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For low cost effective rear suspension, I find adding an adjustable height rear pan hard bar to be very effective. I have one on my car, and have installed them on several other cars. The ones that autocross have found that adjusting the height, which is the roll center height, is very effective on controlling rear grip with varying surfaces. I have found for example, that in San Diego, I like my pan hard bar a bit higher than I like it at Fontana. Now, this is my biggest issue with a lot of the aftermarket suspensions, They do not have adjustable rear roll centers. I race against a lot of these suspensions and they do really well. I have modified a Detroit speed quadra link pan hard bar to be height adjustable. This made a dramatic change for the better on this cars over steering issues. Leaf springs are not ideal, after all they are horse and buggy technology. They can work well but never like a link/coil over. If you pan hard bar leaf springs, you need to either use rubber bushings or swivel bushings. Polyurethane, delrin, and aluminum are all to stiff and will cause excessive bind. For budget road handling suspension, I personally would do the pan hard bar, some lowering leafsprings, probably composite ones from Flex Form, and some single adjustable shocks from RideTech.
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Another note is using a 9in rear housing makes it easier to deal with and cuts some weight housing wise. A friend has installed a 1100 hp Nelson TT LS engine and was getting 10 bolt and 12 bolt breakage. so he did it 9in and found it held BUT the 9in really eats power so he talked to a friend who used to drag race and bought 4 9in center sections that used 12 bolt gears. POWER IS BACK. And he still gets swap ability for track days and breakage, which with braced 9in housing is no longer issue.
And he added Moser axles for included durability.
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I'm definitely in line with bryant and MonzaRacer. The biggest difference I saw in my car was first correcting the front roll center with simple adjustable upper A arms from SPC. Then, adding the Fays2Watts link was like night and day. Having an adjustable rear roll center allows a simple bolt change to shift the car from throttle oversteer to excessive understeer(if thats your thing lol) and anything in between. I then made the big leap and went to a Fab9 housing and designed my own 3 link with adjustment in Antisquat from 40 to 160% in 30% increments. If you have a big company behind a design that knows what they're doing, then the adjustability probly isn't that needed, but since I had no idea how it would affect my car, I made sure I had the option (btw, not a fan right now of 100% AS).
So from a budget setup, I say an adjustable roll center in form a panhard bar, torque arm from a sponsor here that knows what they're doing, and some great adjustable shocks. the shocks are the brain of the suspension, so being able to tune lift/dive and roll characteristics will take the car that's fun but tiring because it's a constant handful, to a car thats fun and fast because it's Stable and Predictable. In fact just last week I received my DA shocks from Chassiworks and am very much excited to tune out some characteristics at the track I've been wanting to get rid of, anyway.. Good luck man, hope this helps.
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I learned a fair bit reading this thread, good discussion. As someone budget minded and having to progress in steps as such, I'm running full Hotchkis front suspension (non-coil overs) w/ a 2" drop along with Hotchkis SFCs/leafs/rear shocks and CalTracs to a Quick Performance Ford 9". That's just all I could afford far as where I am with my 69 Firebird. In a few years, I'd love to go proper coil overs on the front and a 4 link in the rear w/coil overs. I'll probably go with the RideTech setup.
Now my 92 Trans Am GTA is a different beast. It has every goodie Spohn makes for a 3rd gen, tubular k-member and all. I haven't significantly increased my power, but I am already capable of driving at the limit of the torque arm setup. As mentioned, having an adjustable Panhard bar helps. The next step will be a 4 link setup if I ever take the next step. I'm thinking I'll just leave the 92 as-is and focus on my 69. After 20 years of playing with the 92, the 69 that I've always wanted just has my heart right now.
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Fays2 watts link while keep the leaf suspension is an option. Global West Cat-5 leaf springs might be something you want to consider before you start getting crazy with changing suspension. Besides Global West and Hotchkis, I don’t know who else would offer a spherical spring eye 1st gen leaf spring. I think that would be your most cost-effective setup along with being easy to install.
My assumption for people on forums like this (not to be taken as offense) is most just want a lowered car with big wheels, LS engine, etc. That can be done on leaf springs at the best price point in most cases.
What rear end is in the car? Rebuilding the stock GM rear end is usually the easier option than getting a whole new setup. Detroit Truetrac is preferred by many on that subject.