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Discussion Starter #1
I'm currently running a AOD with a 3.80 ratio posi (8' rear) in My 64 Sprint. The rear wines starting at aprox. 60 MPH. - It's, new, Motive gears, and I've had the set-up changed 2 times by reputable builders, and same result. -- This is, strictly, a street car, and I'm considering switching to a 3.0, ratio, open rear. My tires are about 26" tall. -- Performance is not a big issue, it's just a cruiser. -- Will this (3.0 ) work OK ? -- The other option, is to try a 3.80 ratio US Gear set-up, and keep the posi. -- Everyone will likely say stay with the 3.80 ratio, but, again, will the 3.0 set-up work OK ?
 

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I am not sure I understand the goal..

Given this statement, "The rear wines starting at aprox. 60 MPH."
Is the goal to reduce nose?
Or is it to get street gears or engine RPM down?

Posi (track-lock) does not impact anything except slow vehicle speed turning and very slow vehicle speed conditions where one wheel is turning faster than the other, So leave all things the same you coudl swap out ring and pinion instead of an entire differential.

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Hello RPMagoo,

Are you asking along the lines of will the 3.0 gears physically fit the carrier or more general like is this a good idea?

If you are asking about the physically fitting, check with the carrier manufacturer. If you go with an open differential carrier they'll probably fit as there is less carrier in the way to hit the larger pinion gear.

If you're asking if it's a good idea in general, you'll need to consider where is the horsepower and torque curves with respect to engine RPM. In other words is your engine set up for more high RPM horsepower or more low RPM torque. It would be a bad idea to use a lower ratio gear set such as a 3.0 if you have a large duration camshaft in a small displacement engine as low end torque suffers and loading the engine further with a smaller ratio rear end will make it feel like a dog from a stop and labour the engine.

Other information to consider would be does the AOD have the stock narrow ratio gear set or was it retrofitted with a wide ratio gear set (similar to a 4R70W) and what is the stall speed of the converter. All these factors really come into play on changing rear axle gear ratios.

A note on gear whine. I did a lot of research when selecting parts for my Ford 9" conversion in my Chevrolet Caprice Classic. Many people have gear whine problems from aftermarket gear manufacturers. I sourced a brand new Eaton True Trac differential and Richmond gear set for a brand new Moser Ford 9" made for the 5th generation Caprice Classic and I set up the gears myself and the rear end is dead quiet cruising around town or interstate speeds (70 MPH).

Another thing to take note is if the 3.8 gear set is a hunting, semi-hunting or non-hunting gear ratio. Off the top of my head I don't remember. But when the gears are lapped at the factory they usually have timing marks if they are semi-hunting or non-hunting gear ratio set. If they are not timed when installed in the third member they will make noise.

Good luck with your decision.

Cheers
 

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The axle ratio for the period that used AOD was likely in the 2.75 to 3.55 range. Depending on body style and engine

The ratios for the AOD are
  • First: 2.47:1
  • Second: 1.47:1
  • Third: 1.00:1
  • Overdrive: 0.67:1
  • Reverse: 2.00:1
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Discussion Starter #5
I'm currently running a AOD with a 3.80 ratio posi (8' rear) in My 64 Sprint. The rear wines starting at aprox. 60 MPH. - It's, new, Motive gears, and I've had the set-up changed 2 times by reputable builders, and same result. -- This is, strictly, a street car, and I'm considering switching to a 3.0, ratio, open rear. My tires are about 26" tall. -- Performance is not a big issue, it's just a cruiser. -- Will this (3.0 ) work OK ? -- The other option, is to try a 3.80 ratio US Gear set-up, and keep the posi. -- Everyone will likely say stay with the 3.80 ratio, but, again, will the 3.0 set-up work OK ?
I am not sure I understand the goal..

Given this statement, "The rear wines starting at aprox. 60 MPH."
Is the goal to reduce nose?
Or is it to get street gears or engine RPM down?

Posi (track-lock) does not impact anything except slow vehicle speed turning and very slow vehicle speed conditions where one wheel is turning faster than the other, So leave all things the same you coudl swap out ring and pinion instead of an entire differential.

Action
The noise is a P I T A, and I have the 3.0, open, gear set, and I am trying to decide if I should try it. That would be a inexpensive way to cure the issue, if it would work. I believe that the RPMs will be Ok, as long as I choose to shift to OD at a higher speed than currently. The lack of posi is not an issue, with Me. It's not a race car.
 

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Discussion Starter #6
The noise is a P I T A, and I have the 3.0, open, gear set, and I am trying to decide if I should try it. That would be a inexpensive way to cure the issue, if it would work. I believe that the RPMs will be Ok, as long as I choose to shift to OD at a higher speed than currently. The lack of posi is not an issue, with Me. It's not a race car.
The 3.0 set is assembled, in a chunk. I assume the set-up is as OEM, and I don't want to disturb it. -- Just switch the 2 complete chunks.
 

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The "chunk" is the differential assembly. Sometimes called a third member, drop out center section or pumpkin.

Replacing a differential to fix a sound may not accomplish that goal. It may. Or the other assembly may have it's own issues But the question is ... are either differential assemblies set up correctly to be quiet? To me it sounds like you do not know the answer.

So you have some clarity - there is gear noise and bearing noise. (Before driving make sure all tires are inflated correctly and uniform. The fluid in the axle is full)
Gear noise tends to be in a somewhat narrow range. Your example of over 60 may be that range. to isolate it more ... the question would be under acceleration, deceleration, maintaining speed???? If you drive faster, say 70 or 75 and then shift into neutral, while the vehicle is slowing down into that range is the sound still there?
Bearing noise tends to remain constant over the broad range. It may change in pitch however it usually does not go away. Specifically, wheel bearing noise is usually more pronounced during coast and may go away with light application of the brakes

Hunting v non-hunting is relevant to the issue.

I do not believe a 3.80 ratio is stock. And not much is stock on the vehicle. If the axle data on the link is correct the answer of 38 ring teeth and 10 pinion teeth solves that unknown.

Here is the 3.0 ratio. 39 ring teeth and 13 pinion teeth

Questions would be -
Is the gear teeth contact pattern correct for either set? Both on the drive and coast side
Is there excessive back lash?
Are the gears worn out?

As you say you can try the other set. But it is already out of the car. Run a pattern and check the teeth engagement. If it is good at least you know you have a good assembly to swap.

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Discussion Starter #8
The "chunk" is the differential assembly. Sometimes called a third member, drop out center section or pumpkin.

Replacing a differential to fix a sound may not accomplish that goal. It may. Or the other assembly may have it's own issues But the question is ... are either differential assemblies set up correctly to be quiet? To me it sounds like you do not know the answer.

So you have some clarity - there is gear noise and bearing noise. (Before driving make sure all tires are inflated correctly and uniform. The fluid in the axle is full)
Gear noise tends to be in a somewhat narrow range. Your example of over 60 may be that range. to isolate it more ... the question would be under acceleration, deceleration, maintaining speed???? If you drive faster, say 70 or 75 and then shift into neutral, while the vehicle is slowing down into that range is the sound still there?
Bearing noise tends to remain constant over the broad range. It may change in pitch however it usually does not go away. Specifically, wheel bearing noise is usually more pronounced during coast and may go away with light application of the brakes

Hunting v non-hunting is relevant to the issue.

I do not believe a 3.80 ratio is stock. And not much is stock on the vehicle. If the axle data on the link is correct the answer of 38 ring teeth and 10 pinion teeth solves that unknown.

Here is the 3.0 ratio. 39 ring teeth and 13 pinion teeth

Questions would be -
Is the gear teeth contact pattern correct for either set? Both on the drive and coast side
Is there excessive back lash?
Are the gears worn out?

As you say you can try the other set. But it is already out of the car. Run a pattern and check the teeth engagement. If it is good at least you know you have a good assembly to swap.

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Thanks
 
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