Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Mentioned: 1 Post(s)
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New springs are the correct way to go. There are different spring rates depending on application. I put brand new springs in my '68 Lemans and experienced the same thing. I believe I got the "softer" ride springs. The factory shocks are typically 50-50, or equal dampening force going up and down. Then you have 70-30 which means it has less dampening force going up (extending) and requires more force going down (retracting). I believe I installed a set of 70-30 drag shocks on my car as the car bottomed out on my exhaust pipes on bumps and ruts. Then there is the 90-10 drag shock which is designed to be loose when you accelerate so the car nose goes up and puts the weight on the rear. It then settles down slowly as it is designed to require more force going into the down position. These are not recommended for street use, so I am thinking I used the 70-30 shock. No more bottoming out, but this is not a cure-all, and may not even work in your situation due to your oversized tires.
The front spring rate for a car having air conditioning is higher and would be stiffer -less bounce, and rides rougher. You also need to know what the factory spring ride height is when its compressed. For example, you could have a 17" spring height uncompressed and install it to become 12" once you have the weight of the car on it so it meets the factory ride height/ground clearance specifications. You could also have a larger diameter spring wire at 15" and compress to 12". A heavier diameter spring wire will need more weight to compress it to 12", so this is factored in and the spring is set at 15" uncompressed. If it was set at 17", it would cause your factory ride height to increase and so would your ground clearance.
My guess would be that you want a set of coil springs for a GTO with A/C or possible a Lemans wagon with A/C to get a higher spring rate and keep the stock ride height. If you want a little more lift, then the Chevelle's and El Camino's had some taller springs -Moog #5400 & 5450, but they are also stiffer. I am using #5400 on my '68 Lemans build for hopefully a higher stance. Many choices, just have to do some research and maybe make a few phone calls or send a few emails. It can get confusing.