Above is a suspension upgrade I performed on my C4. The shocks are HAL-DR4855P QA1's with 375 pound spring rate 10 inch springs. To allow for the lowering of the car about 2 inches it required that I move the sway bar forward about 7/8 of an inch. For most I suggest 350 -400 pound springs. Due to the angle of the springs and shocks the actual spring rate is only about .66 times the spring pounds per inch. (375 * .66 = 247 lbs/in.).
UPDATE---- I relocated the sway bar back in it's original position and picked up these Helms joints from Exotic Muscle, they were about $220...
This is an image of the rears, they are using the HAL-DR5855P QA1 shock with coil over springs. The picture shows a 275 pound Spring rate 12 inch spring. I suggest 275-300 for the rears. The spring rate at the wheel is about .95 ( 275 * .95 = ~261 lbs/in.). The top of the shock mount I cut and welded a large flat washer to strengthen the mounting tab. I moved the top mount forward about 3/4 of an inch to allow the additional circumference of the spring to clear the half shaft when the wheel is fully in a "bump" position. I retained the original rubber bumper to keep the shock from completely bottoming out. I also had to shave the trailing arm bolt heads about half the height of the cap on the bolt to allow extra clearance. I also reversed the way the bolt is run, note the nut is now to the outside of the rear end knuckle.
I shimmed the shock to allow it to stay clear of the trailing arm bolt heads. These were inserts for the shock bushings that had to be removed. The setup requires using the stud type top mounts. For those concerned about how they hold up; to date I have over 15,000 miles driving from California to Ohio on some of the worst roads in the country everything has held together great.
Competition Engineering makes clevis mounts that could be used instead of the Stud ends on the shocks. I have not tried them but they seem to measure out about the same they are 1.5 inches deep wide and the height is 1.360 and inside diameter perfect fit for the QA1 eyelet ends and less expensive than the studs for the shocks. I have not checked the shock length either to see if that 1 inch is enough to bottom out the shock. The c-3422 has a 5/8" mounting hole, the c-3423 a 3/4" mounting hole. Summit numbers are CEE-3422 and CEE-3423 respectively. I have not measured the mounting hole on the front but the bushings seem to be 5/8", so I'm GUESSING the holes are 5/8". AGAIN I HAVE NOT TESTED THESE! Rough measurements show that at 26.75" inches in the front, the shock length is about 12.5 inches. The DR4855 is from 10.1 to 14.0 inches so it looks like there is still 2 inches of shock travel, since the wheel moves about 3 inches to 1 inch of shock travel. I'm planning on buying and testing this out...
This is my current setup the part numbers are from Summit Racing:
Qty Part number desc.
(2) HAL-12-275 275# springs
(4) HAL-SS110SDM Studs for top of all 4 shocks
(2) HAL-DR4855P shocks new HAL-DS402
(2) HAL-DR5855P Shocks new HAL-DS502
(2) HAL-BAR350K T-Bars for front bottom shock mounts
(2) HAL-10-375 375# springs
(1) HAL-T114W spring adjustment tool
My best ride was with the front springs at 350 to 400 and rear at 275 - 300 these are pretty much the minimum I suggest going. Anything less you will be riding the rubber snubs. The ride height is adjusted for 27" in the front and 27 3/4" in the rear with a full tank of fuel. This is measured from the ground to the center of the fender wells, both front and rear. Lower than 26.5 in the front the steering gets very stiff and it seems like the tie rods start to bind in the rack , this would probably require buying the zero bump steer tie rod ends. In the rear as you drop the car the shock start to lean towards the back further getting closer to the half shafts the car will ride only about an inch above the rubber bumpers.