I am doing the Mark V111 IRS install in my 71 F100. I have a 97 Mark V111 IRS. I want to install a limited slip differential and change the gear ratio which is currently 3.07. There is a lot of confusing info out there due to the fact that the IRS doesn't use the same differential as a straight axle truck. This has a groove cut in the side gears to accept a C clip for the axles. If you know for sure of part numbers for the proper differential, ring and pinion gears so I can change to a limited slip in the 3.50 range I would appreciate some help.

 

I don't know about part numbers, but I've done a bunch of research on this subject and I do know a little bit about interchange. To find a direct drop in trac-loc differential you need to be looking at Thunderbird Super Coupes. The automatic transmission cars up to 1993 had trac-loc as did all of the manual transmission cars. It sounds like you already know that the difference between an irs differential and a regular one is a groove that is machined into the side spider gears to retain the circlip on the end of the half shaft. What some have done is hunt up a regular trac-loc diff and have the groove machined into the spider gears. Or buy a trac-loc and replace the spider gears with irs ones. Maybe ND would have the part numbers and availability of the gears. A guy that I have bought some parts from over the years who drag races Mark VIII's told me that he doesn't even bother machining the groove into the spider gears because when the irs is all bolted together, the half shaft can't come out anyways. Personally, I didn't trust that program, and I searched up an SC diff.

As far as ring and pinion gears go, they are the exact same as any other 8.8 diff. So just about any ratio that you would want is available aftermarket. Otherwise you might do a little research and find out what ratios the SC's came with and maybe you could kill two birds with one stone.

 

Thanks. This is helpful information. So I can be looking for a SC trac-loc. If I get one salvage, go for up to 93. Does that include 93 as well? I am still interested in the Ford Part # or Auburn Gear if anyone including ND knows.

 

Yeah, that includes 93. Or any year manual transmission SC. In 94 they put traction control in the automatic cars and went to an open diff. Basically the same setup as the MKVIII except for the cast iron housing.

On a side note, when I started researching and buying parts for my own F100 project, I could have kicked myself right square in the butt. 4 years ago I scrapped a 93 SC that was my DD for about 5 years because the body was shot and I moved and didn't have room for all of my cars any more. Now I've got about twice as much money in buying the same parts than I got for the whole car for scrap. If I still had that car sitting around, I would probably have swapped the complete powertrain into my truck. Some day I will have the money to build a big pole barn and I will never get rid of another car again.

 

Hate when that happens.

 

How's this?
FORD:
old number E9SZ4026B.....loaded posi case assembly (SZ = t-bird)
replaced by 2L5Z4026AA....loaded posi case assembly (Ford changed what digits mean so now L5 means late model Ranger little truck)
Ford ring & pinion kits too expensive. Pick a 8.8 part number.

FRPP (Ford Motorsports now called Ford Racing Performance Parts):

M4204A28 Auburn HD 28 spline loaded posi
M4204T28 Torsen28 spline loaded posi
M4204T31 Torsen 31 spline loaded posi
M4204F318C factory Ford 31 spline loaded traction lok

 

Thanks for the information.

 

After some extensive searching I see that National Drive train offers a kit that includes everything necessary to go limited slip including Auburn Gear and ring and pinion choices. Trying to decide between 3.55 and 3.73. I will eventually be putting a 500 plus horsepower stroked out and stack injected 4.6 that I was building for another car. Most likely a Lentec 4R70W behind that. Any input?

 

Any preferences over Traclok, Auburn, or Truetrac(which is an Eaton brand)? National Drivetrain will put together a package deal for me with everything I need and choice of gear for a reasonable price to change my Mark V111 rear over to limited slip.

 

With an OD tranny like you mentioned I vote 3.73's. Sounds like you're building a motor that's going to like higher RPMs. It's going to scream more than lug. Besides, the RPM difference between those two ratios at highway speeds is only about 700 RPM anyways. I'd like to see a video of you launching that truck once it's done!