sounds to me like you need to pick a better engine builder...like one that knows miatas.

At the least call supertech with your cam info and they'll tell you which spring to run.

 

supertechs are 56 for the single springs and 74 for the duals (seat pressure).

eibachs are 66 for single, 75 for dual.

who knows what stock is? It's not on solomiata.

 

I am trying to understand how this all works, vs just paying my builder to do it for me and not know why i choose what.

Thanks for the suggestion, ill shoot them a call.

Does the length of springs not matter when the strength of them is being changed?

I am confused because the springs are different lengths, different strengths and i am still not grasping why the length needs/doesnt need to be changed.

I guess this brings me to the question of:
Which supertech springs would be best for a high revving, high hp track motor and why?

 

length is somewhat irrelevant. you need to know rate and force required to get it to the installed height (which will be the seat pressure in lbs--technically not pressure at all).

so in that sense, length does matter, but it's not useful information by itself.

I can tell you that soft springs will have to be longer than stiff springs in order to reach the same seat pressure (force in lbs).

the math:

Hooke's Law: F = k * x
Where: F = force in lbs, k = spring constant in lbs/inch, and x = displacement of the spring.

if you compress a spring with a constant of k=10 lbs/inch by x=1 inch, you will exert F=10 lbs of force.

if a spring has a k = 5 lb/in you will have to displace it by x=2 inches to get 10 lbs of force or conversely, if you displace it by 1 inch, you only need 5 lbs of force. (you can see why softer springs are going to be longer -- you need more displacement to get the same force out of them).

 

Flyin Miata spring sets
1.6 58lbs @ 34.10mm
1.8 56lbs @ 34.20mm

Supertech spring sets
SPRK-MM16D1- 49lbs@36.80mm
SPRK-MM18D-K 74lbs @ 34.20mm
SPRK-MM16D2- 70lbs @ 33.50mm
SPRK-MM18S-K 56lbs @ 34.20mm
SPRK-MM16S-K 58lbs @ 34.10mm


By simple comparison you can see the single valve spring kits (the lower two of Supertech) are the same as FM do. FM spring kits are good for 9000rpm and a bugger load of lift (which you dont have to use). So you I would believe they are the ones to go for but I sure y8s calculations can be applied to find out how suitable they are.

My personal part of this is the worry of HLAs coping with higher RPM as in the UK HLAs tend to die above stock fairly easy. So what do you do to fix this?

 

Nick, why do you want to spend money on springs if you're not increasing cam lift or planning on revving to 8k+? You've got that '95 head in your garage. Have the spring rate on those measured.
IIRC, mine were around 55 lbs. vs around 48 lbs for the bp4w head. That's a 13% increase. Assuming the bp4w head is good to 7250rpm from the factory, a 13% increase in spring rate will get you easily past 7500.
IMO you should be changing the valves. Backcut valves will improve flow and as a bonus, they're lighter too. Lighter = more rpm