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Diff Ratio Question and LSD Ford 9inch


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Hi Team

 

My 73 vert was originally a 302 coded car with a C4 and Axle 3 2.79:1 ratio

 

The car when I bought it and imported it to NZ just over 2 years ago had a 351W C4 and what I believe to be a 28 spline 9inch diff. The previous US owner purchased the car 6 years earlier with this engine in it. 

 

When replacing the shocks, I also noticed the spring hangers had extra plates with an extra tounge and hole fitted which I believe are for a rear sway bar assembly.

 

I am about to remove the Diff carrier assembly as my mechanic is going to fit a 2nd hand rebuilt 28 spline LSD that I bought as she is a single spinner. 

 

My question is I was wondering if I should change the crown wheel and increase the ratio.

 

I had noticed that the speedo is not accurate.  Using a phone speed app 100km (62mph) is showing as 75mph on the speedo. 

 

I wonder if the 9 inch is original or swapped out as some feedback is a 302 car would not normally be fitted with a 9 inch. I cant recall the tyre size but she is also on 16inch rims and 215 tyres (cant remember ratio). I doubt the speedo head on the cable was changed if the diff was changed when the 351 went in. 

 

Would this error in speed be tyre circumference only or have I actualy got a different ratio diff than on the tag?

 

I will be taking the car down the 1/4 mile again hopefully in Sept again (if group events allowed) at our Fathers day drags hence why I am putting in the LSD. 

 

I had traction issues with standard tyres and best ET was a 15.2 at 94mph

 

I am happy for it to remain a cruiser for mustang club runs and local car shows so dont need an accelerating monster as the engine HP is a modest 400 only so she isnt going to break any 1/4 mile records but a 14 and 100mph over the line would be nice!

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Ballaratcastle,

your speedometer error signals that there is more than tire size at issue here.

You had mentioned that you had recently replaced the rear shock absorbers, do you recall if your rear shocks were (staggered)

Staggered rear shocks will have 1 shock forward and one aft of the rear axle.

The reason that I ask is if you are going to use a LSD locking axle you are going to want staggered rear shocks to prevent wheel hop.

The rear tires you currently have are still not going to help too much with traction if going to a locking differential, most will fit a 255/60R15 and still have traction issues.

The speedometer error is easily dealt with by replacing the speedometer gear in the transmission, the accuracy will depend on BOTH axle ratio and tire size.

As far as the axle ratio, the 351 Q equipped mustangs of the day that had the best suspension could be equipped with a 3.50 rear differential.

I am not a convertible expert by any means, but something strikes me as odd about your sway bar attachment plates, why are they there ?

weather or not you have staggered rear shocks will tell a lot, pretty sure that the rear sway bar only came with competition suspension.

Perhaps a previous owner swapped the complete rear differential and not just the center section ?

Boilermaster

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Sorry I can't offer too much on your rear axles issue, but if the speedo is that far out, I would think the transmission speedo cable gear is incorrect. There are various gears available, you just have to figure out which one you need, BUT your rear axle could also be NOT the ratio you think it is.

Years ago, I owned a Chrysler 5th Ave. It blew out the spider gears, so a mechanic buddy swopped in a junkyard rear axle. As soon  as it was driven, the speedo would show like 70kph when really it was doing 40. Later I found out it was from a 6 cylinder car, not a V8.

LSD, do you mean a Traction-Loc? If so, you might want to go to a 3.50:1. Mine just got a Traction-Loc installed over winter, but I kept the original 3.25:1. It could definitely use a bit more grunt off the line, but still have good highway mileage.

 

Edit: Boilermaster beat me too it , but yes tires could also be in issue. His comments on shocks are also valid. You might want to upgrade your leaf springs to 4.5 or 5 leaf. If you do, BUY Eaton Detroit Springs springs, not Grab-A-Trak junk. (Ask me how I know!!)

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Geoff.

 I learn something new every day!

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I have attached a couple pictures of a staggered shock set up and you can also see the way bar. If you car did not come with competition suspension you cannot stagger the shocks without cutting up the rear floor and getting a donor mount. 

You can see the rear of the housing on the 9" rear has a bump out to clear the gear. The 8" does not. If you go with a traction loc you need to add the friction modifier to the rear end grease. DO NOT use synthetic rear end grease your pinion bearings will go out.

On the gear ratio. My Mach 1 had AC so you got a 3.25 gear. After I broke the rear 3 times under warranty Ford sent a factory rep in to see what was up. Ford had put the wrong rear in the car at the factory. It had small u-joint and the yoke was breaking. This was with a stock engine and F-70 tires not fat ones. The factory rep told them to put a 3.50 gear in the car when they put back. 

If you were just going to drag race you would go even farther with the gear but you are driving on the street most of the time so the 3.50 would be a good choice.

You could probably add the sway bar if you have the plates that have the holes for it.

One of the old hot rodder's cheap tricks to reduce wheel hop was to add a half leaf to the springs. We would take a junk main leaf spring and cut the eye off the front then we would cut the spring off just behind where the spring perch is. Take U bolts off the centering bolt out of the rear remove the clamps around the spring pack and add the half spring on the top. We made new clamps to put back on the front half to keep the springs tight together. This gives the front half of the spring some more strength so they do not wrap up.

DSC-0937.jpg

 

DSC-0938.jpg

 

DSC-0939.jpg

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When a man is in the woods and talks and no women are there is he still wrong??:P

David

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If the car was originally a 302, then it *shouldn't* have the upper mount for staggered shocks, those were only used on the 4V cars. It's entirely possible, as Boilermaster noted, the rear axle came from a 351-4V car and the sway bar plates were kept in place.

 

With a simple math calculation regarding the speedo error, it's possible your car actually already has a 3.25 or 3.50 gear in place already, unless you have some ridiculously low profile tires. This would make sense regarding the sway bar plates, since the vast majority of 4V cars had 3.25 or 3.50 gears.

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All good points here fella's,

I totally whiffed on the small driveshaft and rear yoke, that WOULD be a deal breaker for me.

On the staggered shocks, there is info on the net to do a bolt on upper mount and different lower plate to convert to staggered shocks.

I did this mod, very easy, just some square tube and flat stock, used one of the discarded lower plates for the top shock thru hole,

Lets see what Ballaratcastle has for shocks, driveshaft and diff flange first.

Boilermaster

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FYI, crappy picture of the extra 1/2 leaf on a Grab-A-Track 4 1/2 leaf spring, red arrows as David alluded to.

 

IMG-1278-LI.jpg

Yep that was the poor mans way of making it a little better but not as good as bars for sure.

When a man is in the woods and talks and no women are there is he still wrong??:P

David

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FYI, crappy picture of the extra 1/2 leaf on a Grab-A-Track 4 1/2 leaf spring, red arrows as David alluded to.

 

 

Yep that was the poor mans way of making it a little better but not as good as bars for sure.

Works for me as I dislike the look of traction bars on a street car. There are better suspension systems without that "look", expensive though I'm guessing.

Geoff.

 I learn something new every day!

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Ballaratcastle,

your speedometer error signals that there is more than tire size at issue here.

You had mentioned that you had recently replaced the rear shock absorbers, do you recall if your rear shocks were (staggered)

Staggered rear shocks will have 1 shock forward and one aft of the rear axle.

The reason that I ask is if you are going to use a LSD locking axle you are going to want staggered rear shocks to prevent wheel hop.

The rear tires you currently have are still not going to help too much with traction if going to a locking differential, most will fit a 255/60R15 and still have traction issues.

The speedometer error is easily dealt with by replacing the speedometer gear in the transmission, the accuracy will depend on BOTH axle ratio and tire size.

As far as the axle ratio, the 351 Q equipped mustangs of the day that had the best suspension could be equipped with a 3.50 rear differential.

I am not a convertible expert by any means, but something strikes me as odd about your sway bar attachment plates, why are they there ?

weather or not you have staggered rear shocks will tell a lot, pretty sure that the rear sway bar only came with competition suspension.

Perhaps a previous owner swapped the complete rear differential and not just the center section ?

Boilermaster

 

Thanks. My car is in storage at a friends factory during lockdown so I havent been able to access it until tomorrow. I will have a look and check. As I assembled the car over a year ago I cant remember but think both the shocks went the same way into the floor behind the seat where I did the nuts up from memory. Because the car had the extra plates and a 9 inch made me wonder if the diff had been swapped. I googled what they are there for and saw photos showing a rear sway bar but my chassis doesnt have the holes where the sway bar rubber bush mounts would be so one has never been fitted. Friends have some old rims with semi slicks to borrow for the 1/4 mile so wont be running on the old tyres.

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Sorry I can't offer too much on your rear axles issue, but if the speedo is that far out, I would think the transmission speedo cable gear is incorrect. There are various gears available, you just have to figure out which one you need, BUT your rear axle could also be NOT the ratio you think it is.

Years ago, I owned a Chrysler 5th Ave. It blew out the spider gears, so a mechanic buddy swopped in a junkyard rear axle. As soon  as it was driven, the speedo would show like 70kph when really it was doing 40. Later I found out it was from a 6 cylinder car, not a V8.

LSD, do you mean a Traction-Loc? If so, you might want to go to a 3.50:1. Mine just got a Traction-Loc installed over winter, but I kept the original 3.25:1. It could definitely use a bit more grunt off the line, but still have good highway mileage.

 

Edit: Boilermaster beat me too it , but yes tires could also be in issue. His comments on shocks are also valid. You might want to upgrade your leaf springs to 4.5 or 5 leaf. If you do, BUY Eaton Detroit Springs springs, not Grab-A-Trak junk. (Ask me how I know!!)

 

Thanks Stanglover. Yes I did add a leaf to the old springs as the cost of importing springs from the US paying for the weight was beyond my budget! When I got the car assembled and took it to my first car gathering, with passengers in the back the diff hit the drivers side exhaust pipe over gentle bumps. The extra leaf and bend in the springs lifted her an inch as she was a bum down stance before. I have attached a photo from that car show where she is looking like a drag car with the nose up!

I am dropping the carrier out tomorrow when I can access the car. I will disconnect the drive shaft while the car is on the hoist and I presume if I rotate the wheel one complete circle I can count the revolutions on the front of the diff to estimate the ratio?

resize-8.jpg

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I have attached a couple pictures of a staggered shock set up and you can also see the way bar. If you car did not come with competition suspension you cannot stagger the shocks without cutting up the rear floor and getting a donor mount. 

You can see the rear of the housing on the 9" rear has a bump out to clear the gear. The 8" does not. If you go with a traction loc you need to add the friction modifier to the rear end grease. DO NOT use synthetic rear end grease your pinion bearings will go out.

On the gear ratio. My Mach 1 had AC so you got a 3.25 gear. After I broke the rear 3 times under warranty Ford sent a factory rep in to see what was up. Ford had put the wrong rear in the car at the factory. It had small u-joint and the yoke was breaking. This was with a stock engine and F-70 tires not fat ones. The factory rep told them to put a 3.50 gear in the car when they put back. 

If you were just going to drag race you would go even farther with the gear but you are driving on the street most of the time so the 3.50 would be a good choice.

You could probably add the sway bar if you have the plates that have the holes for it.

One of the old hot rodder's cheap tricks to reduce wheel hop was to add a half leaf to the springs. We would take a junk main leaf spring and cut the eye off the front then we would cut the spring off just behind where the spring perch is. Take U bolts off the centering bolt out of the rear remove the clamps around the spring pack and add the half spring on the top. We made new clamps to put back on the front half to keep the springs tight together. This gives the front half of the spring some more strength so they do not wrap up.

DSC-0937.jpg

 

DSC-0938.jpg

 

DSC-0939.jpg

Thanks for the photos. 

I will chuck the car up on the hoist tomorrow and take some photos from the same angle as yours. I only have a couple of shots at the moment to show the diff. There are welding lines showing where the diff was added to the axle tubes. Are these normal. Also one of the rebuilt LSD I am putting in. Diff-1.jpg

 

diff-3.jpg

 

LSD.jpg

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The weld lines are normal. When you get your carrier out count the teeth on the ring gear and the pinion gear, divide the ring gear number by the pinion gear number to get the ratio.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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The weld lines are normal. When you get your  carrier out count the teeth on the ring gear and the pinion gear, divide the ring gear number by the pinion gear number to get the ratio.

 

Ok thanks Don will do. I will get back to everyone tomorrow after I remove it and let you know.

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Sorry I can't offer too much on your rear axles issue, but if the speedo is that far out, I would think the transmission speedo cable gear is incorrect. There are various gears available, you just have to figure out which one you need, BUT your rear axle could also be NOT the ratio you think it is.

Years ago, I owned a Chrysler 5th Ave. It blew out the spider gears, so a mechanic buddy swopped in a junkyard rear axle. As soon  as it was driven, the speedo would show like 70kph when really it was doing 40. Later I found out it was from a 6 cylinder car, not a V8.

LSD, do you mean a Traction-Loc? If so, you might want to go to a 3.50:1. Mine just got a Traction-Loc installed over winter, but I kept the original 3.25:1. It could definitely use a bit more grunt off the line, but still have good highway mileage.

 

Edit: Boilermaster beat me too it , but yes tires could also be in issue. His comments on shocks are also valid. You might want to upgrade your leaf springs to 4.5 or 5 leaf. If you do, BUY Eaton Detroit Springs springs, not Grab-A-Trak junk. (Ask me how I know!!)

 

Thanks Stanglover. Yes I did add a leaf to the old springs as the cost of importing springs from the US paying for the weight was beyond my budget! When I got the car assembled and took it to my first car gathering, with passengers in the back the diff hit the drivers side exhaust pipe over gentle bumps. The extra leaf and bend in the springs lifted her an inch as she was a bum down stance before. I have attached a photo from that car show where she is looking like a drag car with the nose up!

I am dropping the carrier out tomorrow when I can access the car. I will disconnect the drive shaft while the car is on the hoist and I presume if I rotate the wheel one complete circle I can count the revolutions on the front of the diff to estimate the ratio?

resize-8.jpg

 looks nice in "All Black" very appropriate for a NZ car don't you think!

NPD (National Parts Depot) sell Eaton 1" longer shackles to raise the back end to a more desirable stance. They are like 20 buck each, you need two of course. I'm getting a pair for my car as perhaps you had read a past post of mine talking about the GTA's being too arched and I needed to use 1" lowering blocks. Well I took the springs to a local spring shop to get de-arched 1", but the guy gave the job to a worker who took them down 1 3/4". They didn't want to re-arch them if case they cracked, so until I can buy Eaton springs, I'm stuck for now, but the longer shackles will bring it back to where I want it.

Geoff.

 I learn something new every day!

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Sorry I can't offer too much on your rear axles issue, but if the speedo is that far out, I would think the transmission speedo cable gear is incorrect. There are various gears available, you just have to figure out which one you need, BUT your rear axle could also be NOT the ratio you think it is.

Years ago, I owned a Chrysler 5th Ave. It blew out the spider gears, so a mechanic buddy swopped in a junkyard rear axle. As soon  as it was driven, the speedo would show like 70kph when really it was doing 40. Later I found out it was from a 6 cylinder car, not a V8.

LSD, do you mean a Traction-Loc? If so, you might want to go to a 3.50:1. Mine just got a Traction-Loc installed over winter, but I kept the original 3.25:1. It could definitely use a bit more grunt off the line, but still have good highway mileage.

 

Edit: Boilermaster beat me too it , but yes tires could also be in issue. His comments on shocks are also valid. You might want to upgrade your leaf springs to 4.5 or 5 leaf. If you do, BUY Eaton Detroit Springs springs, not Grab-A-Trak junk. (Ask me how I know!!)

Ha exactly. 

I bought the car already painted black. The previous owner in the US bought it around 2013 and it was blue with blue interior. He got it panel and painted and then it sat as a body shell for 5 yrs before he sold it. He did have 11 cars in various states of rebuild. I am undecided whether to change the seat colour to dark red. A lot of people who have seen it like the white and with the roof down the seats dont get hot. 27484554-1733974323288313-1963462197-o.jpg

 

28171443-1754039434615135-334165956-o.jpg

 

28191003-1313399428761947-1437930246-n.jpg

Thanks Stanglover. Yes I did add a leaf to the old springs as the cost of importing springs from the US paying for the weight was beyond my budget! When I got the car assembled and took it to my first car gathering, with passengers in the back the diff hit the drivers side exhaust pipe over gentle bumps. The extra leaf and bend in the springs lifted her an inch as she was a bum down stance before. I have attached a photo from that car show where she is looking like a drag car with the nose up!

I am dropping the carrier out tomorrow when I can access the car. I will disconnect the drive shaft while the car is on the hoist and I presume if I rotate the wheel one complete circle I can count the revolutions on the front of the diff to estimate the ratio?

resize-8.jpg

 looks nice in "All Black" very appropriate for a NZ car don't you think!

NPD (National Parts Depot) sell Eaton 1" longer shackles to raise the back end to a more desirable stance. They are like 20 buck each, you need two of course. I'm getting a pair for my car as perhaps you had read a past post of mine talking about the GTA's being too arched and I needed to use 1" lowering blocks. Well I took the springs to a local spring shop to get de-arched 1", but the guy gave the job to a worker who took them down 1 3/4". They didn't want to re-arch them if case they cracked, so until I can buy Eaton springs, I'm stuck for now, but the longer shackles will bring it back to where I want it.

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Ballaratcastle,

Here is a link to add staggered shocks to your stang, so easy I did it myself.

 

https://www.fordmuscleforums.com/threads/staggered-rear-shocks-for-any-mustang.489854/

Just need some square tube, plate steel and an extra lower shock plate to locate on the square tube and all but eliminates wheel hop.

Boilermaster

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Ballaratcastle,

Here is a link to add staggered shocks to your stang, so easy I did it myself.

 

https://www.fordmuscleforums.com/threads/staggered-rear-shocks-for-any-mustang.489854/

Just need some square tube, plate steel and an extra lower shock plate to locate on the square tube and all but eliminates wheel hop.

Boilermaster

 

Thanks. I definately dont have the staggered shocks as I removed the carrier the day before yesterday and dropped it to my mechanics workshop to pull apart. He will check my gear ratio. 

So did all 71-73 with a trac loc 9 inch have the staggered shocks? I see they were added to an earlier model which looks a bit different. I dont have those skills to do that unfortunately so it would cost me more $$$ than I have right now to mod it... was way over budget getting it back on the road! Lol

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Thanks. I definately dont have the staggered shocks as I removed the carrier the day before yesterday and dropped it to my mechanics workshop to pull apart. He will check my gear ratio. 

So did all 71-73 with a trac loc 9 inch have the staggered shocks? I see they were added to an earlier model which looks a bit different. I dont have those skills to do that unfortunately so it would cost me more $$$ than I have right now to mod it... was way over budget getting it back on the road! Lol

 

Staggered shocks were standard issue on all of the 4 barrel cars - 351 & 429. The TracLoc was not included with unless it was specifically part of a package or engine option (ie: Boss 351 or Drag Pack for 429CJ)

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So I removed my carrier/diff head and the oil was quite black and smelled burnt. I had sucked out the old oil and put new oil in just over a year ago.

My mechanic didnt have the chance to disassemble the diff head until yesterday. Turns out the ratio is 3.25 so this proves the diff is not original in the car as the code said 2.79 ratio. I am happy to stick with the 3.25 as she is 99.9% a street car and glad she has a little bit better acceleration for that once a year on the 1/4 mile.

I am replacing all the bearings, collapsable spacer, seal and o-ring.

Question are they always a square profile shape o-ring? Will a round profile one do if I cant source a square profile?

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