This is the first installment of the “Royal Scamp” street\strip A-body build. The main objective for the Scamp is going fast, after all this is supposed to be a 10-second street machine. One of the key elements to building a street machine is reducing weight while adding adjustability and reliability. Let’s face it, torsion bars just are not the most reliable suspension design. They are heavy, awkward and prone to breakage, especially when subjected to the stresses of a high-powered launch (and the subsequent return to earth) down the drag strip.
A torsion bar functions on a similar principle as the coil spring, but instead of rolled into a coil, the spring bar is left straight and mounted laterally with the subframe. As the lower control arm moves up and down, the torsion bar twists in the middle (the ends are secured by large socket-like fittings in the frame and control arm), resisting the movement. The best thing about a torsion bar is that they are easy to change out and easy to adjust. As the vehicle ages, the bar eventually stretches and bends, which requires periodic adjustment for proper ride height. This also means that it is easy to lower a torsion bar car by simply loosening the adjuster.
The biggest problem with torsion bars is reliability and spring rate. Coil spring breakage is fairly rare, but finding busted torsion bars is fairly common. Adding the additional weight of a big-block to the car will only compound things. The Scamp will have some serious torque on tap, and the thought of breaking a torsion bar is not pretty. Then you have the problem with torsion bars and the lack of variable spring rates. We could swap in some big-block torsion bars for load capacity, but then the front end won’t lift as much during the launch, which will reduce the weight transfer. Slant-6 torsion bars release faster for a better launch, but the car will be harder to control on the street. The only real solution is a coil-over conversion.
This is one of those tricky swaps, where you can either spend $5K on a tubular K-member for a simple bolt-on, or you can do it low-buck and get many of the same benefits. It may not look as pretty, but it only costs a fraction of the price. We decided to keep the cash in our pockets and go for the home-brew method. It is more fun than just bolting on a kit and it leaves the budget intact for other projects.
There are several key elements to this swap so pay attention. The parts you need to order are critical. The part numbers listed here are QA1 components that we sourced from Summit Racing (except for Reilly Motorsports, we ordered those direct). They had everything in stock, even though it was not all listed in the catalog. We spent some time on the phone with Bob Reilly at Reilly Motorsports researching the components. They manufacture the AlterKtion tubular K-member as well as the tubular A-arms and strut rods we used in this swap.
You need the following components to make this work-
QA1 adjustable coil-over shocks- single or double adjustable, we opted for the doubles (576 potential settings), but this depends on your intended purpose and tuning needs. The main point is to get the right shocks. We used part #DDR-5855P, which are QA1 Proma Star 17” ext, 11.625 collapsed double eyelet shocks.
SS100SDM eyelet to stud adapter- This is the key component. Whether you use QA1 shocks or something else, you have to have an adapter. The adapter converts the upper eyelet mount to a stud mount like the factory shock. Yes, the rubber will wear out faster and you will have to change it, but for a weekend car like this, that could take 3-5 years. For a daily driver, you will be changing it once every 1-2 years.
Coil-Over Spring- We used the QA1 10-450 spring, which has rate of 450 lbs./in., and is 10-inches long and 2.5-inches in diameter. DO NOT use the 12-inch spring, it is too long. The 2.5-inch diameter barely fits, so nothing larger will work there either.
Tubular Upper A-Arms- The stock A-arms won’t clear the coil spring, they need to be swapped out. We used a set of Reilly motorsports tubular arms, which not only clear the coil-overs, but also add a fair amount of adjustability to the castor and camber, all of which mean greater tuneability at the track.
Adjustable Strut-Rods- These components are not required for the coil-over swap, but they are a good idea. Our strut rods came from Reilly Motorsports and allow you to the tune the movement of the lower control arm. Using a rod-end and solid bushings, the play in the factory version is removed, leaving you with only the movement you want. Beside, our stockers looked more like an ocean wave than a straight bar.
The installation was simple, but as we are working on Mopars that do not have a great reputation for tight tolerances, trimming some stock components was necessary. We used an ESAB Handi-Plasma to make quick work of the offending material, but a sawzall or oxy-acetylene torch works too. If this is your driver, you need to plan ahead as getting the entire job done will take two days, including adjustments. As with any front suspension work, the car needs an alignment afterwards.
Don’t forget to torque all the suspension components. If you do not have a service manual handy, here are the basics-
Pivot shaft (lower control arm) 145 lbs Strut to lower control arm 105 lbs Cam Bolt (upper control arm) 65 lbs Ball joint stud upper 55 lbs Ball joint stud lower 85 lbs steering knuckle bolts (upper) 55 lbs steering knuckle bolts (lower) 100 lbs