When making big horsepower there comes a point when the 8.8-inch rearend which has been a mainstay in the Mustang for nearly 30 years, just won’t cut it anymore. In our case we long ago upgraded the rear differential on Project Wild E Coyote to a complete nine-inch unit from Moser. What we didn’t realize when that upgrade occurred is that we also needed to update our upper control arm to one designed specially for nine-inch rearends. To do that we turned to the guys at BMR Suspension.

Learning Curve

BMR actually built one of the first eight-second quarter-mile capable Mustangs on the S197 chassis. The car served as a test mule for the company’s suspension components and along the way BMR found a problem.
When making the switch to the nine-inch rear BMR discovered a clearance issue between the and the top of the nine-inch differential and the upper control arm (UCA). “When we put the nine-inch in our 2005 we noticed the upper control arm was hitting the differential,” says BMR’s Kelly Aiken.

The unique curves to the BMR design are what allow it to work flawlessly with a nine-inch rear diff in a S197 chassis.

The nine-inch center section is considerably larger than the 8.8 and the shape of the center chunk is different as well. The top of that center chunk actually makes contact with the UCA under the right conditions. Aiken says that in theory you could raise the height of the rear of the car to compensate, but if you bottom out the suspension damage to that control arm will still occur.

With a straight design control arm and a nine inch axle the suspension can not react and do its job properly due to interference from the rear housing. The housing also interferes with the UCA and the forces exerted on the UCA actually bend it. This leads to a change in pinion angle, and a suspension that is damaged and can’t function properly.

“The only solution that we found, which would still allow us to run the ride height we wanted to was to angle that control arm up and then down,” says Aiken.

The adjuster in the center of the arm is unique to BMR and is not only tough but also allows for proper adjustment of the pinion angle.

Street and Strip Proven Design

BMR designed their upper control arm for S197 Mustangs with a nine-inch rear, part number UTCA027, to some extremely high standards. First the arm had to be incredibly strong. Coming into play here are some of the best materials in the business including 1 5/8-inch .120 wall DOM tubing for the control arm itself. The adjuster in the center of the tubing is unique to BMR. “This is not an off the shelf piece that we buy from a third-party, it’s unique to BMR and it’s the largest in the suspension industry.”

Tough is great but in a marketplace that increasingly sees high output cars, like our Project Wild E Coyote, on the street, they needed more. “We have some very demanding customers, so we designed the bushing to have very good NVH qualities but still be stiff when it reacts on the track.”

While many enthusiasts have gone to the more rigid and less forgiving spherical bushings for high output cars Aiken says they have some very fast customers still running this poly-bushing. “I probably have 20 customers with cars that run eight-second quarter-mile ETs using that arm and bushing, it’s just a beast.” This is possible due to the high mass of the control arm’s bushing. “What we do is run a 95 durometer material which is about as rigid as you can get for a polyurethane upper control arm bushing application.” Aiken says the large mass reduces NVH while retaining strength of the bushing allowing for very little deflection.

Left: The large 95-durometer polyurethane bushing allows for excellent NVH absorption for occupant comfort but has been proven in eight-second cars at the track. BMR also includes a heavy duty upper mount. Right: Here you can see that a 8.8 upper control arm doesn't allow for proper articulation and will bind up at ride height when used on a 9-inch.

The adjustable link of the upper control arm not only allows for proper pinion angle adjustment for hardcore racers, but for high output street car drivers as well. The weakest link in the S197 driveline has always been the two-piece driveshaft. Aiken says when switching to a one-piece design the pinion angle will be incorrect if using the stock upper control arm. The adjustability of the BMR upper arm will allow for properly setting pinion angle to get the driveline back where it needs to go.

Installing this control arm on Wild E Coyote was no different than putting in any other upper control arm. With it in place, and a our pinion angle set we can now drive the car on the street or the track without worry of suspension issues caused by our upper control arm.