FWIW, when I first installed my Godspeed kit, I was trying to stay "legal" for stock class autocrossing. My rule set allows for "any" replacement shocks, but stock springs and ride height must be retained. So, my plan was to just use the adjustable shocks to improve the suspension, since nobody makes performance shocks for a Mirage.
I had to have some custom spring perches made to accomplish this, and it sort of worked, but not really. The problem is that the stroke of the Godspeed shocks is a LOT shorter than the stock shocks. They are designed for a shorter spring and a LOT less suspension travel.
I have some numbers here somewhere...
OE Ride Height: 14-1/8" from center of hub to fender lip. Front and rear.
OE Front Strut Length: Installed static length = 17.5"
OE Front Spring Diameter: 2.5" tapered diameter at each end (fatter in the middle)
OE Front Spring Length: Installed static length = 8"
OE Front Spring Length: Installed fully extended length = 10.5"
OE Front Strut Stroke: 5.5"
GodSpeed Front Strut Stroke: 4.5"
GodSpeed Front Bumpstop: 1-5/8" very stiff, but progressive. 1.5" OD, .75" ID.
OE Rear Shock Length: Installed static length = 22.5"
OE Rear Shock Stroke: 8"
GodSpeed Rear Shock Stroke: 5"
Just look at the front. The stock spring setup has 2.5" of extension travel. And the Godspeed shock has only 4.5" of stroke. That means 2" of compression travel.
What that means is that if you use the stock spring rates, and set it up with only 2" of compression travel rather than the stock 3", you're going to be pounding the bump stops A LOT, and ride quality is going to be awful.
If you adjust things so that you have 3" of compression travel, then you only have 1.5" of extension travel, and you'll "top out" the shock and unload the front tire. (I have photos AND video of my Mirage lifting a front tire under hard cornering... it's not a good idea)
In short, I wouldn't try to fit custom taller and/or stock rate springs to the Godspeed kit.
The Godspeed springs really aren't that stiff only 4k/3k... about half of what you'd find on a typical early Miata coilover kit, and the vehicle weight is about the same. If you want a comfortable ride, just set the Godspeed kit up toward the top of it's height range. Be sure the shock stroke is near the middle of its travel at static ride height. And keep the shock settings towards the very soft end.
Simplify and add lightness.
I think you just need to play with the adjustment range of what you have. It's very possible that you've just got it set up "wrong". (there are a lot of ways to do it wrong)
If you've maxed out the spring perch height to get max ride height, you're probably set to where you have no droop travel, and all compression travel. You might think that would give you a cushy ride over bumps, plenty of travel to take up a bump. But, topping out a shock can be just as harsh as bottoming out a shock. The suspension is designed to go both ways, up and down. If it's not doing that, it causes all kinds of problems with both ride and handling.
Think about what kind of weirdness you're going to feel if the rear shock is fully extended, and you start to turn the car. The outside suspension will compress, the inside suspension will try to extend... but it can't. The car will be pulling up on those inside wheels. Hit a bump during that process, and it gets even weirder.
Preloading the springs too much does weird things, too. if you've cranked the spring perch up so tight that you have a couple hundred pounds of preload on the spring... then the first couple hundred pounds of weight transfer (whether its from cornering or hitting a bump) is JUST overcoming that preload... the suspension doesn't move at all... what you feel is... THE BUMP. The suspension doesn't absorb anything until you overcome that preload. That's good for steering response if that's what you're after. But, not so good for ride quality.
You don't have to "slam" the suspension. But, do get it into the design range of the Godspeed kit. It is very possible to set it up too tall OR too low. (mine is currently too low, but it was a compromise that I made trying to get the CG as low as possible to keep the inside front tire from lifting under extreme cornering in competition) And even at a proper ride height, there are still several ways to screw it up.
You've got 4.5" of travel available, and a 5/8" bump stop. You don't want to ride around on the bump stop that's harsh. So, get yourself about 2" off of the bump stop. Then you'll have a little less than 2" of "droop travel", and a good 2" of compression travel.
With that, set the shocks to within the softest 1/4 of their adjustment range. That's as good as it's going to get. And it shouldn't be bad.
You CAN get replacement springs. They are standard 2.25" ID race springs. You can get pretty much anything you want. I put stiffer springs on the front of my car.
In the US, you'll usually find them sold in lb/in rather than kg/mm. 4k = 224#, 3k = 168#. You could go a LITTLE bit softer than that, but lowering a car without making the springs proportionally stiffer is a bad idea. I'd stick to within 10-20% of those rates. If you go softer, consider going an inch longer. (you could do the math and find out exactly how long a spring should be based on its rate and the weight of the car)
Speedway Motors has a pretty good selection of springs, for example:
Or, once you find out what you want, you can shop eBay and maybe find something used for a lot less money. Even Amazon carries 2.5" ID coilover springs.
But, I'd start by playing with the adjustments that are available on the car as it sits. Go back to the Godspeed instructions. Set them up exactly as they recommend. Make SMALL adjustments from there. Maybe a half inch or an inch taller (extending the shock, not messing with the spring perch height).
Simplify and add lightness.
I set the front springs at whatever compression there was already. I then set the (mount point?) at the lowest the threads existed (so all the threads were used). That was an inch too high so I moved the mount up an inch.
The rear I have set to the highest spring tension and the lowest shock point.
You mention the GodSpeed instructions. Where are those? I did not see any in the box. I just went by my whims. This is all new to me. I obviously know enough to install them, I just have no idea how to set them up.
I knew about the droop. From what I understood the shock should just limit the spring from falling out, mine does. So I figured that bit of takeup (spring preload?) once the car is set down would be my droop? As far as I know the front spring is just under tension as it sits slack, so again, wouldn't the first part of it sitting down takeup that droop I need?
Thanks again for all your help!
This is the picture of my setup before I lowered it an inch (I think)
I don't remember if my kit came with instructions or not. But, I seemed to remember that Godspeed had a decent website, and Google took me straight to it.
I didn't read through them all, but they seem to be pretty thorough. They even have a separate page about setting preload, which takes an AWFUL lot of words and photos just to say that "you want to set them up with zero preload". Enough that the springs aren't loose, but not so tight that you can't rotate them by hand.
Once preload is properly set... from there, yes, as you set the static weight of the car on the springs they will compress and the amount that they compress from fully extended to static load is your "droop travel" or "extension travel" or "rebound travel".
From there, whatever travel is left is your compression travel. Minus most of the bump stop. This kit uses a pretty hard and short bump stop, so it is a true "bump stop". As opposed to what a lot of OE setups are using these days, which is a much taller and more progressive bump stop that acts more like a "progressive rubber spring". Doing it that way allows them to have the ride quality of a nice soft spring, but the handling benefits of a stiffer suspension while cornering, with a buttery smooth transition between.
The key thing, and I'm not sure if you did this or not, is that they're flat out telling you the same thing I was saying: Don't try to adjust the ride height with the spring perches. Ride height is adjusted by extending or shortening the length of the shock body.
Simplify and add lightness.
I'm selling a pair. Checkout my post in for sale. Price drop
Trying to sell them again