So you have an early Mustang and you decide to go Pro-Touring with it. The first part of the Pro-Touring scene is to get the stance lowered, and add fatter tires, right? Those are easier things to do, but when it comes to handling, there isn’t much to do with the rear suspension. What if you could install an independent rear suspension in your Mustang that is a complete bolt-in kit, would that up the ante for you?

Hot on the heels of the Pro-Touring scene, Heidts Hot Rod and Muscle Car Parts has done it once again with their completely bolt-in PRO-G Independent Rear Suspension system for the 1964-1/2 to 1970 Ford Mustang. This unit is based on the same award-winning IRS kit used for early Camaros, and like the Camaro kit this one will give you handling that will leave your competition scratching their heads.

Heidts didn’t want to let the Chevy guys have all the fun, so they decided to add the early Mustang to PRO-G IRS line up. The PRO-G IRS is tunable, giving you the option to go with different spring rates and different gearing. If you want to upgrade the standard Wilwood brake discs from solid to slotted, that can be done as well for an additional cost. The four-piston Wilwood calipers are standard in the kit.

Slotting a brake rotor is done to improve braking performance. During hard braking where heat is increased, there is a build up of gases on the surface of the brake pad. Slotting a rotor swipes the gas build up from the pads giving the pad a better bite on the rotor.

When the Pro-Touring scene hit the streets and autocross courses across the country, the only thing missing was a solid, competitive rear end that could replace the dilapidated leaf-spring set up that hindered racers looking to take the next step. The cars that were performing and winning typically had a completely custom built chassis, leaving stock-suspended muscle cars at a slight disadvantage. But the competition is heating up, and things are changing with Heidts independent rear suspension kits.

Suspension Solutions

Heidts is known for their innovative solutions to old school suspension problems, and this kit goes a little further because you can install it even if you don’t know how to weld because it’s completely bolt-in. If you do want to weld it in, the nice thing about it is that you can bolt it in, and weld it up afterwards to make it more secure.

When we spoke to Mike Hawley, Sales Manager at Heidt’s, he told us that the new PRO-G IRS was a must. “Everyone was adding our front suspension kits to their cars, and with the pro-touring scene hitting hard they wanted to upgrade the rear end, too,” Hawley told us. 

Hawley continued, “These new PRO-G IRS units will handle the kind of horsepower that people are getting out of their muscle cars.” This new IRS unit won the prestigious “Best Product – Chassis and Suspension” award at the 2011 Hot Rod and Restoration Show.

We’ll show you step by step installation of this new PRO-G IRS, and cover some important points along the way. Working with older cars, it’s a given that they aren’t all exactly the same, slight differences will occur from one car to another. The old adage of measure twice and cut once is no different here, and before you drill any holes or make any cuts, it’s always best to take your measurements, test fit your parts, and check your measurements again for good measure.

One thing that must be considered is that these are very old cars, and 40+ years of abuse on the road means that these cars could possibly have tweaked a little here and there. Or if the car was ever in an accident you have to be sure that the frame is square and true. One frame rail out of alignment and you’re not going to have such an easy time getting everything bolted up. Remember: this kit is for a car that is structurally sound, so it must be straight and square in order for this kit to fit properly.

Meet Slingblade

Slingblade - the 1970 Mustang Mach 1 project that is getting the full Heidts suspension treatment: IFS and Pro-G IRS.

We wanted to document the installation process as it happened, so we contacted Michael Young, owner of Street Rods by Michael in Shelbyville, TN. He was kind enough to share the progress of his installation, and to provide pictures of every step along the way. Fortunately, we caught up with him during the installation of the rear suspension, and we were able to see it all come together at the same time he did.

After a stint as an accomplished musician, Michael Young began restoring classics and muscle cars back in 1990 and has been growing his business by leaps and bounds.  He is currently adding another 3,000 square feet to his shop, giving him well over 15,000 square feet of total shop space, including a 5,000 square foot paint shop.

This enormous shop space gives him plenty of room for Slingblade and his many other projects. While he is known by many as the go-to guy for early street rods, Young wanted to build something a bit more modern to reel in the muscle car crowd, too. An avid fan of Heidts suspension products, Young was excited about Slingblade and very eager to share this project car with us.

Michael’s project car for this IRS install is a 1970 Mustang Mach I that he calls Slingblade, which is getting the full treatment: replacing floors, sheet metal, drivetrain, suspension – the whole nine yards. In addition to the PRO-G IRS that is getting installed here, Heidts PRO-G front suspension kit is also being installed.

Third Member, Shocks and Control Arms

Unpacking all of the parts and laying them out, Young encourages us to read the instructions thoroughly. He is no stranger to Heidts IFS or IRS installations, and from the looks of his web site he’s no stranger to custom builds, either. He first laid out all of the parts, and got himself familiar with the order in which all of these parts were to be installed. As with any project of this magnitude, shortcuts and working ahead isn’t advised, stick to the game plan and you won’t find yourself back peddling because you missed an important step.

The first step, of course, is to get the entire rear suspension and all remnants of it removed from your Mustang to start with a clean slate. This means that all tabs and mounting points should be removed because the new IRS is a completely self-contained unit that bolts to the existing sub-frame.

At the uppermost part of the frame, there are supports for the stock suspension that must be removed. This is where the two saddles for the suspension are bolted into place on the frame rails. There are large bolts that are used to bolt these saddles in by drilling through the frame rails. If you are going to weld in your suspension, this is not the time to do it! If you must, tack it into place in case it needs to be moved or adjusted later.

Once the saddles are in place, Young marked the holes for access to the upper shock mount. This is in the instructions that Heidts provides, and they even provide you with covers for those access points. The covers attach with three sheet metal screws to hold them in place, and make access much easier. After the holes are cut, the crossmember is installed, bolting it to the saddle at the frame rails.  This crossmember will support the coil over shocks, upper control arms and the third member housing.

The third member in this kit is based on the 9-inch Ford center section in a custom aluminum housing. This housing bolts to the crossmember to keep it in place and two flanges on the housing are used in conjunction with a mounting plate on the third member as the mounting supports for the lower control arms. The third member is a strong unit built for high horsepower applications, and based on the Ford 9-inch means that parts are readily available for it at almost any auto parts store.

While Hawley states that this is not intended to be a “show piece”, it is a very trick looking set up. Parts can be painted or plated to give your car a more custom look, but he assures us: this IRS is all business and intended to make your car more competitive on the autocross course.

For those who want a little variety to compensate for tire sizes, horsepower ratings and transmissions, Heidts offers six ratios from 3.00 to 4.11 for this 9-inch positraction. You can also upgrade the spring rates, which are customizable from 350 to 550 pounds. This means that you aren’t just stuck with what’s popular for ratios or spring rates; you get to customize when you order your IRS. The best time to do your customizing is when you order, because the upgrade in spring rate or gear ratio doesn’t affect the price on your initial order. But if you want different springs or a different gear ratio later, they are available at an additional cost.

The stub shafts will mount to the housing later in this installation, but it is always a good idea to test fit some components along the way. The front mounting plate bolts to the third member after the third member is installed into the housing. This gives access to the mounting hardware. The lower control arms can be installed here, as well as the front crossmember. With the third member and the front plate installed, you can see the IRS taking shape.

The hubs and the rear uprights are assembled together, and that assembly is installed using the hardware supplied. While the uprights are very trick looking, like the rest of the PRO-G IRS they, too, are all business. The upper and lower control arms attach to the uprights, and the coil over shocks can be assembled and attached at this time. Leaving nuts and bolts slightly loose at this time is best because there will be times when you’ll need to unbolt things, like the upper control arm at the upright, for installation of the axles.

Depending on the stance that you prefer, as well as how much of a road racer you plan to be with your Mustang, the rear coil over shocks are adjustable, and spring rates can also be changed. Quite often, professional racers will experiment with different spring rates for different road courses to get the proper handling out of their car.

You can select stiffer springs, but there are other things to factor in, like vehicle weight and driving style.

Stiffer springs don’t always equate the to the best handling, other factors come into play like the weight of your vehicle, for example. If your suspension is too stiff, handling can get rather choppy because your forcing the car into a turn but the suspension is fighting you. If you go too soft, the spring isn’t fighting back enough and handling gets sloppy. This kit was designed for autocross performance, so keep in mind that a stiffer spring might help the handling, but ride quality is sacrificed.

This is why many hard core racers will tell you that you should start in the middle and make adjustments from there, so you can dial in your springs to suit your driving style and needs. You can adjust the shocks using external knobs that adjust the internal valving. This controls how fast or slow your shock will allow the suspension to rebound from a compression. Heidts can help you make a decision beforehand to help you get the proper spring rate based on your driving habits.

With the popularity of autocrossing these days, adjustable coil over shocks have become very common and were part of the design of this kit. There are always starting points for making adjustments to springs and shocks, and it should always be done when the car is completed with all components installed. It’s recommended that when adjusting shock absorbers you should start at a medium setting, midway though the range, and adjust from there.

The design of the PRO-G IRS is for handling, not just for show. A live axle suspension won’t give you the same cornering ability because both wheels stay perpendicular to the pavement. The design of the PRO-G IRS allows you to get up to a 1/2-inch of camber curve at 3/4 of bump, so hard cornering is not going to be a problem for this set up. As your suspension compresses, the top of the wheels pull inward, that’s negative camber and it assists in cornering. What that means is that further into the compression your rear suspension goes, the better the grip will be during hard cornering.

The long, forward braces allow you to connect the rear suspension to the front sub-frame for stability and strength. This helps make the IRS more effective and greatly aids in cornering stability and overall body strength. This kit was designed for autocrossing, so every effort went into the design to make the Mustang perform, and perform well. This part of the installation is important part because you need to make sure that everything bolts up squarely. Any difference from one side to the other and you’ll find your car doing a little sand crabbing because the tracking will be off.

The master cylinder and proportioning valve isn't included with the PRO-G IRS. But Slingblade is getting the full Heidt's suspension treatment, so a complete Wilwood brake kit was ordered.

Axles and Brakes

Young was building this car as a project, so even though the PRO-G IRS comes with rear brake calipers and rotors, he opted to purchase an entire Wilwood brake kit for Slingblade. Most of what you see above is included in the kit, with the exception of the master cylinder and the proportioning valve. He has also installed Heidts PRO-G IFS kit into this Mustang, so a complete braking system was needed and therefore he purchased the brakes separately.

The brake rotors that come with the kit are standard, vented rotors, but you can upgrade to slotted/drilled rotors for an additional fee. The additional fee is an upgrade fee, taking into account that the other rotors are not in the kit. So you won’t get two sets, just the rotors you decide on. If you want to upgrade later, you would need to purchase a new pair of rotors at full price.

With the arms, third member and shocks bolted into place, the only part of this installation that’s left is to attach the axle shafts, brake rotors and calipers. While it may seem simple enough at this point, there is still a right way and a wrong way, so following the instructions is still paramount.

Assembling the rotors and hats, the supplied thread locker should be used on all bolts. When you install the button head socket bolts, install them loosely to be sure all are threaded into the hat prior to tightening them, and then tighten them per the instructions.

The axle stub shafts are next inserted into the housing, and there are two retainer plates on each side. You will notice a recess in the two retainer plates, this recess faces towards the housing and will hold the axle bearing in place. The brake caliper mounts to the rear plate, and thread locker is also used on the bolts for these plates to keep them from becoming loose. There is a half-moon notch in the flange to allow access to the mounting bolts, which makes it easier to bolt things up.

The next step Young did was to install the drive axles into the uprights, without completely tightening the axle bolt. This allowed some maneuvering for the next step, which is to attach the brake rotor to the flange, and to mount the inner CV joint to the flange. This part, Young states, can be tricky and takes a bit of patience. He recommends that once you get one of the bolts through the rotor and CV joint, to not tighten it down completely.

He used a dowel to help align the other holes in the CV joint so that he can insert the bolts. Once all of them were installed, he then tightened up the bolts completely. The uprights can then be reattached to the upper control arms, and you’re ready for the final process of the installation: the brake calipers and brake pads.

The calipers bolt to the rear, attached to the bearing retaining plate, and the pads are inserted into the caliper from the rear. Using the supplied retaining pin, the pads are held securely in place, and with everything installed all bolts are double-checked and tightened as necessary. There are a lot of bolts and specifications to this installation, and the instructions are very thorough and complete.

Conclusion

We asked Michael what he thought of this kit, the fit, installation and his overall impressions on how well it was designed, and he said he’s happy with the PRO-G IRS. He said, “Heidts engineered it to be self-explanatory, and using CV joints instead of universal joints seems to work a lot better.”

We asked if there was any advice he could give about the installation, and he said, “Be sure not to tighten one bolt down completely until all are threaded.” This was particularly true with the axle shafts, he recommends installing them loosely, then once all of them are threaded in, tighten them in sequence.

Michael told us, “All we used to have were Corvette or Jag rear ends before. Heidts found a way to do this that makes me happy. Not only do they use a third member out of a Ford 9-inch, but they use the Wilwood brakes that are very popular with the hot rod crowd.”

While this installation is very complete and the instructions are very clear and precise, this isn’t a job for someone without experience, nor will it be a one-man job – unless you have all the right tools and the guns to pull it off. We want to reiterate here that should you decide to weld the framework in place, final welding should only take place after the entire kit has been installed, checked and tightened properly.

This PRO-G IRS should help your Mustang get through the cones faster – and a lot easier – than the stock suspension. And don’t be surprised if you find yourself going through those cones a little faster, because that is what this kit was designed for: to make your early Mustang perform with the big dogs.