3.15 Gear Ratio -Why it is different-

[JohnVallo]

I will do my best to explain in detail why the 3.15 ratio setup isn’t compatible or doesn’t interchange with the other higher numerical gear ratio’s (i.e., 3.31, 3.55, 3.73 etc.), setups in the “Super 8.8” rear-end. -What’s Different? -

First of all, the -Bare Housing- in the “Super 8.8”, (the part that holds all the internals, and bolts to the subframe), whether it be aluminum, or cast iron, (ARE ALL INTERCHANGABLE).

Terminology:
If you are referring to the -Bare Housing with everything in it-, this is usually referred to as a “Loaded Carrier”, or “Pumkin”.
The “Differential”, or “Differential Carrier” should only refer to the part that the Ring-Gear bolts to whether it be “Trac-Loc”, or “Torsen” type.

So here are the exact differences between the 3.15 ratio setup, and the others (3.31, and up) in the “Super 8.8” rear-end.
The “Differential” itself is different with the 3.15 than with the others. Why? Read-on
The 3.15 pinion head (the part of the pinion that contains the teeth) is larger, than the others, Why? -It has more teeth- The pinion tooth count on the 3.15 is: 13, The pinion tooth count on the others (3.55, 3.73 etc.) is: 11

Left: a 3.15 Pinion gear: OD at head is: 112 mm, Right: a 3.73 Pinion gear: OD at head is: 98 mm

This larger diameter pinion head presents a problem. The 3.15 Ring-Gear now has to be spaced further away from the larger 3.15 pinion head. How did Ford accomplish this? They used a different Differential, with a different offset.

Left: a 3.15 Trac-Loc Diff., Right: a 3.31, and up, Trac-Loc Diff. The offset difference is: 8 mm

I won’t get into the deeper subject of “Differential Carrier Breaks”, and “Differential Carrier Deck Heights” now, but West Coast Differentials has a good description here:
https://www.differentials.com/technical-help-2/carrier-breaks/

The Ring-Gear’s physical size (in all the Factory ratio’s) are pretty similar in physical dimensions.

All “Super 8.8” Ring-Gears have same ID, OD, and bolt pattern, -they only vary slightly in thickness and tooth count-

Left: 3.15 Ring-Gear, Right: 3.73 Ring-Gear

Ratio, Pinion TC/Ring-Gear TC, Part Number

3.15, 13/41, Ford PN: JR3Z-4209-A
3.55, 11/39, Ford PN: FR3Z-4209-C
3.73, 11/41, Ford PN: FR3Z-4209-B
3.91, 11/43, Ford Performance PN: M-4209-88391A (May be Discontinued)
4.09, 11/45, Ford Performance PN: M-4209-88409A

Aftermarket (Super 8.8) Ring and Pinion gearsets have the following Tooth Counts:
4.11, 9/37
4.56, 9/41
4.88, 8/39

For more complete information on the "Super 8.8" rear-end see:
https://www.mustang6g.com/forums/threads/inside-the-super-8-8-rear-end.150514/

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[WD Pro]

I have a 4:09 off the car if you need any dimensions for comparison

I wonder if a 8 or 9 tooth pinion (aftermarket) is considerably weaker than a 11 or 13 (OE Ford) ?

WD

 

[TeeLew]

@JohnVallo What plate stack height should I be shooting for on either side of the diff? The Ford book gives a relatively wide range and I'm curious if you have a particulary 'good' setting?

 

[JohnVallo]

@JohnVallo What plate stack height should I be shooting for on either side of the diff? The Ford book gives a relatively wide range and I'm curious if you have a particulary 'good' setting?

Some consideration may depend on what preload spring/springs you use. I thought I read about you were thinking of using or had used Belleville springs for preload.?
I don't have a recommended good setting, Go with your best guess.

 

[TeeLew]

I had to punt on the Bellevilles, but I have another little contraption that has a similar function to the S-spring with more oomph. That will be my first shot at preload. It has little coil springs that can be changed to adjust preload should the pain hit. I was more thinking about the individual stack heights, but maybe that will kind of shake out in the wash.

 

[Dieval]

@JohnVallo I have a 2015 V6 which comes with the 3.15 gears… what is the best way to upgrade to 3.73 gears?

 

[WD Pro]

I would guess for you guys where there are a lot of these things on salvage yards - get a full PP1 Torsen pumpkin from a M6 car and swap the full unit ?

WD

 

[Cobra Jet]

I will do my best to explain in detail why the 3.15 ratio setup isn’t compatible or doesn’t interchange with the other higher numerical gear ratio’s (i.e., 3.31, 3.55, 3.73 etc.), setups in the “Super 8.8” rear-end. -What’s Different? -

First of all, the -Bare Housing- in the “Super 8.8”, (the part that holds all the internals, and bolts to the subframe), whether it be aluminum, or cast iron, (ARE ALL INTERCHANGABLE).

Terminology:
If you are referring to the -Bare Housing with everything in it-, this is usually referred to as a “Loaded Carrier”, or “Pumkin”.
The “Differential”, or “Differential Carrier” should only refer to the part that the Ring-Gear bolts to whether it be “Trac-Loc”, or “Torsen” type.

So here are the exact differences between the 3.15 ratio setup, and the others (3.31, and up) in the “Super 8.8” rear-end.
The “Differential” itself is different with the 3.15 than with the others. Why? Read-on
The 3.15 pinion head (the part of the pinion that contains the teeth) is larger, than the others, Why? -It has more teeth- The pinion tooth count on the 3.15 is: 13, The pinion tooth count on the others (3.55, 3.73 etc.) is: 11

Left: a 3.15 Pinion gear: OD at head is: 112 mm, Right: a 3.73 Pinion gear: OD at head is: 98 mm

This larger diameter pinion head presents a problem. The 3.15 Ring-Gear now has to be spaced further away from the larger 3.15 pinion head. How did Ford accomplish this? They used a different Differential, with a different offset.

Left: a 3.15 Trac-Loc Diff., Right: a 3.31, and up, Trac-Loc Diff. The offset difference is: 8 mm

I won’t get into the deeper subject of “Differential Carrier Breaks”, and “Differential Carrier Deck Heights” now, but West Coast Differentials has a good description here:
https://www.differentials.com/technical-help-2/carrier-breaks/

The Ring-Gear’s physical size (in all the Factory ratio’s) are pretty similar in physical dimensions.

All “Super 8.8” Ring-Gears have same ID, OD, and bolt pattern, -they only vary slightly in thickness and tooth count-

Left: 3.15 Ring-Gear, Right: 3.73 Ring-Gear

Ratio, Pinion TC/Ring-Gear TC, Part Number

3.15, 13/41, Ford PN: JR3Z-4209-A
3.55, 11/39, Ford PN: FR3Z-4209-C
3.73, 11/41, Ford PN: FR3Z-4209-B
3.91, 11/43, Ford Performance PN: M-4209-88391A (May be Discontinued)
4.09, 11/45, Ford Performance PN: M-4209-88409A

Aftermarket (Super 8.8) Ring and Pinion gearsets have the following Tooth Counts:
4.11, 9/37
4.56, 9/41
4.88, 8/39

For more complete information on the "Super 8.8" rear-end see:
https://www.mustang6g.com/forums/threads/inside-the-super-8-8-rear-end.150514/

Another awesome rear diff thread by @JohnVallo!,

Thanks for taking the time to not only post the tech, but provide actual comparison images.

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