Rear Axles

The ’94-2004 Mustang came with three main rear axle variants. All the V-6 cars had a 7.5-inch ring-gear solid axle, one width of housing from ’94-’98, and then a wider track from ’99-2004. The ’94-’98 Mustang GT and Cobra had the 8.8-inch ring gear solid axle, while the ’99-2004 Mustang GT and Mach 1 had a wider track version of the same solid axle. Of course, the ’99-2004 Cobras had an independent rear suspension that was still based on the 8.8-inch ring-gear size. The many upgrade options for the rear axle assembly based on the type of expected use are discussed here. The 7.5-inch rear has fewer options and is not as strong an axle assembly as the 8.8, so if your V-6 car is going to be used for serious racing activities, or if you just want a wider range of options available to you, swap out the 7.5 for an 8.8. They’re inexpensive enough and widely available both new and used to make this a logical alternative.

Choosing the Right Rear Axel For Your SN95 Mustang

Axle Ratios

Choosing the Right Rear Axle For Your SN95 Mustang - DIY FordThis Tech Tip is From the Full Book, HIGH PERF MUSTANG BUILDER’S GUIDE: 1994-2004. For a comprehensive guide on this entire subject you can visit this link:


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I get asked several times each week, “What gear ratio should I put in my Mustang?” There is no single answer for this question, but I have my preferences. Many factors that should play into your gear choice: the type of use the car receives, whether or not it’s a daily driver, whether or not fuel mileage is a concern, whether long-distance drives are part of the car’s requirements, etc.

Most Mustangs came from the factory with 3.08:1 or 3.27:1 from ’94-’98 and 3.27:1 or 3.55:1 from ’99-2004. These ratios are mandated in most cases by CAFE regulations and NVH (noise, vibration, and harshness) concerns – not for optimum performance. Believe me, if the Ford engineers could give us all 3.73:1 and meet their other objectives, they would.

An everyday Mustang used for a variety of activities works well with 3.73:1 gears and still gets decent fuel economy. The improvement in acceleration is noticeable, and yet the engine RPM on the highway is still quite reasonable. This is true for V-6, 5.0L, or 2-valve 4.6L engines. The ’96-2001 Cobra and the ’03-’04 Mach 1 can benefit even more from a 4.10:1 ratio because of the extra 1,000 rpm available in the 4-valve engine’s operating range. Also, since the 4-valve engine does not produce as much torque in the 2,000 to 3,000-rpm range, the higher ratio gets the engine into its power band more quickly. I wouldn’t hesitate to run more gear on a daily driver car that does not need to run down the highway at 70 mph all day. A set of 4.30:1 or even a 4.56:1 is not out of the question on a modified Mustang that makes power in the 4,000 to 7,000 rpm range.

As far as the dragstrip goes, a 4.10:1 ratio combined with a 28-inch tall tire is a popular combination, allowing a trap speed up to about 130 mph at 7,000 rpm. Combinations with higher RPM or mph potential may require a different ratio. The objective is to reach peak operating RPM in high gear just as you pass through the timing lights, so the tire size, power output, and RPM limit all play a role in choosing the best gearing. In road racing, it’s a similar deal. You need to gear the car for the maximum speed required on the fastest section of the track. Assuming that your Mustang has a close-ratio fifth gear, a 3.73:1, 4.10:1, or 4.30:1 ratio should be in the ballpark depending on the track.

 

Choosing the Right Rear Axel For Your SN95 Mustang

Gear Installation Tips

Here is an illustrated installation guide detailing the process of installing a new ring-and-pinion and differential in an 8.8-inch axle assembly. Here are some tips to ensure you have a trouble free life with your new gears and differential.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

Once you’ve selected your gears, you have some other components to consider before you get to the installation. A word on purchasing ring and pinions: Buy them from Ford Racing. Period. The Ford ring and pinions are made on the same equipment as their production gears, so they’re dimensionally the same as the stock gears. This means less trouble for you installing them because most of the time the original pinion bearing shim can be reused and is correct for backlash and gear pattern. Whenever someone brings us another brand to install, it inevitably takes longer and is more difficult to install. Just buy the Ford gears – they save you time and money in the long run.

Differentials

Most performance SN95 Mustangs come with Ford’s Traction-Lok limited-slip differential. It’s adequate for moderate performance use, but it has a low breakaway torque, limiting the amount of power that can be applied to both wheels at the same time. Ford actually supplies an additive for their limited-slip differential that allows smooth slippage between the friction discs and the steel plates, since most customers are annoyed by differential chatter while cornering in the city. A dyed-in-the-wool enthusiast trades some degree of noise for the ability to put power to the ground through both rear tires.

We can increase the friction in the Traction-Lok by modifying the clutch pack. I first learned to do this back in 1990, when we had to run the original equipment differential in my Firehawk road-race Mustang. Breakaway torque is the amount of torque that can be applied to one wheel while the other is held stationary, before the wheel breaks free. This can actually be measured with a torque wrench and some suitable adapters. A new stock Traction-Lok is in the 80- to 100-ft-lb range. The factory service manual describes a breakaway torque of 20 ft-lbs as an acceptable service limit for an 8.8-inch Traction-Lok differential. You might as well not even have a limited-slip differential at that level! The standard Trac-Lok’s clutch pack arrangement has alternating friction plates and steel plates. Each side of the differential has three clutch plates and four steel plates. We can add one more friction plate on each side between the two steel plates that come back to back from the factory. This increases the friction surface area, and also increases the preload on the “S” spring between the two halves of the differential. We need to be careful we can still get the axle C-clips in, but as long as we can accomplish, this the axle breakaway torque is increased to over 300 ft-lbs. Now the differential has the ability to transfer a much higher level of power before spinning the lightly loaded inside tire.

 

Choosing the Right Rear Axel For Your SN95 Mustang

Eaton makes another plate-type limited-slip differential. The Eaton unit has carbon fiber friction plates, which can operate at much higher temperatures without the differential losing effectiveness. Ford also started using carbon fiber friction material in the Traction-Lok differentials beginning with the 2003 Cobra.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

Auburn makes a cone-type limited-slip differential. The Auburn unit uses two cones faced with friction material preloaded against each other with a group of five springs. The two versions of the Auburn are the standard and the Pro series. The Pro series has a higher breakaway torque than the standard, making it a better choice for drag racing and serious open-track racing. The Auburn differential was also used in the 1LE Camaro in the early 1990s, and GM provided a service kit at the time that included competition-level springs and clutch cones to service the differentials. When new, the breakaway torque would be 250 ft-lbs, and they would get rebuilt when the breakaway torque dropped below 150 ft-lbs. The winning teams rebuilt the differential every weekend, just as we did on the Mustang.

Recently, Auburn released an electronic differential (ECTED) that incorporates both a friction cone limited-slip, but when you flip the switch, an electromagnet locks the differential solid, allowing zero slip between the axles. This is the best of both worlds for a dual-purpose street/racecar. I recently tested an ECTED in a Mustang buildup and I was impressed. It’s a really good solution for a street/strip car.

Moving on, other differentials on the market that do not rely on friction materials are available. These are less sensitive to changes due to wear. The Torsen differential uses helical gearing to perform differentiation and torque distribution. The Torsen works as an open differential until one wheel starts to lose traction, and then the difference in torque causes the gears in the differential to bind together. The differential has a torque bias created through the design of the gears, which allows more torque to be applied to the wheel with better traction. The torque bias on the T2 Torsen is 2:1. The T2-R Torsen has the addition of steel clutch plates on the sides with spring preload, for a torque bias of 3.5:1. During acceleration through a turn, weight shifts to the outside wheel, and the inside wheel no longer has enough traction to support the torque load. The Torsen differential transfers torque to the outside wheel before the inside wheel has a chance to slip. Assuming the inside wheel begins to slip at 500 ft-lbs torque input, an open differential with a 1:1 torque bias only applies the same 500 ft-lbs of torque to the outside wheel before the inside wheel begins spinning. The Torsen T2R with a 3.5:1 torque bias would allow 1750 ft-lbs of torque to be applied to the outer wheel before the inside wheel begins to spin. By comparison, a clutch-type limited-slip normally has about a 2:1 torque bias from the factory. The Torsen is a good choice for an open-track or street car where you want a high level of traction with virtually noiseless operation. It’s probably not the best choice for a drag car with sticky tires, as high shock loads may inflict wear or damage over time to this type of differential.

The Detroit Locker is an icon for American muscle cars, so of course, it’s available for the 8.8-inch Mustang differential. The Detroit Locker is noisy on the street because it functions like the ratchet in your toolbox. Power is applied equally to both wheels when accelerating, but it ratchets as an open differential when you’re coasting or decelerating. The locker is a favorite of endurance racers because of its durability, and it’s also quite suitable for drag racing.

Both the Torsen and the Detroit Locker offer less influence on a road-racecar’s cornering than a friction clutch type limited-slip differential. When you are turning in on corner entry, the breakaway torque inherent in a friction limited-slip is going to resist differentiating, creating a push or understeer condition during the corner entry phase. Once the power is applied, the car has a slight power oversteer during corner exit. This is not all bad, mind you, because the corner-entry understeer allows the driver to feel the edge of traction with the front tires. The corner-exit oversteer is controllable with throttle modulation and steering inputs, but this does cost some cornering speed. The Detroit locker is neutral during the corner-entry phase, allowing full differentiation of the rear tires, but the transition to the throttle during corner exit can be ticklish. The transition to fully locked rear wheels under power is sudden and requires smooth driver inputs to avoid sudden changes in the car’s attitude.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

Every sort of differential has tradeoffs, and each may require chassis adjustments to exploit the full potential. Driving style also suits some differentials more than others, so some experimentation may be in order. Real gains can be made by effectively transmitting the power to the tires, so long as you don’t exceed the available level of grip.

Choosing the Right Rear Axle For Your SN95 Mustang - DIY FordThis Tech Tip is From the Full Book, HIGH PERF MUSTANG BUILDER’S GUIDE: 1994-2004. For a comprehensive guide on this entire subject you can visit this link:


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It gets much easier with pure drag cars. A spool is a mechanical connection joining the two rear axle shafts and eliminating the differential action. This also removes some additional weight from the rotating assembly, freeing up some horsepower to the wheels. A spool is ideal for drag racing, but not too useful on street cars. It is possible to use a spool on the street, but it’s not recommended, as the car resists turning corners. Spools are never used in road racecars; well almost never – it’s true that one of the most famous road-racecars ever built, the Porsche 917, used a spool. In the 1970s, it was the only solution deemed strong enough for the 1,000-hp sports racer! I haven’t heard of anyone using one in a road race Mustang just yet.

When you install a spool on a Mustang 8.8-inch axle, C-clip eliminators must be installed at the same time. Normally, the axle is retained in the axle housing with a C-clip on the end of the axle inside the differential itself. With a spool replacing the differential, there is no method of retaining the axle shaft using a C-clip, so a C-clip eliminator is used. An outer axle bearing is pressed on the axle shaft and retained with a lock ring that is shrunk fit on the axle shaft. The bearing assembly is restrained inside an aluminum enclosure, bolted to the axle flange. This positive retention method also prevents a C-clip or an axle from failing and the car losing an axle shaft at speed on the dragstrip. The NHRA requires C-clip eliminators for the 8.8-inch axle on any car quicker than 10.99 seconds. C-clip eliminators are prone to leaking oil from the axle seal if used on the street or around corners to any extent. They are not recommended for open-track/road-racecars for this reason.

Even with the power outputs of late-model Mustangs regularly exceeding 600 to 700 hp on the street, and over 1,400 hp at the drag strip, we really don’t see many differential failures these days. We do, however, see component failure of another highly stressed component. See below.

Axle Shafts

SN95 solid-axle Mustangs came with 28-spline axles shafts from the factory. The ’99 Cobra IRS also had 28 splines, while the ’01-’04 Cobra came equipped with 31-spline axles.

The stock 28-spline axles are fragile if used with sticky tires. Even a stock Cobra with drag radials can break a 28-spline axle. Increasing the spline count to 31 increases the strength of the axle by some 35 percent. Adding to this, after-market alloy axles are built of high-strength alloy steel, which makes them even stronger. Most often, an axle fails at the root of the splines, which is the area of highest stress. It’s a really good idea to upgrade the axles at the same time as the differential and/or the ring and pinion are installed. 31-spline aftermarket axles are sufficient for most applications, but if you’re running a high-horsepower drag car, 35- and even 40-spline alloy axles are available from specialty axle manufacturers such as Mark Williams and Strange Engineering.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

The ’03-’04 Cobras, even with their factory 31-spline axles, begin having problems as soon as their owners start adding more power. The splines are strong enough, but the axles themselves are prone to breaking the CV joint when drag raced with slicks. The shock loads are sufficient to crack the cages that contain the CV balls, and then it all goes bad in a hurry. Fortunately, aftermarket axles are available from the Drive Shaft Shop. These 300M axles withstand the shock loads of standing starts and drag slicks much better than the stock axles, and they also offer an outboard CV joint upgrade for the folks who have 900+ hp cars. Most serious drag racers switch out the IRS to a solid axle because it’s just so much easier to upgrade the axle components and to tune the suspension.

In case you think road racers are immune to halfshaft problems, we recently had to replace a 28-spline axle shaft that broke on a ’99 Cobra open-track car. The car left the track at a great rate of speed, and then rejoined the action on the track. The rear tires were spinning at great speed in the grass and then grabbed the asphalt suddenly upon reaching the track. The axle failed immediately, fracturing inside the differential at the root of the splines. When the IRS Cobras began road racing in 1999, there was a problem with the shaft popping out of the differential. The shaft is retained by a spring clip, and there was insufficient tension to retain the axle shaft under extreme cornering loads. Also, depending on the differential used, the end of the shaft inside the carrier sometimes needs a small amount of material removed in order for the snap ring to fully expand in the groove. The same can be true for the solid axle. Sometimes we find the end of the alloy axle must be dressed to allow the lock pin to slide through the carrier.

Fluid Coolers

We didn’t have much problem with the solid-axle cars overheating the differential, but as soon as the IRS arrived in 1999, we sure saw a change. The aluminum case of the IRS expands so much as the temperature rises that the preload on the side bearings goes away, and the next thing you know, you have differential failure. The solution is a good oil cooler for the differential. This is also a dandy idea for high-speed Mustangs in open-road races like the Silver State, even with a solid axle. The high speeds encountered in the Silver State, combined with aero management techniques designed to minimize the amount of air traveling under the car, reduce the cooling effect on the axle assembly.

In addition to ducting some air directly toward the differential housing, an external oil cooler, located usually in the trunk, allows the oil to carry some heat away from the differential. I prefer to use a Tilton pump specifically designed for gear oil. The Tilton pump passes some trash in the oil without causing the pump to fail, which is important in a long-distance event. A large-capacity cooler core, from Earls or Setrab, is located in the trunk area, and supplied with cool air ducted in from outside the car. Provision must be made to exit the air from the trunk area as well, usually through some vents in the rear panel. A temperature gauge should be installed in the differential housing, allowing the driver the opportunity of monitoring the temperature of the differential oil. The cooler pump should be turned on once the differential oil reaches 150 degrees F. If the driver forgets to turn the differential oil pump on, work over his/her fingers with a small hammer until they remember to do it. Before we started cooling the oil in the IRS housing, the cars would return from a race with the front differential mounting bushings melted from the heat, allowing the housing to clunk back and forth as the power was applied and released by the driver. Simply adding a cooler eliminated the problems.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

Speaking of differential temperatures, the oil used in the differential should be tailored to the application. Different types of differentials require different oils. Auburn recommends 80W90 non-synthetic GL5 gear oil for their limited-slip differential. Auburn and Ford recommend that Ford friction modifier be added to eliminate any chatter during differential operation. You already know my opinion on this, so use it if you must in a street car, but never on a track car. I also recommend a non-synthetic 80W90 gear oil on the Ford Traction-Lok differential. The synthetic oils are just too slippery for the clutch plates or cones, and reduce the breakaway torque. Torsen differentials can use a synthetic or a non-synthetic GL-4 or GL-5 gear oil, according to the manufacturer. I think the Redline heavy shockproof oil would be a good selection, since it contains an extreme pressure additive to cushion gear teeth from shock loads, something I would be concerned with on a Torsen. The Detroit Locker can also be used with either synthetic or regular gear oil, GL-4 or GL-5.

If you have a new ring and pinion in the differential, you should use only non-synthetic gear oil initially. The gears need some time to break-in, during which some material is going to be worn from the gear teeth, until all the high spots are worn smooth. A slippery synthetic oil inhibits this process, which is not what we want at all. In fact, a newly assembled differential is going to create a lot of heat during the break-in process. Not only are the gears new, and a bit tight against each other, but so are the bearings. The manufacturer’s own specifications for the bearing preloads are higher on new bearings than used ones. Virtually no one does this, but the proper break-in procedure for a new ring and pinion is to take the car out, drive it for 10-15 minutes until up to operating temperature, then accelerate and decelerate the car under moderate load several times, return to the shop, and let the differential cool down to ambient temperature before racing it. Once the car has 500 miles on it, or a couple of weekends at the track, drain the oil and refill with new oil of the desired specification. This is cheap insurance, just like an oil change on a fresh engine.

Driveshafts

The standard steel driveshaft works forever and requires low maintenance. But being enthusiasts, we can’t just leave well enough alone, can we? A very popular upgrade is the Ford racing aluminum driveshaft. A couple of years ago they were so inexpensive ($180) that I couldn’t figure out how they could produce them for what they were fetching. Today they have gone up about 40 percent in cost, but they’re still a tremendous bargain. The shaft is 14 lbs, half the weight of the stock unit. Reducing the rotating mass frees up a bit of power to the ground. Additionally, the driveline has reduced vibration, especially at really high speeds. The 1999 Cobra in particular had some driveline vibration issues, and an aluminum driveshaft reduces the vibration to an acceptable level.

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

 

Choosing the Right Rear Axel For Your SN95 Mustang

The Ford Racing driveshaft is stout enough to be used on eight-second drag cars, but you should add a driveshaft safety loop if you are involved in any sort of racing activities. The loop contains the driveshaft in case of a U-joint failure, preventing a nasty accident. Driveshaft loops are required by the NHRA on all cars. Composite driveshafts constructed of carbon fiber bonded to aluminum yokes have become popular in the last few years. They are even lighter than aluminum, and stiffer, resisting twisting to a higher degree. They also offer a potential benefit in case of an accident. During a violent crash, composite drive-shafts are less likely to bend and pierce the bodywork; they’re more likely to break into pieces. For this reason, they are preferred for some races like Silver State, where high-speed crashes can occur. The price on carbon fiber drive-shafts has dropped in the past few years as the production methods have improved, and now they are available in the $600 to $700 range. At the end of the day, there is still nothing wrong with a good steel driveshaft, and for many custom applications, we get our local driveshaft shop to make a steel shaft to our requirements, balanced and ready to go.

Written by Sean Hyland and republished with permission of CarTech Inc

 

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