Well, the issue is obviously with the spring lengths. The B-body spring is the same length as the FMJ spring, but the center pin is moved forward 2". The E-body spring has the same pin location in relation to the spring eye as the B spring, but it's 1" shorter overall than either the FMJ or B parts. So why does the aftermarket offer a spring hanger for the E and not the B? Because to put the center pin in the same location as the original axle, your rear spring eye would be 2" further back putting your shackle at an undesirable angle. The further off vertical the shackle, the less effective the spring will be. So, short answer, if you're not going to use an FMJ spring you should use the E-body piece. Or do you?
Now, let's add to that confusion: Super Stock springs! There are B, E, and A-body variants of the spring according to the old literature, but they're all the same in the length department. The "application information" regarding body style is just there due to expected weight of the race car. In all applications, the front segment is 20", and the rear is 36". Period. As it happens, A-bodies used a 20" front segment from the factory, which is why there's no Super Stock hanger for them.
Still got it straight? The Mopar oval-track springs have a 20.5" front segment, and a 35" rear segment, so they're a half-inch shorter than a Super Stock but actually move the rear axle back half an inch (when used with a SS hanger). The dimensions give a little extra wheelbase for stability while allowing a shackle angle that best lets the spring work for roundy-round work.
OK, enough confusion. The skinny on spring lengths:
A-body = 55" overall; 20" front segment, 35" rear
B-body = 58" overall; 22" front segment, 36" rear
E-body = 57" overall; 22" front segment, 35" rear
F/M/J-body = 58" overall; 24" front segment, 34" rear
Super Stock (all applications) = 56" overall; 20" front segment, 36" rear (illegitimate love child of an A & B-body love affair)
Circle Track (all applications) = 55.5" overall; 20.5" front segment, 35" rear (all the other springs look at it weird)
The FMJ car uses the longest front segment as a concession to noise, vibration, and harshness, along with its miserable oval front spring eye and the iso-clamp setup, the latter of course being a hand-me-down from later B & C bodies. It also makes it the worst setup of the bunch for any other application other than floating down the road. I consider the iso-clamps a "must go" situation personally, suitable only for 100-point restorations. The "positive" effect on ride quality is negligible but they allow way too much slop in the springs for decent handling or launches. They allow too much side loading of the springs, causing bushing pinch and not allowing the spring to work. Yes, even with poly isolators. Does poly flex less? Yes. The operative word is "less". They still flex, and flex is bad.
The Mopar design, being an asymmetrical semi-elliptical spring, allows for good cushioning of minor road irregularities to be handled by the long, thin rear segment while the short, stout front segment acts more like a control arm. This is why I laugh every time I see "slapper" traction bars on a Mopar: they don't do anything. Ever. I have yet to see one with worn snubbers, but if I did it would probably be on the rubbery FMJ design. The FMJ spring is still plenty asymmetric, though, and a lot can be gained by just eliminating the iso-clamps and using higher-durometer front eye bushings. For better launches, as with any solid-axle Mopar RWD car, an adjustable pinion snubber is a great idea, too. If you're considering one of those, go with the American Muscle one with the threaded adjuster. Experience has taught me a couple of times that there's no happy place with the pin-type snubber MP sells. It bashes the floorpan pretty hard at stock ride height with stock springs, to the point where it will tear the rubber off and deform the floorpan. Once that rubber's gone, the banging on the floor is loud as hell, too.
It all comes down to ultimate goal. In the case of my Imperial, I went with the FMJ springs because I'd wanted to keep the ride quality a little soft as a concession to daily driver usage, and improve handling with a round spring eye, solidly-mounted springs, and a rear swaybar. The B-body shocks are a much better design than the stud-mount FMJ for both function and longevity, as well as offering a much greater array of choices for any application.
Another scenario would be using a stock A-body spring with the correct FMJ Super Stock hangers. In that scenario, you'd likely want to use a spring from something like a 383/440 car or a cop application ('76). That would give you the short, stiff front segment and the same rear length as the E-body. This is one of the potential alternatives I'm still considering, since I already have a set of brand-new Super Stock hangers in my basement, left over from the LeBaron project. I think for the moment I'm going to use the springs I have with the factory hangers, B-body shock setup, and the spring sliders replacing the shackles. If I'm not happy with the way it works, I'll investigate de-arched Super Stock springs or the A-body option.
Trickery:
- If you have oval spring bushings with the holes on either side of the hanger bolt, you can run bolts & nuts with washers through them to compress the bushing, increasing your functional bushing durometer. It tightens up the handling and decreases wear on otherwise-good bushings. Obviously, you'll want to make sure your setup clears the hangers. The bolts need to be a tight fit in the holes, and the tighter the better. If you really squeeze a large-diameter bolt in there, you can cut off the bolt heads and threaded end flush. My new springs have solid bushings so unfortunately this doesn't apply to me.
- Removing the spring clamp from the rear segment and reinstalling it ahead of the axle centerline allows the rear leaves to fan out on hard launches, letting the front segments and pinion snubber do their job more effectively. There is absolutely no downside to this modification from a ride or durability standpoint unless you drag race on salt-covered roads and get residue between the leaves.
- Fastener torque spec is critical in suspension work to increase effectiveness and component longevity. "Tight enough" is probably too tight. Find and use the torque spec for the fastener you're tightening. This applies to every fastener in the suspension, both front and rear. In the case of leaf spring shackles, they should never be torqued until the car is resting on the ground on its own weight. You might be surprised how much better a totally-stock, all-factory suspension works when it wasn't assembled by UAW guys with zip guns who consider torque specs a suggestion at most.
