Open diff to trackloc

Al rumble · Sunday at 6:37 PM

My car is a 73 Grande. Mild built 351c FMX transmission.. 28 spine 2.73 gears. I’m getting a vibration when coasting down hill. I figure it is the rear end, as it is the only thing I have yet to rebuild or replace on my car. I figure if I’m doing the rear end I may as well upgrade from open diff to trackloc. I’m in Canada and have found a supplier with replacement center sections for around $1750. These are 28 spline with 3.00 gears.
I’m asking the experts out there if this is a good plan? 3.00 with FMX? Stay 28 spline?
Anything else I should consider?
Thanks.

Sheriff41 · Sunday at 7:00 PM

Not sure about the vibrations issue. What makes you think the diff is causing the vibration? Do you hear anything in the rear axle making noise, like a gear whine, or bearing noise? Make sure you don't have any play or worn parts in your driveline.

Your choice of diff sounds like a good plan for a fun cruiser, though you may want to consider a 3.25:1 gear, also. The 3.00 and 3.25 were factory available gears with the FMX transmission. 28-spline axles are fine.

I think you're paying premium price for a traction loc. In US dollars, you could get that setup for about $900-1000 ($1300-1400CDN). You may find the 3.25s are less expensive than the 3.00s since the 3.00s are a less common choice from builders.

Here's an example: https://www.ebay.ca/itm/391952274722?_skw=ford+9+inch+rear+end+3rd+member&itmmeta=01JM8K4V7H0K40PTZFZF60116C&hash=item5b422d1522:g:8KkAAMXQj9xRlBpy&itmprp=enc:AQAKAAAA4FkggFvd1GGDu0w3yXCmi1ekSamwccao9VvAgLaRx3TZsnyX9xkQUzMfm3LHfTNy3TYv0Ct5zWhRSD1njfFWmrAQ+nCCXeNEJkbhponog5qDgxgGEjjGkOV8/7laDWGGxDeZzT9wndLNur0iiibuqfAWq73/9XOStf0g6sXHSy2qzniiy/lMuFiRquJI08Z79fl2kEm4a/XQuySACTJYTddQ7t5KN9JAOMPOjLEucEqdp9vmyPP/pYnOX5+fITugSZ0SaRm6OBDhN0NNLlFcTE0MJaJ7INKwUCHeJkHZBs6D|tkp:Bk9SR_Czk5OiZQ

I've used Quick Performance with good results.

You'll also want to change your axle bearings and seals while you have the axle disassembled.

giantpune · Sunday at 7:06 PM

Are you launching the car hard a lot? Are you running wide or sticky tires? 28 spline hold up fine to moderate abuse. If you are really beating the snot out of them, then maybe think about upgrading.

If you have a local shop, they can swap the carrier in your center section. You don't need to buy an entire center section.

71coop · Sunday at 7:07 PM

Agree with Sherriff41. I think you'll be fine with a 3.00:1 ratio. It should still pretty good for highway driving, and if you have any low end torque will be a bit better from the stoplight. 28 spline setup is fine for street driving, especially if you aren't hammering the **** out it.

Al rumble · Monday at 12:13 AM

Thanks for the advice.
I had a mechanic friend dive it and he figured the vibration was the coast side of the gear.
Of course I like to hit it hard off the line……especially if the lady’s are watching.

mrgmhale · Monday at 8:01 AM

Al rumble said:
My car is a 73 Grande. Mild built 351c FMX transmission.. 28 spine 2.73 gears. I’m getting a vibration when coasting down hill. I figure it is the rear end, as it is the only thing I have yet to rebuild or replace on my car. I figure if I’m doing the rear end I may as well upgrade from open diff to trackloc. I’m in Canada and have found a supplier with replacement center sections for around $1750. These are 28 spline with 3.00 gears.
I’m asking the experts out there if this is a good plan? 3.00 with FMX? Stay 28 spline?
Anything else I should consider?
Thanks.

The prior owner of our 1973 Mach 1 replaced the original 2.73:1 rear axle gear with a 9" housing with 3.5:1 TractionLok gears. He told us the rear axle ratio change made a very notieable difference in performance, especially off=the-line and at lower speeds. We also have a 1973 Mustang Convertible with a 2.73:1 rear axle rattio gear set. It performs adequately well for a around-own driver and highay cruising. But, the steep 2.73:1 rear axle ratio definitely holds back the performance the car would have with a lower rear axle gear ratio.

With the vert;s 2.79:1 ratio we run at just about 1,850 ROM at 60 MPH in 3rd gear (C4 Auto Tranny). Our 73 Mach 1, however, would run at about 2,800 RPM at 60 MPH with the TractionLok 3.5:1 rear axle ratio and the C4 being in 3rd gear. Although the 2,800 or so RPM did not cause a lot of anxiety, I felt an AOD would be a nice upgrade. Now that the AOD is in the car it runs the engine about 1,900 RPM in 4th, OverDrive gear. Were we to not have gotten the AOD it would have been fine with a Mustang sporting a fairly low rear axle ratio. Personally I would not go with a lower ratio than 3.5:1, as the engine RPM when driving on a highway qs it is.

Bill Kanode · Monday at 9:29 AM

Al rumble said:
My car is a 73 Grande. Mild built 351c FMX transmission.. 28 spine 2.73 gears. I’m getting a vibration when coasting down hill. I figure it is the rear end, as it is the only thing I have yet to rebuild or replace on my car. I figure if I’m doing the rear end I may as well upgrade from open diff to trackloc. I’m in Canada and have found a supplier with replacement center sections for around $1750. These are 28 spline with 3.00 gears.
I’m asking the experts out there if this is a good plan? 3.00 with FMX? Stay 28 spline?
Anything else I should consider?
Thanks.

For a street car the idea of a posi unit sounds good. In my past experience for a little racing I prefer an open differential so I can preload the body at the spring until I get the same amount of tire burn on each side. The open differential would have 1 long tire burn Mark and after the body preload the car made 2 short burn marks to show that I was getting the power to the pavement.

Hemikiller · Monday at 9:50 AM

A vibration that changes with load on the drivetrain is usually a bad u-joint. Before you spend $1700, check the $25 part.

Al rumble · Monday at 10:22 AM

It’s been 10 years since I did the u joints so I will check them first.
Thanks again

INPete · Monday at 10:53 AM

I also have a '73 vert, mild built 351C with an FMX with 2,000 stall converter. I purchased my pumpkin from 8and9inchfordrears.com Been really happy with it so far. I did upgrade to a nodular case, track-lok, 3.25 gears, and went ahead and put in new rear axle bearings and replaced the axles with Moser 28 spline. Driveshaft was also rebalanced with new U-joints.

Bill Perkins · Monday at 8:59 PM

Hemikiller said:
A vibration that changes with load on the drivetrain is usually a bad u-joint. Before you spend $1700, check the $25 part.

If I may add this comment, my 72 Mustang 302 with C4 had a slight vibration for many years during highway speeds. I changed out the U-Joints and it was still there. A few years back I took my driveshaft out and took it to Atlanta Driveline in Doraville, Georgia and they found the vibration was in my driveshaft. The Tech said the center of the driveshaft had some loose packing that was installed by the vendor who built the driveshaft. The loose packing was causing the vibration as the driveshaft wasn't balanced. Sounds crazy but that is what I was told. My driveshaft was fixed and re-balanced with new U-Joints and should be good.

Open diff to trackloc

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