Springs discussed will have end types "squared" x 1 and "tangential" x 1. This will be a direct swap and require no adapters or isolators other than stock. It's also important to remember that the nose weight I mentioned earlier is not all "sprung" weight. I'm not a suspension guru or specialist: If anyone sees a mistake please point it out, as I don't wish to spread misinformation but it's been a long day. My old info is lost in a sea of 6k screenshots so I pulled new part numbers from the Moog website.
Alright, so a 5608 has a rate of 424 lbs (required to compress it 1"), a free height of 15" (14.88), and it takes 1640 lbs of compression to reach the installed height of 11". It has a 4.08" I.D. and a bar diameter of 0.68".
Spring rates are dictated by material, bar thickness, and overall coil diameter as well as how many coils are present per a given distance.
So...
Theoretically, a spring of the same material, an equal rate, the same bar diameter and same I.D./ O.D. but a taller free height should garner a lift, regardless of what the listed installed height is- the car weighs the same and can only compress the spring so much. The springs listed for G Bodies typically show an installed height of about 11" but the "load" it takes to get there varies quite a bit, accounting for various options, packages, etc.. I've also heard rumors that factory coils were cut during installation but have my doubts.
I'd want to be between about 300 to 500 lbs/ inch to avoid a stiff ride and make use of all that travel, keeping in mind the travel limits of the shock, tire, and upper & lower bumpstops. Moroso Trick Springs are around 250 lbs. just for a point of reference.
Plugging some different numbers into the Moog site brought up spring # 5390: free height 16.6", I.D. 0.65", dia. 3.63", rate 336 lbs., requiring a load of 1952 lbs. to reach 11". The diameter is a bit off but we're guessing here anyways and the shocks should still fit inside, so I'd say that the two springs are quite similar in physical specifics aside from the rate and therefore the installed height, or amount of load required to be 11" tall. The load figure is higher for this spring since it must be compressed further to get there.
The 5390 is 1.8" taller (free height) but your car would compress it roughly 5/8" more than it would the 5608 due to the 336 lb/inch rate. It would achieve a theoretical lift of just over 1" above the 5608 and be about as stiff as a normal, non F41 G Body spring which has a rate of 346 lbs IIRC.
Moog spring models 9046 or 9114 each have the F41 spring rate and a free height of 16.25 to 16.5. so they'll compress to nearly the same height of 1" over a stock F41 spring, and would probably end up being upwards of 4" taller than super saggy springs.
An 80998 should get about a 3/4" lift and has a rate of 472 for a firm yet not harsh ride. A 5278 would provide a soft if not spongy ride at a rate of only 298 lbs but would yet again be about 1" taller than F41. Same goes for a 5372, 6330, and 5276 with the latter being 0.5" shorter. A 5030 or 5556 would be the same height but have rates of 420 and 442 lbs., respectively.
Let's not forget that other people like Hyperco, Afco, and Swift make coil springs or you could possibly have your own coils made and specify the diameter, rate, etc. or even use a 1" spacer on top of the spring if you're bold. The taller springs will be more difficult to install even if they're of a lighter rate, and can still take your head off. Shock stud extenders are recommended since you want to keep the shock in its "sweet spot" or designed operating range.
Bumpsteer is another issue entirely, since going taller moves the tie rod ends in the wrong direction, but you could run heim joints for outers and 5/8" bolts instead of tapered studs. Requires drilling the attachment point. I can't speak to how much the stock ball joints can articulate or when would be appropriate to bolt in some taller ones @ $70 each.
gbodyforum.com
