going from 315 gears to????

[stang1971]

I cant seem to find a for sure answer... can you switch the gears out of the 315 diff to say 373? Or.. is a whole new diff needed that the 373 will fit in?

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[EXP Jawa]

There are answers for sure in the drivetrain subforum, but it boils down to this: with the 3.15:1 gear set, there is a unique differential case that only fits the 3.15 ratio. To change that, you also have to buy another diff. The actual install should be fairly standard though. Or, you can buy a complete carrier that already has the 3.73:1 gearing in it, either from Ford Racing or maybe someone that's parting a PP car out. That cuts the work in half and removes the fussy part (proper gear set up) from the job.

 

[stang1971]

There are answers for sure in the drivetrain subforum, but it boils down to this: with the 3.15:1 gear set, there is a unique differential case that only fits the 3.15 ratio. To change that, you also have to buy another diff. The actual install should be fairly standard though. Or, you can buy a complete carrier that already has the 3.73:1 gearing in it, either from Ford Racing or maybe someone that's parting a PP car out. That cuts the work in half and removes the fussy part (proper gear set up) from the job.

i put it here figuring it would get seen here more..... ty for the quick answer... thats what i figured...... was hoping to avoid that..lol

 

[craigsix]

im going to 3.31 got new posi unit to go with it. 3.15 and 3.73s have different posi unit. and 3.31 and 3.55 use the same part number.

 

[craigsix]

also i have bmr diff lockout cause the 3.15 auto is aluminum housing and the ears can break with lower gears.

 

[craigsix]

There are answers for sure in the drivetrain subforum, but it boils down to this: with the 3.15:1 gear set, there is a unique differential case that only fits the 3.15 ratio. To change that, you also have to buy another diff. The actual install should be fairly standard though. Or, you can buy a complete carrier that already has the 3.73:1 gearing in it, either from Ford Racing or maybe someone that's parting a PP car out. That cuts the work in half and removes the fussy part (proper gear set up) from the job.

not true rbs550 has run a 3.73 and 3.31 gears in his 3.15 diff.

 

[boosted4life]

Wish someone would trade me there 3:15 for my 3:73s

 

[stang1971]

Wish someone would trade me there 3:15 for my 3:73s

im interested..lol

 

[EXP Jawa]

not true rbs550 has run a 3.73 and 3.31 gears in his 3.15 diff.

I'm sorry, but that won't work. The ring gear flange on the 3.15:1 carrier is in a different location than it is on the 3.31 and numerically higher carriers - its farther to the left side.

When the ratio gets lower, numerically, the pinion head becomes larger because it has more teeth on it. That means that the ring gear teeth have to move farther away to mesh with it. At some point, the ring gear will get too thin, and you have to step the whole gear - and the mounting flange - farther away. On this axle, that cut off is between the 3.15 and 3.31 ratios.

You can physically bolt the higher ratio rings to the 3.15 diff case, but you can't run the gears in the axle that way. The teeth won't be in mesh, or they'll be in a very, very poor mesh. If you change the ratio to anything higher from a 3.15:1 set, you have to also change the differential case.

 

[craigsix]

I'm sorry, but that won't work. The ring gear flange on the 3.15:1 carrier is in a different location than it is on the 3.31 and numerically higher carriers - its farther to the left side.

When the ratio gets lower, numerically, the pinion head becomes larger because it has more teeth on it. That means that the ring gear teeth have to move farther away to mesh with it. At some point, the ring gear will get too thin, and you have to step the whole gear - and the mounting flange - farther away. On this axle, that cut off is between the 3.15 and 3.31 ratios.

You can physically bolt the higher ratio rings to the 3.15 diff case, but you can't run the gears in the axle that way. The teeth won't be in mesh, or they'll be in a very, very poor mesh. If you change the ratio to anything higher from a 3.15:1 set, you have to also change the differential case.

check rbs550 on this forum

 

[EXP Jawa]

Sorry, but I'm not chasing your hearsay. If you want to, that's on you. But the parts are the parts; there's a carrier break in this axle design, and there's a reason why Ford states that differentials work with 3.31:1 and up gears or for the 3.15:1 gear. You can't cross that bridge without swapping diffs.

 

[craigsix]

Sorry, but I'm not chasing your hearsay. If you want to, that's on you. But the parts are the parts; there's a carrier break in this axle design, and there's a reason why Ford states that differentials work with 3.31:1 and up gears or for the 3.15:1 gear. You can't cross that bridge without swapping diffs.

thats fine ill let you know how they work out einstein

 

[The Great Gazoo]

im going to 3.31 got new posi unit to go with it. 3.15 and 3.73s have different posi unit. and 3.31 and 3.55 use the same part number.

You're putting a Chevy differential in your car?

 

[AmericanLegend]

thats fine ill let you know how they work out einstein

I'd listen to EXP Jawa... he is correct.

Application Note. Ford Racing 3.73 gears are NOT compatible with a 3.15 ratio differential - use differential M-4204-MT or M-4204-M.

Ford Racing 3.55 gears are NOT compatible with a 3.15 ratio differential - use differential M-4204-MT or M-4204-M.

 

[EXP Jawa]

thats fine ill let you know how they work out einstein

Hey, that's fine - do what you want. If you're willing to spend money on something the manufacturer says won't work (for reasons that have been explained) based on something that you heard on the internet, that's up to you. Good luck.

You're putting a Chevy differential in your car?

FWIW - "posi" is simply a shortened form of "Positraction", which is actually an Eaton trademark. Granted, GM relied heavily on Eaton diffs over the years and advertised the name. Now days, its often used a bit more universally, like Kleenex is...

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