Heidts IRS

General Motors’ F- and Y-body cars have always had an interesting sibling relationship. While the Camaro and Firebird could have a V6 engine, which wasn’t offered in the “higher-performing” Y-body Corvette, the optional, upper-end F-body engines were squeezed between the frame rails of GM’s two-seater.

It’s no secret that Chevrolet played with horsepower numbers so that those who paid a premium for a Corvette, wouldn’t play second fiddle to F-body owners. They also enjoyed the additional handling availed through Corvette’s independent rear suspension (IRS).irs-front-angle-complete

Even if they didn’t have an independent rear suspension, the third-gen Camaro had a lot going for it when it was first introduced for the 1982 model year. It was the first Camaro to feature electronic fuel injection — although not until 1985. GM also gave buyers the choice between a four-speed automatic or a five-speed manual transmission for those who chose to row their own gears.

Technology has a way of filtering down throughout the automotive realm, and many times earlier generations are the benefactors of modern technology upgrades. This helps keep those previous generations in the running for the hearts and minds of enthusiasts who prefer a vintage body style. Late-model engine swaps keep the horsepower numbers steadily on the rise, and in this case, late-model technology and careful engineering by the folks at Heidts has afforded ‘82 through ’92 F-bodies with an IRS that will be the envy of anyone, including Y-body Corvette owners.

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The Heidts kit is comprehensive. Everything you see here comes together to create a super-handling rear suspension package. All units are sent unfinished so the end user can paint or powdercoat to their liking.

Heidts has offered its Pro-G line of independent rear suspensions for years, and recently introduced this system for third-gen F-body owners, so we decided to find out what it takes to bring a better handling solution to the back-side of a third-gen Camaro. It goes without saying that other models within this generation of F-bodies would require similar steps, and we hot-footed over to Hawk’s Motorsports where they already had a Camaro waiting to get the IRS treatment.

The Test Mule

The folks at Hawk’s Motorsports are no strangers to third-gens and have specialized in GM performance since 2000. Their diehard love for F-bodies, and the fact that they were committed to putting the IRS under its ’88 Camaro, cemented the worthiness of the modification. Clearly, the Pro-G IRS for third-gen F-bodies is taken seriously by those in the know.

Our guinea pig vehicle was initially built by Hawk’s Motorsports for both the autocross and dragstrip, as well as normal street operation. It has a 427 cubic-inch LS engine with a Tremec six-speed transmission. The suspension used common upgrades found in the late ’90s and early 2000s.

Upper and lower control arms allow the necessary travel, while still keeping the contact patch of the tires parallel to the road's surface. The toe is adjusted and kept by tie rods that follow the same arc as the control arms. This prevents bumpsteer when experiencing undulations in the road's surface. Heim joints and urethane bushings allow movement, but not unwanted flex.

Those upgrades consisted of bolt-on parts such as subframe connectors and strut tower braces that help to create a much tighter and better handling car. As good as the car handled, the crew at Hawk’s wanted to create a true sportscar that can handle the autocross or race track with newfound vigor. The ride with the higher-performing IRS is actually very smooth and compliant on the street, while giving a much better feel through corners. Even more so, the results with the IRS are bolstered even more, thanks to the already-installed subframe connectors and strut tower brace.

Aligning The Rear Suspension

While IRS suspensions have a reputation for keeping proper geometry when handling differing road surfaces and conditions, they are also known for having a higher level of adjustability. With the increased adjustability of the IRS, proper set up is crucial for proper performance and long tread life.

Here are the recommended alignment specifications for the Heidts IRS suspension:

Camber: Zero to .5 degrees negative

Toe-In: Zero to 1/16-inch


Each Pro-G IRS assembly is built by the folks at Heidts using DOM tubing of various thicknesses, depending on the suspension part. The systems feature urethane bushings to limit unwanted flex and noise, but for those seeking the ultimate in performance, they also offer spherical bearings in the uprights for a “no-deflection” option. Other options include upgraded brake packages featuring Wilwood discs and four-piston calipers. But even those who opt for the standard brake package will benefit from Corvette technology through the same rotors and calipers found on the C4. All kits are shipped unfinished so that the enthusiast can paint or powdercoat the parts to match their application.

Choose Your Kit

The system comes in one of two different ratings, keyed to a vehicle’s horsepower requirements. The standard kit is rated for 400 horsepower, and the high-horsepower kit is designed for cars with up to 800 horsepower.

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The kit is designed around DOM tubing of various thicknesses, depending on the suspension part.

The 800 horsepower-rated kit also comes with upgraded half-shafts to handle the additional twisting power, and the Wilwood brake kit is standard. All complete Heidts kits come with a Currie 9-inch center section in varying ratios of 3.00, 3.25, 3.50, 3.70, 3.90, and 4.11 gears wrapped around a positraction differential.

Heidts’ Pro-G IRS for third-gen cars is the only kit made specifically for these cars. It doesn’t require any modification to the car’s interior or fuel tank placement. It can be installed over the weekend and affords third-gen F-bodies with sportscar handling and stability on the track, without sacrificing ride quality on the street.

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Each Heidts kit receives a Currie 9-inch differential, which is then bolted solidly into the center carrier. On each side of the differential spins either the standard CV shafts or the optional high-horsepower versions. Other choices include factory Corvette brakes or aftermarket units from Wilwood.

Scott Diedrich at Heidts best describes it, “There is a noticeable difference on the street, as the car rides smoother than in stock form, while offering the ability to enter corners at greater speeds and maintain your line throughout the corner,” he said. “On the track, the new setup allows drivers to push the car beyond what GM ever imagined a third-gen F-body doing. The IRS offers a solid, planted-to-the-road feeling without compromising the everyday drivability of the car.”

On the track, the new setup allows drivers to push the car beyond what GM ever imagined a third-gen F-body doing. – Scott Diedrich

The kit brings a lot to the table for a weekend upgrade, but there are a couple of steps between running a jack Friday night and a Sunday afternoon driving the car. Removing the original rear suspension is obviously a nut-and-bolt procedure, but installing the Heidts IRS does require some new mounting arrangements, as you can imagine, with some cutting and trimming also required.

For starters, the frame rails on each side need to be modified to allow for fitting and and welding in of the new saddle assemblies to which the center cradle and suspension components will attach. Since the original Panhard rod is no longer needed, the mounting surface under the body of the vehicle will need to be cut away to allow room for the new suspension pieces.

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The standard kit uses half shafts rated for 400 horsepower, and the 800 horsepower-rated kit comes with upgraded half-shafts.

As always, careful measuring and remeasuring before wielding a die-grinder or cut-off wheel is the best guarantee of a quality installation. Once these steps are completed, the center cradle and other components will have a solid foundation, and the rest is a bolt-on process.

Adjustability Built In

Whereas the original suspension relied on trailing arms and Panhard rods — all with varying arcs of motion, the new suspension relies on upper and lower control arms and toe adjustment rods, all travelling in the same arc of rotation, to keep the tires squarely planted to the road surface and pointed in the proper direction under all conditions.

The suspension under our car was upgraded over the years with the best technology that was available at the time. The first step was to remove the factory rear suspension and all related components to make way for fitting the new IRS. The fuel tank also needs to come out.

The differential is mounted to the cradle assembly and makes use of either the factory driveshaft or a custom driveshaft designed to mate to the 1350-size yoke at the differential. Other custom steps include fitting new brake lines, since the old units will no longer apply. Support for the rear of the vehicle is provided through varying spring rates; enthusiasts can choose between 300-, 350-, and 400-pound spring rate coils wrapped around single-adjustable shocks.

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After trial fitting to make sure that everything lines up, you can then paint or powdercoat the Heidts pieces in your desired shade. Here you can see how the assembled differential rides in the cradle assembly. Whereas the original differential moved with the travel of the suspension, the IRS unit is solidly mounted under the vehicle.

Heidts informs us that a double-adjustable shock kit will soon be available, giving spirited drivers the option to control both shock compression and rebound characteristics. Diedrich informs us that the Heidts shop vehicle is equipped with the 300-pound springs, and does fine since they don’t load the rear of the car with stuff.

If you’re seeking an even higher-performing ride, or plan on hauling several Sumo wrestlers in the back seat of your F-body (a feat we would love to see photo documentation of), you might consider stepping up to a higher rate spring. Calling the Heidts tech line will get you dialed into the best application to suit your needs.

The only body modification necessary is mounting the side saddles onto the frame rails. You need to remove the inner metal panels of the frame rails by drilling out the spot welds, and cutting a section out of the upper radius to allow for the upper control arms to pass through. The new side saddles then mount into place and are secured with bolts and welds. The parts can then be painted after welding.

With so much benefit residing on the table, the next question surely becomes, “What does it take to put this under my third-gen?” We’re glad you asked! Check out the images to give you a sneak peek of what it takes to put a performance-capable IRS under the backend of GM’s third-generation F-body. If you would like more information, feel free to visit the Heidts website, or check out its online instruction sheet for a comprehensive step-by-step of the process.

On each side of the differential, heavy-duty DOM tubular control arms keep the wheels true, while the CV shafts carry the torque through the suspension's range of travel. Coil springs are tunable to ride expectations, and standard brakes are C4 Corvette units, but are upgradable to Wilwood kits.

So there you have it, an upgrade for the third-gen Camaro that not only improves handling, but will deliver a great ride, and help your car get the respect it deserves — especially when it passes a newer-model car on those twisty roads. Stay tuned, as the guys from Heidts have some serious track testing planned, and we will definitely bring you the results so you can see how much of a performance-enhancing upgrade this kit is when added to your car.

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The entire unit fits snugly up under the vehicle and is designed to allow for upgrades to engine or transmission, or work with factory equipment like the driveshaft.