The better question to ask is what your intended purpose with the car is? Are you just wanting the car to sit a little lower? What's your ride comfort requirement? How is the car going to be driven?
Other's disagree, but coil-overs for these cars are a compromised design and if you're not looking for every 10th at a competitive auto-x event, are probably not only not necessary, but may be a waste of money. The big issue with retrofit coil-overs is that they aren't a true coil-over at all. They are a standard large body shock with a threaded perch. The shock mount is virtually identical to that of a normal shock, but you're now asking that portion of the shock to take the entire load of the car, rather than just damping loads which are significantly less. This doubles for the factory lower control arm which is not designed to have the shock mount carrying the car's load. So while you can get the coil-overs for around $360.00, you also really need to be thinking about lower control arms that are designed for a coil-over as well. It snowballs from there depending on which lower control arm you purchase. If it has built in positive caster (some do, some don't) you may also need a geometry corrected upper control arm to keep the wheel centered in the wheel well.
Now, instead of spending $360.00, you're over a grand deep in many cases and that's just the front.
If you don't need to continuously change the ride height of the car, or make corner weight adjustments, there's not a lot of value in the adjustable feature, when you can achieve a lowered stance through use of springs or drop spindles.
If your goal is just a lowered stance, but you want to retain a ride similar to what your car currently provides, while improving handling a bit, I would spend that money on addressing the car's geometry issues.
Choose a spring with just a small amount of drop and just a slightly firmer rate than your factory springs. Then add tall upper and lower ball joints to the control arms. If you go with a 1" lowering spring in the front, with the addition of the tall lower ball joint, you'll lower the car about an inch and a half. Then do a spring in the rear to get it to sit how you want. A 2" lowering spring in the rear will get the car sitting fairly level, while that same 1" lowering spring will give it a bit of a rake.
Add a quality shock of your choice and give the car a modern alignment. If you need more wheel rate in the corners, add a larger front sway bar and a rear sway bar.
This setup is cost effective, doesn't re-engineer the wheel and will provide you with a noticeable increase in handling potential, while being as reliable as your factory suspension setup and not destroying your ride quality.
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-Jason
1969 Pontiac Firebird
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