Tech Video: Heidts’ PRO-G IRS Makes This Firebird A Contender

When someone mentions a second generation Pontiac Firebird, thoughts of a weathered Trans Am sitting on cinder blocks next to a double-wide might come to mind. They’ve been a popular car for the southern musclecar enthusiast, and films like Smokey and the Bandit only perpetuated that notion.

The cars gained in popularity thanks to the movie, but for many others the mention of a second gen F-body might remind them of the SCCA series of that era, where the Pontiac held its own against the likes of the Sunoco Camaro, the Boss 302 Mustang, a certain lime green Challenger T/A, or a red-white-and-blue AMC Javelin.

The mullet machine; say goodbye to the factory suspension, and hello to a whole new attitude.

But, as great as its heritage is, the Firebird’s suspension is now a bit outdated. In order to keep up with today’s crop of autocrossers and weekend warriors, the Firebird is going to require a little bit more than memories of races gone by. It’s going to need some serious upgrades in the suspension department.

Sidebar title

Heidts PRO-G IRS

Part #IRC-201-S

  • Complete, bolt-in independent rear suspension
  • Ford 9-inch, iron third member with posi-traction
  • 3.00 to 4.11 gear ratio available
  • Aluminum housing
  • Constant velocity joint halfshafts
  • Tubular upper and lower control arms
  • Steel uprights
  • Front pinion support
  • Sub-frame connectors
  • 10.5-inch Wilwood brake rotors – drilled/slotted optional
  • Black Wilwood four-piston calipers
  • Billet, single adjustable coilover shocks
  • Chrome coil springs
  • 350 lb. to 550 lb. spring rate
A True Bolt-In IRS?

We brought this 1971 Firebird Formula into the Power Automedia shop with the goal to make this car a contender on the autocross track. Our task was to completely remove the leaf-spring rear suspension from this F-body, and to swap the entire live axle rearend for an independent rear suspension using nothing but hand tools.

It might sound like a tall order to install a complete rear suspension with an entirely different configuration – without any cutting or welding – but we had a little help from Heidts Automotive Group. So we kept the sawsall packed away, and the welder was never plugged into the 220. The tool cart was rolled out, and removing the factory suspension was the first order of business.

The reason we were able to swap the suspension without cutting or welding is because we installed a complete, bolt-in, PRO-G IRS from Heidts. And when we say that it’s a bolt-in rear suspension, we mean just that: the entire IRS can be bolted into place. To further reinforce that claim, once the IRS is installed it can even be removed as a complete unit – without any cutting. If you’re still a little skeptical, then keep reading, and watch the video above – it will make a believer out of you.

Why Swap In An IRS?

When it comes to handling, the Heidts Pro-G IRS offers plenty of benefits over the factory live-axle set up. The first thing you’ll notice is the looks, and we won’t deny that it looks a lot meaner than a leaf-spring suspended rear axle.


The Pro-G IRS reduces unsprung weight.

But, as they say, looks aren’t everything. There needs to be some purpose, and some gains, in order for this to be a valuable swap. What good is form without function?

There are a few areas that make this IRS superior to the live-axle suspension it replaces. A live-axle suspension makes for a lot of unsprung weight – from hub to hub.

A reduction in unsprung weight won’t increase your top end or improve acceleration, but it will increase lap times with improved cornering abilities.

Some of these benefits to installing the PRO-G IRS include:

  • Unsprung weight – the IRS reduces unsprung mass by moving the brakes and calipers inboard, and with the third member mounted to the chassis.
  • DOM tubing – the Drawn Over Mandrel for strength and for more consistency in the tubing.
  • IRS is symmetrical – unlike a four-link suspension with a panhard bar, the suspension is not only independent but it’s also symmetrical.
  • No welding – as hard as it is to believe, welding it not required to install any of the components.
  • Better handling – as the suspension compresses, the camber decreases and provides a better contact patch with the pavement, giving better control during hard cornering.
  • Adjustable – Not only can the ride height be adjusted with the single-adjustable coilover shocks, but the camber can be adjusted with the upper control arm, and the track can be adjusted with the lower control arm.

Pre-assembling The Components

With most of us gearheads, we can’t wait to open up the box and “play” with our new parts when they arrive. You know you do it, and so do we. Heidts knows it too, and they even suggest that some parts are pre-assembled on the workbench prior to installing the IRS.

This pre-assembly serves two purposes: it helps to get more familiar with the components before installing them on the car, and it also helps get the smaller assemblies completed first so that attention can be focused on the actual IRS install. Completing the pre-assembly makes the rest of the installation process quicker, and it’s also quite a bit easier to pre-assemble these parts when they aren’t hanging from the car.


There are lots of boxes full of parts, but you can cut down on actual installation time by pre-assembling some of the components prior to installing the IRS.

Looking at all of the parts laid out on the floor can be a little intimidating. But once we began pre-assembling the components, it started to come together nicely and gave us sections that we could install as assemblies.

The control arms are DOM, mandrel bent tubing. It takes longer to make and costs more, but it’s more consistent and precise. -Jim Shaw

The parts that can be assembled prior to the installation include: the lower control arms, uprights, and hubs, the brake rotor and rotor adapter, the upper crossmember can be attached to the third-member housing, and the coilover shocks can also be assembled.

Heidts spends a lot of time perfecting the installation process by giving clear, concise instructions. Jim Shaw, the lead engineer at Heidts, said, “When someone buys this kit, they’re spending a lot of money. We owe it to them to make sure that our instructions have the same kind of dedication and quality as the IRS setup.”

Shaw also said, “This is not a ‘box of parts’ approach, it’s systematically engineered as a whole. The control arms are DOM, mandrel bent tubing. It takes longer to make and costs more, but it’s more consistent and precise.”

Pre-assembly includes mating the hub to the upright, and attaching the lower control arm. The brake rotor adapter is bolted to the brake rotor, and the upper crossmember is bolted to the third member housing. Assembly of the coilovers requires disassembling the adjuster to slip the coil spring and adjuster rings over the shock body.

When the parts arrive, they’re bare, raw metal, and there’s a reason behind that. It’s much easier to apply a finish to the parts after the installation, rather than risking damage to the finish during installation. But Heidts recommends that you wait until the entire IRS is completely installed and tightened down before you paint or powdercoat the parts. As with any custom installation, some parts may have to be modified or adjusted during installation, and it’s best to make sure everything fits before you disassemble the parts and apply the finish.

Along with removing the suspension, the front perch for the leaf spring needs to be removed. This can also be unbolted - no cutting required.

Out with the Old, In with the New – Installing the PRO-G IRS

We began by removing the entire rear suspension. The brake lines were disconnected and the driveshaft was removed and set aside. The solid rear axle, leaf springs, shackles and front leaf spring mounts are all unbolted from the car, and no cutting had to be done at all.


The first parts to install are the saddles that support the upper crossmember. Proper fitment of these is very important because the rest of the IRS is supported by them.

With these parts out of the way, we began the installation, starting with the two saddles that are mounted to the rear subframe of the Pontiac. These two parts are crucial to the install, as they will support the entire IRS.

The saddles were mounted utilizing the existing three weld nuts that are on the stock framerail where the bump stop was mounted. We left the saddles loose initially, so that they can be pressed up against the framerail as tight as possible, and then the bolts are tightened and the additional holes were drilled for the lower bolts.

The upper crossmember relies on these parts being mounted solidly, and once both saddles were in place the crossmember and third member housing was bolted in as a unit from the pre-assembly steps mentioned above.

With the upper crossmember and housing in place, the Ford 9-inch third member was installed. Our Firebird is running a 3.50:1 gear ratio; the available ratios are from 3.00:1 to 4.11:1, and since it’s a Ford 9-inch the parts are readily available.

Installing the saddles and the crossmember, the basic support system for the IRS.

With the third member installed, the pinion support crossmember is attached to the saddles, keeping the third member from moving.

There are a couple reasons for using the Ford 9-inch unit as opposed to manufacturing something unique or specific for this IRS: the Ford 9-inch is a very strong unit, and it’s completely serviceable because of its popularity. This ensures that you won’t have to hunt around for Ford 9-inch replacement parts if you decide you want a different gear ratio later, and you’ll have plenty of choices when shopping for a new ring and pinion.

The pinion support plate was bolted to the front of the third member, and the pinion crossmember was attached to the plate and the two saddles. This lower crossmember helps to firm up the third member because it needs to be mounted solidly into the car, without any possibility of brackets or supports flexing from the torque in the driveline. The pinion support crossmember was bolted to the saddles with the bolts provided by drilling holes in the factory subframe.

When we installed the stub shafts into the third member, we made sure to follow the directions precisely. The bearing on the stub axle is held into the housing by the caliper plates, and the tolerances are very tight. The bearing must seat into the housing completely, but in order to get the caliper plates installed the stub shafts need to be inserted as the caliper plates are bolted down. The bearing will seat into the recessed area and the caliper plates retain the bearing in place.

We specifically designed these parts and many of the components in house, we have a high percentage of ownership in this kit. -Jim Shaw

With the third member bolted into place, we started the process of attaching the lower control ams, uprights, and the hub. This was another step that was pre-assembled earlier, and installing them as an assembly saves time with the overall installation.

Many of the components have both mig and tig welding done. The welds are far superior to what we’ve seen on some factory parts, and Shaw is very proud of the components they put together. He said, “We specifically designed these parts and many of the components in house, we have a high percentage of ownership in this kit.” Shaw also feels that Heidts has some of the best welders in the industry. Looking at the quality of these components, we’d have to agree.


The quality of the welds on these parts is top notch. They need to be strong, and we were very impressed at how well they were put together.

These lower control arms have quite a bit going on, too. They bolt to the third member unit as well as the uprights, and there are additional supports for the coilover shocks and the subframe connectors.

Since much of the weight of the car will be resting on the rear coilover shocks, the shock supports need to be strong; and since the subframe connectors bolt to that same lower control arm, they need to be able to handle all the torque the Pro-G IRS is putting these parts through. This is why the quality and strength of these welds are important.

The front and rear tie bars were installed with both lower control arms with the tie bars to the outside of the control arm. We kept all bolts snug, but not completely tightened down. The toe-in needs to be adjusted when the installation is completed, and these bolts may have to come back out for adjustments.

The LCA and uprights are left hanging so we could install the CV shaft to the stub shaft.

Installing the CV shafts required an extra set of hands, because the brake rotors were attached to the stub shaft at the same time as the CV joints. The rotor adapters were installed to the rotors during pre-assembly, and the long socket head bolts for attaching the inner CV joints also passed through the rotor adapters and into the stub shaft.


The articulation and the plunge on the CV shafts helps when inserting the axle through the hub.

It’s imperative that the washers are placed on the bolts before they’re inserted. Without the washers, the bolt could interfere with the hardware on the caliper adapters.

When we finished installing the CV joints, it was pretty easy to insert the outer CV joint through the hub. The CV joints are heavy duty units that will not only handle the kind of horsepower and torque that modern engines are putting out, but they also have far better articulation, and there is a couple inches of plunge that allows the joint to move through that articulation.

Plunge is the amount of moving that the joint makes inward and outward. Consider the shaft at rest, with the travel of the suspension, the axle will need to extend and contract as the suspension moves. The plunge of the CV joints allows the articulation of the CV shafts without any binding on the joints themselves, and that relates to a solid transfer of power from the differential to the wheels.

An extra set of hands to hold the axle steady is needed to tighten the nut. Heidts warns us to not use an impact gun here.

We installed the nut onto the outer CV joint shaft after it was inserted into the hub. The shaft and hub are splined, so they have to be finessed into place if there is resistance. We applied a small amount of grease to the splined shaft to keep the two mating surfaces from binding, making the installation go much smoother. It’s important here that the shaft is able to slide through the hub entirely by hand, don’t be tempted to use the nut to draw the axle through. If it is binding, wiggle the shaft a little bit to work it’s way into position, then install the flanged nut and tighten to 100 lb/ft.


The inboard brakes are a great idea on this kit, leaving plenty of wheel clearance at the hub.

Heidts cautions us to not use an impact gun on the nut, and if there is difficulty in tightening the nut, it can be left snug and when the brake calipers are installed, and the brakes bled, they can be used to hold the shaft from turning while the nut is tightened.

The upper control arms were installed next, and this was where we could see the camber change when the suspension was put through it’s travel.

Heidts builds negative camber into the rear suspension at 3/4 bump, meaning that when the rear suspension is compressed 75%, the camber will change about .050-inch negatively, which helps with hard cornering at speed. This negative camber forces the axis of the wheel to push at an angle, providing more force and contact with the pavement.

We installed the Wilwood four-piston calipers next and bolted them to the rear caliper plate, and inserted the brake pads from the back side of the calipers. The locking pin was inserted and held in place with a cotter pin. With the basic kit you can have solid or cross-drilled/slotted rotors. The cross-drilling will help to cool the rotor surface, while the slots on the rotor help to swipe built up gasses between the rotor and the brake pad.

These links are a great help, and easily allowed us to see where suspension travel is going to be at its limits.

Before installing the coilover shocks, which were also assembled prior to installing the kit, Heidts suggests fabricating 12-inch links in place of the coilovers so the IRS can be handled as a complete unit. Shaw told us, “The kit should be set pretty close to accurate when it’s installed properly, but it can be adjusted based on driving style with the adjustable coilover shocks.”

These links also provide another function of this Pro-G IRS, a fact that we touched on earlier in this article. Once the toe-in and settings are all made, and all of the bolts are tightened properly, these links can be put in place of the coilover shocks and the entire IRS can be unbolted from the car as an assembly. The links keep the suspension rigid, which makes it easier to remove the IRS in one piece for final coating.

The sub-frame connectors and strut rods keep the rear axles and wheels stable, in the same manner the leaf springs did, but allow for independent travel.

The last part that connects the IRS to the front sub-frame are the sub-frame connectors. They are necessary to help stabilize the rear suspension and stiffen up the chassis. Since this car was not designed for a central mounting point on the rear end, there’s a need to strengthen the chassis and keep the rear wheels in place. The connectors bolt to the front sub-frame rear bolt, and are bolted to the rear sub-frame by drilling the necessary holes. To connect the sub-frame to the IRS, there are two strut rods that bolt to the sub-frame connectors and the lower control arms. This keeps the wheels in place and adds the necessary stability.


When making adjustments on coilover shocks, the full weight of the car should not be on the suspension, and a spanner wrench will help make the task a lot easier.

The coilover shocks are single-adjustable, allowing for variances in dampening the rate of the absorber. At the time we ordered the kit, we were also given a choice of spring rates: from 350# to 550#. Depending on whether you’re driving a big or small track, the available spring rates can help you to dial in the best combination for the track.

This Firebird is going to be raced, but it’s also going to be driven somewhat regularly, so the various spring rates allowed us to choose a good, middle-ground, for spring rates that will accommodate the street driving this car is going to see.

If the autcrossing gets more serious and stiffer springs are needed, additional springs can be ordered from Heidts, and swapping them out prior to a race or event would be a simple task. Each time that the springs are changed, they will need to be assembled just like we did prior to installation – on the bench. When we installed the coilovers, we used a spanner wrench to adjust the tension on the spring while the weight is off of the suspension. Never try to adjust a coilover for ride height with the suspension compressed, it should always be at full extension.


There is sufficient travel in the suspension, as can be seen by the pictures above, plenty for a car like this.

With the adjusters set, the car is lowered and full weight is put on the suspension, and we checked the toe in, we checked the ride height, and we measured both sides. Heidts recommends that the CV joints are parallel to the ground at ride height, and a level can be used to check the angle. If we needed to make adjustments, we raised the car and made the adjustments, and once the coilovers were adjusted properly, we tightened the jamb nut on the shock body.

No Welding Needed, What Else is Needed?

With the complete PRO-G IRS installed, there are a couple of other issues that need to be addressed. These issues are typical of any brake or axle swap, so they shouldn’t be too difficult to work through. The first and foremost part to consider is the driveshaft. Going from a GM third member to a Ford means there will be some differences; plus there’s the issue of length for the driveshaft. The third member yoke is also going to be in a different location fore/aft than the original.

FbodyPRO-GIRS246What do we recommend? We recommend a new driveshaft. With a brand new IRS and a 30+ year old driveshaft, it doesn’t make sense to modify an old part when a new, stronger driveshaft can be made. It’s a small expense compared to the job that was just completed, and with autocrossing in mind, strength and safety should be factors to consider.

A second thing to consider is the braking: with the switch from drum brakes to four-piston calipers, the brake bias is going to be a bit different. An adjustable proportioning valve would be a good choice here because it allows us to change the brake bias with the turn of a knob. It’s not anything new, since disc brake conversions have been done for years. This eliminates the need for finding the proper proportioning valve and master cylinder combination.

Overall Impressions, Is it Really a Bolt-In Kit?

FbodyPRO-GIRS241When we shared with others that we were installing a bolt-in IRS, the skeptics came out in droves. We heard it all, from “there’s no way you can bolt in an IRS” to “wait until the first time you drive it.” It almost seems too easy; unlike all the custom work that we needed to do in years past: welding, fabrication, cutting, designing, and more welding. It doesn’t seem like an independent rear suspension can be bolted in place. But Heidts has nailed it with the PRO-G IRS, and this kit gave us no difficulties during the installation.

However, this isn’t just a bolt-in IRS, this is a well engineered bolt-in IRS. The level of ownership, the attention to detail, and the fit and finish of this IRS is what brings it all together. Countless hours are spent on the design, and this design is engineered around the existing platform, not in spite of it. Utilizing existing bosses in the rear sub-frame to install the first component, the saddles, and using the existing bolts on the rear of the front sub-frame for the final component to be installed, everything else simply bolts into place between these two focal points on the Firebird.

The PRO-G IRS can be described in two words: Bad ass.

It’s the same way with the several other PRO-G IRS kits that Heidts has manufactured. The principle behind this installation is that it can be done by someone who has no welding or fabrication experience. It’s designed for the hobbyist; the enthusiast who likes to work on his, or her, own car.

If you decide this is the kit for your early Mustang, Nova, F-body, or Tri-Five Chevy, then Heidts has a great staff on hand to help you through it. We found the instructions easy to follow, and it shows that some time was spent on making sure the instructions matched the installation. The instructions are more of a booklet than a couple of sheets of aged, generic photocopies. They matched this kit in both procedures and illustrations.

This is not a small task, so the instructions shouldn’t be a small task, either. If the instructions are followed from top to bottom, and you don’t try to outsmart the installation process and get ahead of yourself, you won’t have any problems. We didn’t.

A Little More to Think About

After the kit is installed, there are some additional steps that can be taken. The first is completely optional, and that is to weld the saddles in place and make this a permanent installation. If you’re tempted to weld them prior to installing the entire kit, it’s highly recommended that you merely tack them into place, and complete the weld later.


We measured the distance at each fender lip to set the springs at the same height.

The second thing that can be done is to finish the parts, either with paint or to even have them powdercoated. The color/finish is entirely up to you, and that was part of the plan.

Heidts put a lot of thought and engineering into this award-winning, patented design, and that is what helped us to bolt in the PRO-G IRS – they thought of everything first. We’re looking forward to seeing how well this former “mullet machine” performs at the track now.  Check out the PRO-G IRS at the Heidts web site, and put the welding mask away – you won’t need it.

About the author

Michael Harding

Michael is a Power Automedia contributor and automotive enthusiast who doesn’t discriminate. Although Mopar is in his blood, he loves any car that looks great and drives even faster.
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