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Tech: 2015 Mustang Bolt-In 9-inch IRS

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Tech: 2015 Mustang Bolt-In 9-inch IRS

To the Nines

Watson Racing developed a new 9-inch IRS for straight-line S550s

By Steve Turner
Photos courtesy of Watson Racing

From the moment it was announced that the 2015 Mustang would feature an independent rear suspension, many of us started to fret about how the new pony car would handle drag strip launches. As it turns out, the cars have performed quite well with moderately enhanced power levels, but as we head into the first full racing season since the S550s hit the streets, some cars might need a more durable option, and that’s what Watson Racing has created.

Do you have a high-powered S550 destined for the drag strip? Do you want the undeniable durability of a 9-inch rearend in a bolt-in package? Would you like to skip cutting up your car? If you answered all of the above, you will be quite interested in Watson Racing’s new bolt-in 9-inch IRS.
Do you have a high-powered S550 destined for the drag strip? Do you want the undeniable durability of a 9-inch rearend? Would you like to skip cutting up your car? If you answered all of the above, you will be quite interested in Watson Racing’s new bolt-in 9-inch IRS.

“The factory IRS is very nice but was built for handling, with a few compromises made for NVH. Straight-line racers will know that drag racing with an IRS system can be risky; component failures can lead to serious trouble at the big end of the track,” Patrick Lee, who handles dealer development, sales, and support at Watson Racing, explained. “Because of the new chassis design, there will likely never be a bolt-in solid axle option for this car. At ride height, the axle is just too close to the frame rail. Installing a solid rearend is possible, but requires a lot of serious fabrication and cutting-up of the car.”

In order to add the strength of a 9-inch, Watson Racing developed a bolt-in solution for high-powered S550s that retains an independent rear suspension, but greatly enhances durability.

Weighing in at only 45 pounds, the Watson Racing rear subframe is considerably lighter than the factory subframe, yet its chrome-moly construction is plenty durable to retain the 9-inch IRS setup.
Weighing in at only 45 pounds, the Watson Racing rear subframe is considerably lighter than the factory subframe, yet its chrome-moly construction is plenty durable to retain the 9-inch IRS setup.

“The 9-inch is the drag racing gold standard when it comes to strength and gear ratio selection,” John Phillips, Business Development Manager at Watson Racing, said. “Mating it with heavy-duty IRS components and a lightweight tubular cradle is the perfect mix for a true bolt-in-upgrade rearend system.”

While builders of all-out race cars might be fine with the cutting and welding necessary to transplant a solid rear axle, owners of street and street/strip Mustangs might not want to make that sort of commitment.

So far just a prototype, the Watson Racing rear subframe is designed to mount a fabricated 9-inch housing under and S550 without any cutting. With the vaunted 9-inch onboard, the rearend will not only be far more durable, but you’ll have access to the myriad gear ratios available for this storied housing. It’s worth noting that you can drop out the center section without removing the subframe.
So far just a prototype, the Watson Racing rear subframe is designed to mount a fabricated 9-inch housing under and S550 without any cutting. With the vaunted 9-inch onboard, the rearend will not only be far more durable, but you’ll have access to the myriad gear ratios available for this storied housing. It’s worth noting that you can drop out the center section without removing the subframe.

“It comes down to three things: strength, ease of installation, and weight savings. The day has come where 1,000-horsepower street cars are common, these guys don’t want to cut up the rear of their cars, our bolt in setup is the perfect solution,” John added. “Also, the S550 can certainly stand to shed some weight and if it can be done while gaining strength, it’s a no-brainer.”

If you have one of these high-powered street cars or are planning to build one, the only thing you must be wondering is when this system will be available and how much it will cost. Neither has been determined yet, but Watson Racing will be testing the prototype system this weekend at the NMRA season opener in Bradenton, Florida. If the testing goes well and there’s enough interest in the system, you could see it available sooner than later.

“While the system we will debut in Bradenton is a prototype, don’t assume it will be next year sometime before it is available,” John said. “Once we pull the trigger on production, there’s no reason the system couldn’t be available mid-season.”

Common on drag cars with shorted solid axles and wide rear tires, inboard brakes are employed on the Watson Racing setup. They chose Wilwood four-piston calipers with drilled and slotted rotors to handle the braking.
Common on drag cars with shortened solid axles and wide rear tires, inboard brakes are employed on the Watson Racing setup. This 9-inch IRS utilizes Wilwood four-piston calipers with drilled and slotted rotors to handle the braking.
Transferring the power from the 9-inch diff to the rear wheels are a pair of D.S.S. Racing halfshafts rated for 2,000 horsepower. There is no question this arrangement is more durable than the factory IRS rearend. All of the joints feature a double-shear construction, and all the control arms are fitted with Aurora Bearing rod ends.
Transferring the power from the 9-inch diff to the rear wheels are a pair of D.S.S. Racing halfshafts rated for 2,000 horsepower. There is no question this arrangement is more durable than the factory IRS rearend. As you can see here, the control arms are fitted with Aurora Bearing rod ends.
The ball joints in the control arms are Howe precision units, which are fully serviceable via an integrated grease fitting.
The ball joints in the control arms are Howe precision units, which are fully serviceable via an integrated grease fitting.
To allow for the use of double-adjustable Strange coilovers, Watson utilizes this bolt-on upper mount.
To allow for the use of double-adjustable Strange coilovers, Watson created this bolt-on upper mount.
Supporting the halfshaft are these billet spindles. They are as beautiful as they are strong.
Supporting the halfshaft are these billet spindles. They are as beautiful as they are strong.
Both the upper and lower control arms are completely adjustable, and the Watson Racing setup allows you to dial in the suspension geometry, including bumpsteer.
Both the upper and lower control arms are completely adjustable, and the Watson Racing setup allows you to dial in the suspension geometry, including bumpsteer.
Likewise the double-adjustable Strange coilovers allow for the adjustment of damping and ride height. Obviously, you can also easily swap out the springs to select the ideal rate for the track conditions. You can also see that the Watson setup allows for the uses of a rear sway bar.
Likewise, the double-adjustable Strange coilovers allow for the adjustment of damping and ride height. Obviously, you can also easily swap out the springs to select the ideal rate for the track conditions. You can also see that the Watson setup allows for the use of a rear sway bar.
The complete Watson Racing setup not only drastically improves the durability of the S550 rearend, but it shaves a whopping 170 pounds off the car. What’s even more compelling is the nature of this bolt-in systems means you can return the car to stock if you ever desire to do so.
The complete Watson Racing setup not only drastically improves the durability of the S550 rearend, but it shaves a whopping 170 pounds off the car. What’s even more compelling is the nature of this bolt-in systems means you can return the car to stock if you ever desire to do so.
With the Watson setup installed, you can run a 15x10-inch wheel with 28x10.5-inch wide rear tire, which will help apply the kind of four-digit power that Watson Racing’s 2015 Mustang GT is capable of producing.
With this setup installed, you can run a 15×10-inch wheel with 28×10.5-inch wide rear tire, which will help apply the kind of four-digit power that Watson Racing’s 2015 Mustang GT is capable of producing.

Watson Racing Bolt-In 9-inch IRS Gallery

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