A little google search can show you the calculation but it is basically a matter of the overall diameter of the spring measured from wire center to wire center, the diameter of the wire, the coil material, and the number of coils.  On a basic spring, they are relatively linear and the info from bearing bob is spot on.  A 600 pound per inch spring means that if you apply 600 pounds on a spring, it will compress 1 inch, 2 inches will take an additional 600 or a total of 1200 pounds.  So, the load increases as the spring is compressed but the spring rate remains constant.  A variable rate or progressive rate spring is wound differently and therefore is not linear.  In most cases the spring rate will become larger as the spring is compressed.

If you take a look at the linear spring calculations the number of coils directly affects the spring rate.  Contrary to what your brain may tell you, as you cut coils the spring rate actually increases.  It may be shorter, but it is now stiffer.  Cutting coils to obtain a lowered ride height is generally discouraged but a 1/2 coil may not hurt too much.

Not all 600 pound springs are the same.  They may have the same stated spring rate but as wound you may find the uninstalled height varies a lot.  If you put a tall 600 pound spring in, the car will sit high.  If you put a shorter 600 pound spring in, it will sit lower.  In most cases, and there are always exceptions, you are probably better off with a slightly shorter springs using spacers if required rather than hacking coils off of a taller spring.  As long as you don't get coil bind you are good to go.

I think the setup discussed with the 6 cylinder springs was tied to also doing the perch relocation.  A shock absorber or more correctly a damper dues not add stiffness.  It changes the speed/velocity that something happens but doesn't change the spring rate.  I suspect that if you used 6 cylinder springs with the lower rate the car will sit lower but would not handle well.  On a hard left turn, the right spring would want to compress more throwing off your handling.  But, with a good anti-sway bar, who knows.  Never say never....  I'd prefer a shorter spring with a rate suitable for a V8 rather than a softer spring - unless you do the perch relocation.

What about a lower rated spring for a V8....when it gets cut shorter to adjust ride height?

Let’s say uninstall height = 15”
Installed height = 9”
Spring rate = 300#

How do ya calculate the new spring rate?

I changed a 1966 Ranchero from a six to a 302. I added Kyb shocks and a larger sway bar at the same time. It sat lower but I didn't notice any added bounce to the front end and handled good cornering.

pbrownrk wrote:

I changed a 1966 Ranchero from a six to a 302. I added Kyb shocks and a larger sway bar at the same time. It sat lower but I didn't notice any added bounce to the front end and handled good cornering.

 
Ahhhhhhhhhh-HA!!!!!!
6sal6

Yes, IMO if you're buying springs anyway get the right spring rate and ride height for your application.  Its where talking to a company like Eaton Detroit Spring, etc. can make a big difference.  They have blueprints for literally millions of springs, and decades of experience to inform your decision about what will be best for your application. 

Cutting springs is really only a reasonable course of action when you are just trying to mock things up, set ride height, etc.  Sometimes it works well enough that guys just leave them alone, but most times it's a crap shoot.