Moser M9 9-inch Rearend Upgrade with Baer Brakes Into Our ’11 Stang

We you start making a lot of power, a stout drivetrain is needed to help support that power. We’ve set some lofty goals for our 2011 Mustng GT Project Wild E Coyote, and we need to fortify the drivetrain further. With plans that call for over 1,000 horsepower in a few months, we need to make sure we can reliably transmit as much of it to the tires as possible.

At this level we know that the factory 8.8-inch rearend is only a ticking time bomb, waiting to let us down. We needed something bullet proof, able to get us around town without a lot of fuss, but hardcore enough to handle brutal drag strip launches on slicks. That’s why we turned to Moser Engineering for a complete M9 fabricated rear differential housing, Wavetrac differential, and Moser axle package. “The 9-inch can handle larger torque and horsepower loads when compared to the 8.8-inch rearend,” says Jeff Anderson of Moser Engineering. We’ll also be changing our rear brake package with help from Baer Brakes.

We’re upgrading the tail end of Wild E Coyote with a Moser M9 housing, nodular center section, 4.30 gears, Wavetrac differential, 35 spline axles, and Baer SS4 brakes.

Left: Among the well thought out features of the M9 housing are the drain plug and fill cap making servicing easier. Right: the nodular third member is cast by Moser and machined in house.

The M9 Housing

The nine inch can handle larger torque and horsepower load when compared to the 8.8-inch rearend. -Jeff Anderson, Moser Engineering

We’ve used Moser’s M9 housing on Mustang project cars before and you may remember seeing us install one on our Project 666 Mustang Coupe. Moser has recently introduced the M9 for the S197 chassis, part number M9HP3. This housing is a direct bolt in, and an ideal upgrade for those late model Mustangs taking advantage of the Coyote’s capability of cranking out insane power levels.

The M9 housing is truly a speed parts engineering marvel; constructed from 0.125 inch thick mild steel. Rather than assembling the housing from several pieces of steel, it is built as one piece of triangulated steel and then combined with a 0.375 inch thick faceplate. The housing’s internal gussets give it additional strength, and the entire design reduces the number of small welds needed, which in turn reduces warpage of the steel during manufacturing and ensures symmetry in the final product.

“With the right geometric design you can make a housing that will last longer, weigh less and have a higher stress load capacity while using only the material needed to control weight,” says Anderson. All housings are checked for quality with a CMM machine to ensure that they are uniform and up to specifications before being shipped. The M9 also features drain and fill plugs making service simple.

Left: Our M9 housing features lower control arm tabs with multiple mounting points, allowing us to adjust for maximum traction and lowering springs. Center Left: The housing also features multiple mounting styles for our rear sway bar, further adding to the adjustability of our rear suspension. Center Right: We specified 8.8-housing ends, which will allow us to retain the factory ABS system. Right: The custom alloy axles we ordered, already have the ABS exciter ring pressed on. We also had the axles drilled for both 1/2 inch and 5/8 inch wheel studs should power levels require a wheel stud upgrade. These 35 spline units should handle anything Wild E. Coyote can throw at them.

The axle tubes are built from three inch diameter 0.25 inch wall DOM tubular steel. Our housing also included a pair of Moser 35-spline axles. We also specified 8.8-inch axle housing ends. This was necessary for us to retain the use of our rear ABS sensors. The Moser M9 uses 8.8-inch style ends, though modified where the axle bearings are still pressed onto the axle shaft and then the brake backing plates hold the axle in place, eliminating the OEM style C-clip axles.

Back Brace

We also added the the back brace option for our M9 housing. The reason for this is that we plan on putting slicks so that we will be able to launch hard at the drag strip. In doing this, we introduce the potential for flex into the rearend assembly by transmitting a shock load through it at launch. According to Anderson, “The back brace controls flex without making the housing too stiff to cause cracks.” Having the back brace not only helps ensure consistent performance at the track, but also long life for the housing. “With the design of the M9 housing, and the back brace you can easily eclipse the power handling capabilities of the ring and pinion gears, so the housing is no longer your weak link,” Anderson adds.

The M9 housing is a highly engineered design, which is lightweight, and strong. The M9 back brace was designed specifically for this housing. It reduces flexing, but is not so rigid that it causes cracking.

Center Section

We stuffed our new M9 housing with one of Moser’s nine inch, nodular center sections. Made in the USA, it is cast using Moser owned tooling and casting boxes. “We control the process using premium nodular steel and then heat treat and stress relieve before we do any machining,” says Anderson.

Top Left: We used blue thread lock on the studs for the center section before threading them into the housing. Top Right: Black RTV is used for a gasket, making sure we go underneath all the studs. Bottom Left: It takes two people to slide the housing and center section together, one will hold the housing while the other puts the center section in place. Bottom Right: Once the RTV has had some time to set up, we tighten all the fasteners securing the center section.

Our housing included a set of Moser Pro Street 4.30 ring and pinion gears. We’re running this lower ratio to compensate for the higher gearing in our recently installed Tremec T-56 Magnum XL transmission. We chose to have our center section powder coated silver to add a nice contrast between it and the black finish of the M9 housing.

Left to Right: To remove the 8.8 we disconnected the shocks, followed by the driveshaft, lower and upper control arms, ABS sensors, and rear brake hoses. The last thing we disconnected was the panhard bar.

With the use of a transmission jack, and a few free hands the 8.8 is lowered from Wild E Coyote.


With horsepower at this level we needed a suitable differential that could handle the abuse of brutal drag strip launches, while still allowing us to navigate the cones on the auto-cross, or cruise comfortable to work and around our neighborhood. With those requirements in mind we opted to for a Wavetrac differential.

The Wavetrac design is different in that it is capable of directing torque to the wheel with the most grip when slip occurs. This is done using the patented wave profiles inside the differential. When one wheel spins, the wave profiles begin climbing each other, this creates the necessary load in the differential to direct power to the wheel with the most traction. This design also means that during zero axle-load conditions, such as between shifts, the differential stays engaged. What that translates to on the track is that there is never a delay for the differential to lock again when shifting gears, thus there is no delay in driveline reaction.

The Wavetrac differential is constructed from forged steel, with 9310 steel gears. There are no clutches, springs, or friction materials to wear out. This differential is not only tough enough for the drag strip, but it’s docile enough for our every day use, making it ideal for our application in Wild E Coyote.

New Brakes

We’ve been running Baer 14-inch rotors with their 6S six piston calipers on the rear of Wild E. Coyote that we installed last summer. Our current plan to swap to a 15-inch wheel at the drag strip and run a 28-inch slick definitely won’t work with our massive 14 inch rear rotors. So we turned again to Baer for a braking solution.

Baer’s SS4 calipers feature four pistons. They are CNC machined and assembled at Baer’s Phoenix, Arizona facility. The calipers offer the same features of Baer’s larger brake systems in a smaller package to fit 15-inch wheels.


Everything we do with our bigger brakes, we shrink it down to fit in a smaller wheel. -Rick Elam, Baer Brakes

We’re replacing our rear brakes with a brand new Baer SS4 rear system, part number, 4162676S. This system utilizes 12-inch rotors and four piston calipers. It will still stop Wild E Coyote in plenty of time, especially since we’ll still be using our 14-inch 6S, six piston setup in the front.

The SS4 brake system is designed with drag racers in mind, retaining the ability to run a 15-inch wheel. “Everything we do with our bigger brakes, we shrink it down to fit in a smaller wheel,” says Rick Elam, of Baer Brakes. Like all Baer brakes the SS4 are made in the USA at Baer’s Phoenix, Arizona facility. They feature staggered caliper piston sizes to maximize pad wear, while the stainless pistons transmit less heat back to the brake fluid.

Left: The rotors feature directional vanes, as well as being slotted and cross drilled to maximize cooling. Right: The brake backing plate not only holds the parking brake assembly and caliper mounts, but also the rear ABS sensor, and acts as the axle bearing retainer.

Wavetrac Differential

We’re using a Wavetrac rear differential for our upgrade. This new concept in rear differentials promises to deliver power to whichever wheel has the most traction. This includes when one wheel is in a zero load situation. The Wavetrac does this by using special wave patterns machined into each side gear and hub. Under zero load conditions the two side gears rotate causing them to move apart, away from their hubs, climbing the wave pattern. This creates an internal load that stops the zero load condition, allowing the Wavetrac differential to apply power to the wheel that still has traction.

Baer also built the brake backing plates to work with our M9 housing, while at the same time allowing us to retain our ABS and parking brake functionality. The backing plates will actually act as our bearing retainers as well, just as on a standard nine inch housing. Our new SS4 brake kit included everything we needed to install it including calipers, pads, rotors, hoses, parking brake assembly, and the brake backing plates.

Installation and Assembly

We received all of our parts and began by assembling our M9 housing to get it ready to go in the car. This involved setting the nodular center section in the M9 housing, and sealing it. While a gasket is included with the housing, we opted instead to use RTV sealant instead. This will provide a leak free seal, avoiding potential problems that can come from paper gaskets.

With our housing and center section together we removed our old 8.8 and installed the new M9 into place. We ran into two fitment issues with this installation both of which were easily corrected. The first issue was our panhard bar.  Since the back of the M9 is physically larger than the 8.8, our Lakewood bar would not run across the housing and back brace to meet the bracket on the driver’s side. We put a quick call into Granatelli Motorsports for a panhard bar relocation bracket, which solved our problem. This relocation kit is normally designed for 8.8 rearend owners looking install girdle-style rear cover. The same function held true for the 9-inch, which was a direct bolt on piece.

The second issue we knew about going in, but could not address until the housing was in place. Our driveshaft would need to be longer by about three inches. We enlisted the help of a local driveshaft shop to handle swapping out the axle tube for a longer one. Long story short, the local driveshaft shop to us managed to screw up the entire driveshaft in the process.

Our Dynotech shaft is made from 6061 T-6 aluminum tubing, .125 inches thick. Dynotech’s Steve Raymond, tells us, “Our general rule of thumb for 3.0” and 3.5” diameter shafts respectively would be, up to 400 hp and 900 hp.” Our Dynotech driveshaft is not only stronger than the stock two piece system, but also lighter as well. It saved us another 20 pounds over the stock unit it replaced, which is 33% less weight than our original. While rotating mass on this small of a diameter is not as critical to overall performance in our application, every bit of overall vehicle weight saved is always a positive.

In an effort to save time, we had our driveshaft retubed three inches longer by a local driveshaft company, who actually ended up screwing the whole thing up. With our head in our hands, we went back to Dynotech and begged for forgiveness.

Left: In goes our Moser M9 assembly. Center Left: As the new housing comes up we carefully install our springs. Center Right: We then connected the shocks. Right: We installed the control arms and panhard bar next. We set the lower control arm on the middle locating hole.

With everything reconnected, we filled the housing, bled the brakes, and took Wild E Coyote out for a test drive. We can definitely tell a difference in the gearing. We also noticed that the Wavetrac differential provides quick grip no matter how hard we push it, doing it’s best to keep both wheels biting in a straight line and around the corner. Best of all, no dash lights as our ABS still functions perfectly.

Lakewood sent over a prototype of their new Evolution Series upper control arm, which is a more aggressive, rod end-style upper control arm with a new heavy duty mount.

Top Left: We had the driveshaft lengthened for our M9. Top Right: Rear Baer SS4 brakes will provide plenty of stopping power while allowing us to fit a 15-inch wheel so we can run slicks at the track. Bottom Left: You can see the finished housing sitting in the car looks better than the original and has an aggressive appearance. Bottom Right: Our final task was to set the car at ride height so we could adjust the pinion angle to negative two degrees.

We’ll be heading to the track again soon to get some hard launches and new times on sticky tires. We plan to put our new M9 and nodular center section through the paces. While our current combination would not have killed the old 8.8 on street tires, it may have on slicks, and what we have in the works certainly would have. Stay tuned because Wild E Coyote is about to get a whole lot wilder.

About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
Read My Articles

Hot Rods and Muscle Cars in your inbox.

Build your own custom newsletter with the content you love from Street Muscle, directly to your inbox, absolutely FREE!

Free WordPress Themes

We will safeguard your e-mail and only send content you request.

Street Muscle - The Ultimate Muscle Car Magazine


We'll send you the most interesting Street Muscle articles, news, car features, and videos every week.

Street Muscle - The Ultimate Muscle Car Magazine


We will safeguard your e-mail and only send content you request.

Street Muscle - The Ultimate Muscle Car Magazine


Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

Classic Ford Performance
Drag Racing
Classic Chevy Magazine

Street Muscle - The Ultimate Muscle Car Magazine

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • fordmuscle Classic Ford Performance
  • dragzine Drag Racing
  • chevyhardcore Classic Chevy Magazine

Street Muscle - The Ultimate Muscle Car Magazine


Thank you for your subscription.

Thank you for your subscription.

Street Muscle - The Ultimate Muscle Car Magazine

Thank you for your subscription.

Thank you for your subscription.