ordered, none of the MN12s/FN10s came with 3.55s from the factory.

The options now are to pick up an open diff and swap the gears out.

New gears aren't terribly much - FRPP gears can be bought for $150 from reputable dealers out of MM&FF magazine (Tommy Vaughn has a nice listing in every issue I've gotten over the past year.)

Some of these places will also have the rebuild bearing / seal kits.

RwP

Thanks for the quick response, so any 89-95 posi or open diff case should work? can you use the stock stubs? they are 27 spline right?

RalphP
06-30-2012, 12:30 PM
Thanks for the quick response, so any 89-95 posi or open diff case should work? can you use the stock stubs? they are 27 spline right?

28 spline, but yes, if you use a stock diff, the stock splines will fit. And you can use up to 1997 Tbird/Cougar, or 93-98 Mark VIII cases.

The gear doesn't matter to that, that's part of the tracloc or spider gear setup.

If you're building one up and aren't going for "insane" horsepower/torque levels, you may want to grab a Mark VIII aluminum housing for the reduced weight. Grab it out of a 97-98 Mark VIII, and keep the halfshafts (they're hardened at the weak end - I forget which one, but at the end that's more prone to breaking on standard half shafts.)

RwP

grey500se
06-30-2012, 12:48 PM
I am planning to run high torque, high hp, whats the weak link here, the case, the stubs, or the half-shafts?

Those mentioned models all have bolt on half-shafts?

RalphP
06-30-2012, 01:21 PM
I am planning to run high torque, high hp, whats the weak link here, the case, the stubs, or the half-shafts?

Those mentioned models all have bolt on half-shafts?

Huh? NONE are bolt-on. They're all the same half-shafts (or interchangable!) with a factory set on a SC.

The weak link varies.

Define "high torque, high HP" please. Also, auto or standard?

RwP

grey500se
06-30-2012, 03:05 PM
700ftlb torque, 800hp, I must be remembering something else, which IRS am I thinking of that has bolt on axles?

mr_josh
06-30-2012, 04:31 PM
Certain Jags use a bolt-on half shaft, Nissan / Datsun does on certain Z cars, sure there are many others out there.

grey500se
06-30-2012, 05:43 PM
I think my only option is a Dana 44, Dana 60 or a Chevy 14 bolt w/ a 10.5" but those don't come in a 3.55,

either way, I would cut the tubes, machine the bores to accept the bearing and seal.

What do you know about Ford 9"? splines? and models.

RalphP
07-01-2012, 01:59 AM
For that, if you stay with the IRS, I'd HIGHLY recommend (and if Dalke or one of the other pros chimes in with other suggestions, ignore these recommendations!) that you:

a) Go auto.

b) Use DLF's Delrin bushings on the LCA and spindles both.

c) Upgrade to Cobra 31 spline half shafts and a Cobra 31 spline rear end.

d) Use something besides a tracloc - one of the Detroit family, possibly. Or even a spool instead of a tracloc design.

e) Buy up all the Mark VIII half shafts you can find and have them resplined to run 31 spline

f) Hold on and pray every time you launch :D:D

RwP

grey500se
07-01-2012, 12:19 PM
For that, if you stay with the IRS, I'd HIGHLY recommend (and if Dalke or one of the other pros chimes in with other suggestions, ignore these recommendations!) that you:

a) Go auto.

b) Use DLF's Delrin bushings on the LCA and spindles both.

c) Upgrade to Cobra 31 spline half shafts and a Cobra 31 spline rear end.

d) Use something besides a tracloc - one of the Detroit family, possibly. Or even a spool instead of a tracloc design.

e) Buy up all the Mark VIII half shafts you can find and have them resplined to run 31 spline

f) Hold on and pray every time you launch :D:D

RwP

Okay, thank, let me give you the background of this project, if you search "700hp Homemade Dyno, who likes math?" that's what I'm using the diff because I need to spin my water brake 3.25-3.5 times faster than the, so its opposite of a diff, the power goes through the shaft then back through the pinion. It won't have the same shock loading as launching a car but still should be stout enough to not break, the numbers I listed above is the rating not what the car actually produces.

http://i8.photobucket.com/albums/a42/onebadgixxer/brakesetup_001.jpg

Here's a setup picture, I have two options use a 3.55 diff or triple #50 chain setup (same as your high power motorcycle chains) but the issue with chains is the fact the bearings are spinning up to 7000rpm, I don't think dry bearings would last very long here.

The remaining part of the system is easy, it only spins up to max 2000 rpm, dual #50 chains share the load and the splines I'm using are 1.375" 16spline out of a diesel Dodge, more than enough. Its the step-up part I'm having the issue with. So which one do you all like, chain or diff?

RalphP
07-01-2012, 04:00 PM
Huh.

I like the idea of an open diff (you don't need tracloc for this job!)

If you can find 7.5" ring/pinion gears, that may be better for this. And there's a bunch of 3.8NAs in the local yards (mostly 7.5" diffs in them.)

However, I'm not an engineer and don't even play one on TV. So if someone with "PE" or "ME" after their name shows up, listen to them.

RwP

grey500se
07-01-2012, 04:37 PM
Thanks, I'll just weld the spider gears or buy a mini-spool (hard to find for a dana 60) that's why I needed a open diff but I'm still worried about the stocker 28 spline breaking on me, the dana 60 uses a 35 inner shaft spline and comes in a 3.54 and 4.10, dana 44 uses a 31 although I wanted the stubs out of a Corvette, however, they are a 28spline.

In my opinion rarely does a ring or pinion break, its usually the spider's are the one's to go, anyone disagree?

Like a BMW 530 uses a 3.45ratio with bolt on stubs but strength is another issue, are can be bought for less than $150.