[#1WideTrak]

I need to replace my rear shocks, and I think this is the best time to finally replace the sagging rear springs. If anyone can post photos of their 69-72 GPs and advise which brand/PN springs you have, it would be a big help.

I think these GPs look good with a little lift in the back, but I don't want it to looked jacked up either. I've been told 70 GTO springs give more height. I've also seen comments/answers on Summit's page about the Moog "cargo" springs giving more lift than the 70 GTO springs. Based on the MOOG #s, I would not expect the cargo springs to ride higher than the 70 GTO springs.

Lengths are compressed / relaxed
MOOG #5385 - Stock constant rate 70-72 GP - 9" / 12.6"
MOOG #6319 - Stock constant rate for 70 GTO - 9" / 14.17"
MOOG #CC501 - "Cargo" springs with variable rate for 70 GP - 8" / 12.63"

Any advise is appreciated!

[Joe's Garage]

We haven't put them on a GP, but most 1970-72 GTOs with old, tired springs will go up about 1-1.5" at the bumper, even without any extra load.

It just depends on how worn-out your original springs are.

Height is only one part of the equation. Grand Prixs were designed to ride smooth, and some of them had the factory air-shock system to keep them level. THOSE have really soft rear springs and used the shocks to compensate for it. And they ride really smooth.

The other part of the equation is SPRING RATE, or how much weight it takes to compress the spring a certain amount - I think it is 1".

Here are your three choices:

5385 wire diameter = 0.59"
6319 wire diameter = 0.53"
CC501 wire diameter = 0.61"

Thicker spring wire is harder to compress, thinner is easier.

The Grand Prix is heavier than the GTO, so it kinda makes sense that the factory GP rear springs would be slightly thicker than a factory GTO rear spring. And it also makes sense that the Cargo Coil 'CC' springs would be even thicker.

If you can find someone with an older MOOG catalog, from the late 80s or early 90s, they used to have a specification section with spring rates and anticipated load for each part number, along with the free and installed heights.

When I was working the parts counter at my local speed shop, we could use all those specifications to calculate how much higher or lower the car would sit with various springs.

We used to install a lot of LeMans station wagon rear springs into GTOs back-in-the-day. As I recall, the back end came up about 2" and the ride firmed up.

Good luck!

[#1WideTrak]

Hi Joe's Garage,

Another member here, Harvey ssj71, suggested I try https://www.springsnthings.com/ . I contacted them, and was told they no longer carry 5385, but CC501 is listed also listed as a replacement for my GP. I had the specs I posted above where the 5385 compressed is taller than the CC501. Hmmm.

Thank you for the advice to look for more MOOG specs. I didn't know where
to start to find an old catalog, but I found MOOG's website is quite nice. Many sellers only showed the spring rate of the CC501 variable rate spring. It had a spring rate listed - 175 lbs/in. I assume that 175 rate applies to the first inch of compression on a variable rate design. MOOG's website stated the spring rate for the 5385's is 168 lbs/in. But there is still that compressed height difference. What gives?

The MOOG site also stated "load." Apparently, MOOG bases the 5385 9" compressed height on a load of 602 lbs. They base the CC501 compressed height of 8" on 810 lbs. If you use the same load on both springs, based on spring rate, there is approximately a 1/4" difference with the CC501 being a tad taller. So, I ordered the CC501s with confidence I'd be pleased with them!

Springsnthings shipped them the same day I ordered, and RockAuto did the same with my new shocks. I had all the parts before the weekend, and I spent Saturday afternoon installing them.

Before I started the work, I was thinking "maybe it's not sagging as bad I kept picturing in my mind?" My pictures are so-so, but the top of the wheel well molding was at 24.75" with the old springs, and it's now 26.25:" with the new. The 1.5" lift is just right!

I have a before and after pair close up and one after shot from a distance.

[ssj71]

Brent,

Super glad this worked out for you.
Laura at Springs is great!

Harvey