As a rough rule of thumb and trying to minimise the many variables involved….A coil spring is a helically wound torsion bar, that is to say that the wire/rod twists as the coil compresses or expands, this twist is what provides the spring force.
If the free length of the coil is the same, the outside coil diameter is the same, the wire/rod cross section diameter is the same and the metallurgy of the wire/rod is the same….then the wire/rod length will be longer to accommodate those two extra turns and therefore the 8 turn spring will be softer.
But of course there are other factors, such as variable spring rate springs….this effect can be achieved by a few ways, varying the heat treatment along the length of the spring, tapering the wire/rod used to make the spring, and tapering the outside coil diameter of the spring.
All of the above will only apply to new, unused springs, as springs will soften with use over time, the more compression/ expansion cycles a spring goes through over time the softer it will become.
Aren’t you glad that I simplified the whole thing for you?
Dealing with new springs the only practical way to know is get the actual factory specs for the springs in question.
Dealing with used springs, then the only practical way to know is have them tested by someone who has the equipment to test them.
Or just use the suck it and see method of putting the bloody things on the car
The latter is recommended for all of those who’ll be using the car in a relatively normal fashion… only those who are chasing precious tenths of a second off their lap times are likely to need to get all scientific about it.
"Keep calm, relax, take a deep breath, focus on the problem & PULL THE BLOODY TRIGGER"
'80 Triumph TR7.
'97 Ford Falcon Longreach 'S' ute,
'98 MG-F.
'83 Jaguar XJ6 Sovereign S3.
....?