It didn't take long before Mustang enthusiasts found the limit of the SVT Cobra's IRS. A set of sticky tires and aggressive launches resulting in wheelhop has prematurely ended the lives of many independent rearends. This is a huge problem when it's so easy to make big power with a Four-Valve and some boost.
It takes a certain amount of finesse to get power to the ground through the Cobra IRS. Anyone who's driven a modded Cobra can tell you it doesn't take much throttle application to induce wheelhop. With a solid axle, wheelhop is almost a non-issue, but with an IRS, when the rear tires fight for traction, the grip and slip of the tires can wreak havoc as they try to find traction.
Carl McGill is the owner of this Screaming Yellow '04 Mach 1, which has been upgraded with an '03 Cobra IRS. Besides its suspension upgrades, McGill's Mach has a 1.7-liter Kenne Bell Twin-Screw supercharger, full exhaust, JLT cold-air intake, and custom tune from Big Daddy Performance (Lakewood, New Jersey). This daily driver laid down a very respectable 504 rwhp and 446 lb-ft of torque. Unfortunately, getting that power to the ground resulted in significant wheelhop in First and Second gear before it finally hooked up in Third.
"First and Second gears are just about useless unless you baby it," comments McGill. "It's not an easy car to drive aggressively on the street. As soon as the tires start to spin, you have to be out of the gas, or wheelhop starts." To remedy this, we turned to Maximum Motorsports and QA1.
Maximum Motorsports has a full line of components designed to fortify the IRS and greatly reduce wheelhop, leaving you with the ability to plant the tires harder on the street or track without the fear of killing your IRS. Maximum sent us its Super Street/Competition IRS Grip Package, which consists of subframe bushings, upper and lower A-arm bushings, differential bushings, adjustable swaybar end-links, adjustable tie rods, and all of the removal and installation tools to make quick work of swapping the bushings.
We finished off the IRS fortification with a set of double-adjustable QA1 rear shocks. The TD707 shocks have 18 compression adjustments and 18 rebound adjustments for a total of 324 settings per shock. This may sound overwhelming, but QA1 does a great job of providing base setting for different styles of driving, which we'll get into later.
When it comes to the IRS, there is nothing quick or easy about it. With this in mind, we turned to Big Daddy Performance in Lakewood, New Jersey, to handle the install. But before we ripped out the IRS, we headed to Englishtown Raceway Park for a few 60-foot passes to establish a baseline for our testing. Once the Maximum Motorsports Grip Box and QA1 shocks were installed, we headed back to the track.
Follow along as we add some strength to this IRS Mach 1, and be sure to check out www.muscle mustangfastfords.com for exclusive photos and video of our track test.
"Before the Maximum Motorsports and QA1 stuff, the car was balanced and handled well, but wheelhop was a serious issue in First and Second gears," explains McGill. "When I got the car back, it was totally different. The handling characteristics of the rear suspension were night-and-day better than before. Although wheelhop is still present when you're hard on the gas, it's much more controllable than before. When the car gets sideways, it recovers much better than before, and with less effort. The only downfall is the rear suspension is now much better than the front and the two feel disconnected. The difference in handling made it a new car, and it's amazing to see how much bushings and shocks can do."
Our IRS-equipped Mach 1 belongs...
Our IRS-equipped Mach 1 belongs to Carl McGill, and with 504 rwhp, wheelhop has made throttle application a bit tricky.
1 We started our IRS upgrade...
1 We started our IRS upgrade journey at Englishtown Raceway Park. You can see in the tire marks that the rear wheels bounce violently as it claws at the track trying to find traction. After throwing away our best and worst 60-foot time, we averaged the other five times to establish our baseline of 2.062 seconds. This was achieved with a Falken Azenis tires at 32 psi. We plan on improving that!
2 We headed to Big Daddy...
2 We headed to Big Daddy Performance in Lakewood, New Jersey, where Dwayne Gutridge and Mike Rozman went to work fortifying McGill's independent rear suspension.
3a Once the IRS was dropped...
3a Once the IRS was dropped out of the car, the subframe bushings were the first to be replaced. Maximum Motorsports supplied us with its Super Sport/Competition Grip Box for the IRS...
3b ...The rubber bushings...
3b...The rubber bushings get swapped for polyurethane versions that are much harder. Be sure to use plenty of silicone grease on all of the bushings.
4a The next part of the kit...
4a The next part of the kit was the upper and lower A-arm bushings...
4b ...The white Delrin bushings...
4b...The white Delrin bushings are much harder than even a polyurethane bushing, giving you far less deflection.
5 Rozman removed the lower...
5 Rozman removed the lower A-arm and prepared to remove the stock bushings.
6 There are a few modifications...
6 There are a few modifications that need to be made before the A-arms can be reinstalled. The inside tab has a spacer tack welded to it, which must be removed.
7 Next, the tabs must be...
7 Next, the tabs must be checked for straightness. This is to ensure the tabs are parallel to each other. Maximum Motorsports supplies a tool to flatten any tabs that may become concaved.
8a Using the supplied bushing...
8a Using the supplied bushing removal took...
8b ...Rozman pressed the stock...
8b...Rozman pressed the stock bushings out of the A-arms.
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10 Maximum Motorsports includes...
10 Maximum Motorsports includes an abrasive pad with the kit to clean the A-arms prior to installing the new bushings.
11 To keep the A-arms located...
11 To keep the A-arms located properly, Maximum includes a set of shims. With the A-arm reinstalled, pry the bushings apart do determine the proper amount of shims for a tight fit.
12 Next was the upper A-...
12 Next was the upper A-arm.
13a After removing the A-arm,...
13a After removing the A-arm, Rozman used a hacksaw to cut a section out of one side of the bushing.
13b This it to make room for...
13bThis it to make room for the bushing removal tool to sit flat on the A-arm. Once the bushing is removed, install the Delrin bushing and repeat the shimming process.
14 The rest of the kit includes...
14 The rest of the kit includes the differential bushings, adjustable tie rods, adjustable sway bar end links, and a couple of tools to make the job a little easier.
15 The differential bushings...
15 The differential bushings are straight-forward. The two front bushings and single rear bushing are swapped for the polyurethane versions.
16 The differences in the...
16 The differences in the tie rods are easy to see. The Maximum piece is much beefier than the stock unit. Maximum also uses rod ends to attach each end to the IRS. This does away with the tie-rod ends that can wear out over time.
17 After unbolting the stock...
17 After unbolting the stock tie rod, Rozman measures the length and sets the adjustable version to the same length. Editor's note: Be sure to get a professional four-wheel alignment when the installation is finished.
18 Maximum supplies all the...
18 Maximum supplies all the hardware to attach the new tie rods, including a bumpsteer kit to get the tie-rod angle correct.
19a The stock swaybar endlinks...
19a The stock swaybar endlinks are non- adjustable...
19b ...Maximum's endlinks...
19b ...Maximum's endlinks allow you to adjust preload...
19c ...to fine-tune your...
19c ...to fine-tune your suspension for track outings.
20 Rozman finished the install...
20 Rozman finished the install with a set of double-adjustable shocks from QA1.
21 The QA1 TD707 shocks have...
21 The QA1 TD707 shocks have independent adjustability of compression and rebound. With 18 setting for each, you have 324 adjustment options for each shock, giving you the ability to tune the suspension for the street, strip, or road course.
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23 After putting a few miles...
23 After putting a few miles on the car, we headed back to Englishtown Raceway Park to retest. Our baseline was done in near flawless weather, but when we returned, the track was wet, and we were driving through a dense fog that covered the track. The track staff at Englishtown did a great job of drying the surface, but conditions were nowhere near our baseline. With the tires at the same 32 psi, we began our retest.
24 You could spend days changing...
24 You could spend days changing shock settings to find the best combination of rebound and compression, unfortunately, we didn't have that kind of time. After trying a handful of adjustments, our best results came with the rebound set at three clicks from full soft, and the compression at six clicks from full soft. These settings netted us a new average of 1.979, 0.083 seconds better than before.
25 Wheelhop was inducible...
25 Wheelhop was inducible when we got very aggressive with clutch application, but it was extremely controllable and recovered quickly, allowing us to get back in the throttle without wasting precious time. With better weather conditions and more time to make shock adjustments, we think low-1.80-second 60-foot times are achievable on real street tires.