3.73s & 6R80

Good Lord I know this will cause a lot of questions & controversy, but here it goes I put the 3.73s in my rear this weekend. Now I can't answer all questions at this time(like who did the BCM adjustment), just know that Ford is coming out with a fix in the 4th quarter of this year. The 3.73s work just fine and the speedo is right on. These pics are to show you that even with limited equipment it can be done. Keep in mind I've changed hundreds of gears. You will see the holding tool I made out of scrape to be able to loosen the pinion and to crush the crush sleeve. Keep in mind I'm lucky if I could bench press a 100 lbs. All hand tools(except for the press for the bearings), no air tools. My stock pinion had a .038 shim under the bearing, that's what I put under the 3.73 pinion bearing. Fortunately the existing posi shims worked also. My posi was a super tight fit(the stock one & the new one), probably because of the aluminum housing. The install kit from Ford Racing had more than enough stuff including posi shims (20+yrs. ago we weren't so lucky to get those). Sometimes people get confused on torquing the pinion nut, you don't actually torque the pinion nut, you crush the crush sleeve to preload the pinion bearings and preload reading is in inch lbs of drag that it takes to rotate the pinion gear by itself. Think of it as an older front wheel bearing, where you tighten the nut to preload the bearings. It does take a lot of torque to break loose and to crush, but that torque number has no importance to the install. As you can see I use a long breaker bar to do this, not a torque wrench.

So much for the "the 3.15's are a different diff" crap.

I think you'll regret going to the 3.73's tho...that's going to be super steep with the auto.

1320' said:

So much for the "the 3.15's are a different diff" crap.

I think you'll regret going to the 3.73's tho...that's going to be super steep with the auto.

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Well I did put in the posi that Ford suggested, I could not see any noticeable differences, but used it just to be safe.

Well, it looks to me - and this might a photo angle thing - that the flange offset on the yellow spring diff is noticeably greater than it is on the green-spring diff. There looks to be more clearance between the ring gear bolt heads and the axle case wall. That's exactly what I'd expect, assuming the green diff was the replacement. I know for fact that Ford designed two different diff cases to cover the range of R&P ratios that they intended to offer. I was at the meeting where they gave the quote packages out to prospective differential suppliers. There were actually three differentials at the time, the 3rd being a heavy-duty version of the high-numeric case. That was since replaced with the Torsen and never saw production.

Thanks for the pictures of the install

Really interested in your results. Glad to hear the BCM issue will get resolved. Sucks it is Q4 though.

Gears should be nice for a NA setup.

EXP Jawa said:

Well, it looks to me - and this might a photo angle thing - that the flange offset on the yellow spring diff is noticeably greater than it is on the green-spring diff. There looks to be more clearance between the ring gear bolt heads and the axle case wall. That's exactly what I'd expect, assuming the green diff was the replacement. I know for fact that Ford designed two different diff cases to cover the range of R&P ratios that they intended to offer. I was at the meeting where they gave the quote packages out to prospective differential suppliers. There were actually three differentials at the time, the 3rd being a heavy-duty version of the high-numeric case. That was since replaced with the Torsen and never saw production.

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Great eye, I believe you hit the nail on the head. I was getting tired & blurry eyed lol. That makes total sense as the ring gear on the 3.73 was not any thicker to make up the difference of the smaller diameter of the pinion gear.

1320' said:

So much for the "the 3.15's are a different diff" crap.

I think you'll regret going to the 3.73's tho...that's going to be super steep with the auto.

Click to expand...

Check out what EXP Jawa noticed in the pics, that was a good catch and explains the difference between carriers. I responded with reinforcement of EXP Jawa's findings.

nice

Good Stuff OP!

1320' said:

I think you'll regret going to the 3.73's tho...that's going to be super steep with the auto.

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Agreed. The 6R80 has steep gearing hence the 3.15's. On my 2011 GT I ran 11.2's N/A with my 3.15's. Plenty of gear.

1320' said:

So much for the "the 3.15's are a different diff" crap.

I think you'll regret going to the 3.73's tho...that's going to be super steep with the auto.

Click to expand...

I absolutely love the 4.10's with my 6R80 trans in my '14 GT. Certainly not too steep, IMO. Combined with a big stall converter, and you get quite a performer.

Gearing

For a comparison

000000000Fox Body T-5 Daysooo000000S550 6R80
1st gear oo3.35x4.30=14.41ooo000004.17x3.73=15.55
2nd0000001.99x4.30= 8.550000000002.33x3.73= 8.73
3rd0000001.34x4.30= 5.7600000oo001.66x3.73= 6.19
4th0000001.00x4.30=4.3000000000oo1.14x3.73= 4.25
As you can see very similar, the advantage that the Fox had was being able to side step the clutch at 6,000 RPMs and shift at 6,500, while S550 will be lingering around 4,000 RPMs until the ground speed comes up to 7,300 RPMs and shift. No doubt the 6R80 has way more gear than the AOD days. But when you do a comparison to a T-5 (which ruled the street back then). Believe me I'll be the first in line to get a new posi and 4.10 gears when they're available. It's not like I can't go back, it'll cost me $0 to go back, just a couple of hours of fun wrenching. As a matter of fact when housings come available it'll almost a quick change rearend!

I just purchased a 3.55 from cjpony parts for my ecoboost. Am I SOL on being able to install it until the 4th quarter of this year? I have a tuner but from what I've read that isn't enough. Has anyone swapped ratios without flashing the bcm? Thanks for any help.

Wera750 said:

I just purchased a 3.55 from cjpony parts for my ecoboost. Am I SOL on being able to install it until the 4th quarter of this year? I have a tuner but from what I've read that isn't enough. Has anyone swapped ratios without flashing the bcm? Thanks for any help.

Click to expand...

I've heard that some have done it without flashing the BCM and had no problems, but that's a risk especially if your paying labor for the gear install. In my case it just would be more work. Keep in mind you may need a posi like I did for 3.73s as Ford kept the ring gear thin(good thing), so the posi flange has to be moved inward to get the ring gear to the pinion gear. This is the difference in the posi's from 3.15s to 3.73s, IDK about 3.55s.