Mustang 9-inch Rear End Swap Guide
We all know that big power means big money. Making cars go fast isn’t just an issue of adding ‘go power,’ but making sure the car can withstand the beating. The axle always bears the brunt of the abuse, so let’s compare different aftermarket 9-inch bolt-in rear-ends available on today’s market.
Whether it is piecing together junkyard turbos or burning a 3-day weekend to do your first cam install, the end result is hopefully more power. If you are doing it right, everything else is going to suffer the consequences, and it doesn’t do you any good to build a killer motor if all it does is twist axles and spit pieces of ring gear out. The time-tested Ford 9-inch rear has proven itself in everything from quick street cars to blown alcohol dragsters, and there are a bunch of different companies making true bolt-in 9’s that don’t require a bunch of work to install. We talked to some of the leaders in the rear end game, so they could give us a complete overview of their 9-inch upgrade options for the Mustang.
Whether it came with a 7.5 or an 8.8, your Mustang has a dangerous flaw – C-clip axles, which are retained at the center, are notorious for letting a wheel part company with the rest of the car if one breaks. If you’re lucky, the disk brake assembly may keep you from spontaneous tricycle ownership, but if you’re not (or you never upgraded from rear drums) that wheel may just make it to the finish line before you do. Stronger C-clip eliminator axles, housing braces, rear end covers with cap supports, and a bunch of other fixes are out there for the 8.8 (and even the 7.5 for the true masochist), but the real solution is an upgrade to the 9-inch.
Why a 9-inch?
To learn why a 9-inch solution overshadows an 8.8, we spoke to Brian from Currie Enterprises. “The 9-inch has three pinion bearings. All rearends have two pinion bearings on the pinion shaft, but the 9-inch also has the third pinion bearing on the head of the gear that prevents gear head deflection under heavy load applications, such as launching a drag car,” he explains. “The 9-inch is built with the tapered Timken-style roller axle bearings that have tremendous load capacity, as well as side load capacity for road race cars. It also has positively-retained axles as a standard feature. It has no C-clips and no C-clip eliminators.”
Dollar for dollar, building an 8.8 rearend is not a cost-effective solution when compared to a 9-inch. If one were to buy an 8.8 new, the cost would be greater than a 9-inch while retaining less integrity in regards to strength. While there are some builders who do use 8.8 rear-ends in hobbyist/car show/street applications, they’ll never have the brute strength of a 9-inch or the availability of the world of aftermarket parts that the 9-inch has.
Now, the grassroots builders out there are thinking, “There are several Fords that offer 9-inch rear ends that I can pull from the junkyard.” While it’s true that there are still a few out there left to be found, Strange Engineering points out that, “you must keep in mind that these housings were not made for race cars. They were made for old sedans from the production floor. Not many production cars came with more than 400 horsepower. These days, cars come with far more horsepower and are even further built. The aftermarket housings are designed with that in mind.”
Another factor to consider is ease of installation. A salvaged stock 9-inch housing will have to be adapted to your Mustang’s four-link suspension and shock mounts, which takes some fabrication skill and careful measurement. An aftermarket 9, on the other hand, will arrive at your door ready to bolt in, and can be ordered with just about any combination of features you can imagine. With the scales tipped so far in favor of a new 9-inch, let’s look at what’s available.
- Custom made to order
- 1964 – 1974, 1979 – 2008
- Prices start at $885
- 2 Day Turn-around time
Moser’s Kip Hayden explains, “The 8.8 is a very good rear and will handle what the street/strip type application requires, but for a full time drag car, a 9-inch is your choice. Unlike the 8.8, there are gears manufactured specifically for high horsepower, drag-race-only applications called Pro/Competition gears. The drop-out center of a 9-inch enables the freedom to make gear changes at the track and allows you to have more than one center section that can be swapped out for different types of applications, making the 9-inch more universal.”
Moser offers a Stamped Heavy Duty 9-inch housing as well as a fabricated “M9” housing to replace the stock 8.8 rear-end for Mustangs from 1964 – 1974, and 1979 – 2008. A complete line starts at $1195 for a race-only rear-end and $1255 for a street model. Available options include a $140 installed back brace and an economical Ford Motorsports brake kit for $459, or Wilwood disc brakes with a parking brake for $630. They go as far as to offer custom widths at no additional charge.
“It’s conservatively stronger than stock, it’s got the Mark 2 tubes, it’s a very versatile piece. It comes with a retention system that is far better than an 8.8 with the C-clip axles. If you break a C-clip axle, the axle, your tire, everything comes out – there’s nothing to really hold it in, so you’re not only losing your axle, a wheel, a tire, and a quarter panel, but you’re endangering lives at that point. The 9-inch is a semi-float axle that is retained at the housing end. Therefore, if the axle breaks at the spline, which is the normal point of breakage due to horsepower, your axle stays in the car. You still retain drivability and you can bring it to a safe stop.”
Not unlike a decent burger joint, Moser ‘won’t make it until you order it.’ Because of the extensive list of options for a wheel-to-wheel rearend, the range of pricing is extensive and depends on the options that are needed. The base model housing and axle package starts at $885 and the fabricated version starts at $1000. The average turn-around time of a Moser package is two days and all parts are made in the USA.
“Part of what makes us different from the competition is fact that we have a standard 2-day turn-around. If you place an order in on a Monday, it’ll leave my facility on Wednesday. We also have the largest customer service department in the industry. We have no less than 12 people on the phones every day answering calls, technical questions, and sales.”
- 100% New Content
- 1964 to 2008
- Prices start at $1100
“Dollar for dollar, we don’t recommend the 8.8 at all. If a customer comes in to buy a rearend, an 8.8 is just plain not cost effective when a basic 9-inch is cheaper and stronger. We can do build ups on customer 8.8’s and we do occasionally build an outright 8.8. A Currie-built 8.8, which features 9-inch style housing ends and axles is a much more versatile unit from a performance standpoint. However, the 8.8 will never have the brute strength of a 9-inch or the backing of a world of aftermarket parts like the 9-inch has.”
Currie offers rear-end housing and axle packages that are completely bolt-in for all Mustangs from 1964 to the current 2008 model. While a basic housing and axle package for a Fox-body runs at around $1100, a complete rearend that is ready to run with basic disc brakes and a Posi starts at about $2700. In comparison, rearends for S197 Mustangs start at $1300 and the ready-to-run model starts at $2550. These packages can be upgraded or downgraded and the cost will reflect as much.
One point that Currie stresses is the advantage of not using any reconditioned parts, if the customer so desires. “We offer 100% new content – so the rearend can be a completely new unit. We also offer a lot of the housing and third member components in a reconditioned form – while supplies last – for the budget builder. Also, the ABS and stock rear disc brake retention on the 05-08 rearends is an awesome feature that makes the swap amazingly painless. We also offer high-end adjustable heavy duty control arms for the Fox and the 05-08 models.”
We asked Brian for his thoughts on making an 8.8 rear-end as durable as a 9. He explains, “Yes, we honestly can make it close to as durable, up to a certain horsepower level, but is it worth the cost incurred? We don’t think so – but we build them anyway for the guy that believes that it does make sense.”
- Made in conjunction with Ford for a true bolt-in application
- 1979 to 2008
- Prices start at $980
J.C. from Strange suggests that the 8.8 is capable in street applications. “The 8.8 is a pretty stout unit, and with the right parts in there, it is capable of handling quite a bit,” he explains, “but what I always tell people is that once you start to fail the ring gear in the 8.8, there’s nowhere left to go.” The benefits of the 9-inch, according to J.C, are that it has a stronger ring and pinion, a stronger housing, and a better gear ratio selection. The most important reason to go with the 9-inch, as Strange sees it, is the peace of mind that you get in regards to racing.
The retail cost of the housing is $605 and the optional back brace is $185. The street axle package starts at $379.20 and the race axle packages begin at $439.49. A complete unit without brakes, including the housing, center section, and axles starts out at $2173.85, and the top-of-the-line setup goes up to around $3200. They offer the housing in multiple packages including a 31-spine street application, which comes with a traction-lock Posi or a True-Trac. They also offer a 35-spline setup, which is more of a street/strip application, using a Detroit Locker, and the race version which is available in 35-40-spline configurations with a spool.
“The banjo portion, which is the center of the housing, has internal gusseting to further strengthen the tubes. The tubing that we use is a 3-1/4-inch tube with a 1/4-inch wall which makes the housing much more rigid and helps keep the tubes from being bent; something that the 8.8 is prone to. Not only is the tube welded around the banjo, but a portion of the tube is actually exposed in the center section and is welded to the center banjo as well, making the housing more rigid.”
One thing that sets them apart from the competition is that Strange Engineering designed the 9-inch bolt-in housing in conjunction with Ford, which actually approached Strange to help develop the rearend that they originally sold in the Ford Racing catalog.
Drivetrain Specialists (DTS)
- One-on-one consulting to determine what to build for each customer
- Prices start at $2250
Sandy from DTS notes that the difference between an 8.8 and a 9-inch solely depends on the application that the vehicle is being used for. “It’s not easy to say an 8.8 is better than a 9-inch or vice-versa; there are pros and cons to both, but it still is determined by horsepower and what the customer wants the vehicle to do.” When asked what types of drivers are buying their 9-inch upgrades, we were told, “Drag racing enthusiasts or those who have over 800 horsepower.”
For 1979 to 2004 applications, DTS offers an entry-level 9-inch package starting at $2250 which includes a new housing, a new third member and pinion support, a Trac-Loc or TrueTrac limited slip differential, alloy axles, choice of street gear, yoke and strap kit, bushings in upper ears, and a choice of housing ends. Upgrades include chromoly tubing, a Detroit Locker, 35-spline axles, pro gears, cryo-treating, and several different choices of brake kits.
DTS does not make custom or specific kits. However, the process of buying a 9-inch rear-end is brought to a more personal level, by speaking with the customer to determine what is needed given the expected amount of horsepower. DTS caters to the specific application and helps the racer or hobbyist develop a rear end that suits any particular application.
Both Sandy from DTS and Kip from Moser note that the frictional losses are not drastic going from a 8.8 to 9-inch rearend, and that it is a must to upgrade at a certain horsepower level. The agreed number between the two is that you lose approximately 3% going from the stock 8.8 rearend to a 9.
Chris Alston’s ChassisWorks
- Every part is built in-house
- A complete custom build has a 1-week turn-around time
- Prices start at $650
Chassis Works offers a variety of applications for their bolt-in 9-inch rear-end from 1979 to 2004. Each housing offered by CACW receives billet, ‘late big’ Ford (Torino) housing ends. Doing this allows for use of a stronger, large-diameter axle shaft by reducing excess material normally present for the seal seat. A bare housing in mild steel starts under $650. If you want a chromoly housing with an anti-roll bar, that cost would be about $1700. The axle tubes are three inches in diameter and welded along the internal tube gusset as well as the tapered edge of the center section.
Tim McCain of Chassis Works does his best to explain the 8.8 phenomenon; “There’s always somebody with an 8.8 that lives, no matter how much power you throw at it. However, there’s just far more aftermarket support and stronger components available at a more reasonable price for 9-inch stuff. We don’t have any housings or components for an 8.8-inch rearend. Our bolt-in coilover conversion can be bolted up to your stock housing, but other than that, all we have are upper and lower control arms. When you start to get in to that housing itself, there is so much more stuff available. I haven’t done the research to check dollars for dollars, but I would imagine it would be cheaper to build a 9-inch if you start from scratch. In the grand scheme of things, it’s cheap insurance, I believe. You can build a fully-outfitted rearend that will set you back $4000, but you can put it in and forget about it. You know those parts won’t get hurt. It’s hard to put an arbitrary number on when you need to switch because there are so many variables; the weight of the car, if it is a transbrake car, if it is a footbrake car, if it is a stick shift, if it’s on slicks or street tires, how well it hooks – there are so many variables. You can have a 2000-hp car with an 8.8 and it’ll live because it never hooks up. Or, you can have a 600-hp car that weighs 3400 pounds with a transbrake that will kill it. If you can afford it, I would always recommend a 9-inch rearend. If your car has a ton of power, you are going to find a weak spot, and that’s the first thing to go. You end up buying it twice if you break it.”
According to McCain, “We make as much of our parts in-house as possible. We have millions of dollars worth of automated machinery. We have robotic welding and we have quite sophisticated programming equipment and scanning equipment. With all of our stuff, you can better control quality, the design of things, the supply of products, everything like that. If you make it in-house you’re not dependent on somebody else for your products. Anything that is feasible to make ourselves, we do. Our housings are made from scratch in our buildings. They come in as tubing and flat steel and we make the rest of it here. We can build them in various widths depending on what you’re doing, and besides the Fox-body stuff, we can build them for street cars, full race cars, whatever specs you need in mild steel or chromoly. Usually, the turn-around is very rapid as far as custom ordering. Normally, I’d say a week is about the time a custom housing can be built and shipped. That is because we build it in-house and we have a great system to make sure it is exactly correct and it’s exactly how you want it.”
Every package comes complete with the parts you need to get it done and installed. “Almost everything we ship out, we have all the nuts, bolts, and hardware. We don’t want to send you a part and then say, ‘now you need to go down to your hardware store and pick up these nuts, bolts, and washers.’ We have everything we can possibly put in it with exhaustive instructions for installation,” explains McCain.
A little personal experience…
I am a grassroots racer myself and daily-drive my track car to work day in and out. To keep my car commutable from point A to B, it makes sense to reinforce or buy parts to replace those that are known to go dodgy. For the most part, upgrading to a 9-inch rearend is a necessary precaution. Racers know that in certain conditions, downgrading or detuning to reduce power is the only solution to make a full day of racing possible. With a more heavy-duty rearend, you can have the peace of mind that comes from knowing that the rear end will not be the weak point to your ride. Although there is the 3% additional frictional loss and the minimal added weight that comes with a 9-inch rear end, at a certain point, it is completely necessary to upgrade from the 8.8. The downsides are slight, and in my opinion, are small prices to pay for the durability of a bolt-in 9-inch rearend.