Understanding Lowering Springs
Lowering springs are the most common suspension upgrade made by Camaro owners. They are a simple and relatively inexpensive way to modestly improve suspension performance and dramatically improve the looks of your car. However, there is not much solid technical information out there. The decision often gets made based strictly on ride height or what more people recommend in a particular thread. In this post I am going to provide enough information to help you make a more informed decision but, hopefully, without overloading you with so much that it makes the decision harder. Fortunately, this is a pretty straightforward affair for 5th Generation Camaro owners as it doesn't require much in the way of supporting modifications.
To Lower Or Not To Lower
The allure of lowering springs is obviously to improve performance and aesthetics as stated above. On the performance side of the equation a good set of lowering springs can improve steering response, cornering stability, body roll, traction, and even has aerodynamic advantages. I don't think I need to explain why performance cars look better lowered. They just do. It is the way the car gods intended for them to be, plain and simple.
But like all modifications there are tradeoffs that have to be made. A lower ride height means less clearance. You have to be aware of that and possibly change where and how you drive/park in some situations. Steep driveway and parking lot entrances can sometimes be a problem and you will quickly realize that you fiercely hate speed bumps. Depending on just how low you decide to go you may find that you cannot pull as far forward in parking spots because your front no longer clears the curb. Oh, and be careful about swinging that door open when parked on the street because it may not clear the curb either. The other tradeoff is that the way the car rides will change. Period. Don't let that scare you, it does not mean that the car has to ride poorly but it will be different.
Spring Rate
Spring rate refers to the amount of weight it takes to compress the spring a certain distance. The higher the rate the stiffer it is. The lower the rate the softer it is. To achieve the improvements in performance and to compensate for the reduction in ride height, lowering springs will always have a higher spring rate than stock springs. As I mentioned previously that does not mean the car has to ride poorly. Manufacturers can tune the spring rate and performance characteristics on a lowering spring in a few ways; the diameter of the wire used (thicker is stiffer, thinner is softer), the diameter of the spring's coils (the greater the diameter the softer it becomes), and the number of coils in the spring (the more coils there are the softer the spring gets). Higher spring rates also reduce body roll keeping the car flatter in the corners.
Linear and Progressive Springs
The two categories of springs commonly used in automotive applications are called linear rate springs and progressive rate springs.
Linear rate springs, also known as standard or specific rate springs, are the simplest spring design there is and are relatively inexpensive to manufacture. The spring has a very uniform design from top to bottom with the coils evenly spaced and all the same diameter. The spring is one rate from top to bottom. This has some advantages for performance at the cost of reduced ride quality. You will most often see a linear spring used on coilovers but some stand alone lowering springs do use this design or some variation of it.
On progressive rate springs the size and spacing of the coils varies. Parts of the spring will have tighter coils that are close together and others with have larger coils that are spaced out more. This creates different spring rates in different parts on the spring. The first section is relatively soft to absorb bumps and cracks in the road for a smoother ride but as the spring compresses the rate increases and it becomes more firm. Because it offers the best of both worlds progressive rate springs tend to be the most common type of lowering spring. They do cost more to make so you will pay more to buy them.
Dampers
The other piece of the equation is your dampers, more commonly referred to as shocks and struts. They tame the movement of the springs as you go down the road so that you don't bob and bounce all over the place. It is important that your springs are designed to play well with your dampers otherwise their service life and your ride quality will suffer. This is especially true on the ZL1 which is equipped with a marvel of modern engineering, Magnetic Ride Control, rather than traditional shocks and struts. The quickest way to trash them is to try to go too low. 1-1.5" is a safe drop on the stock parts and is well within the range of alignment adjustments available.