It's possible that your friend had the wrong spring rate when he bought the Eibachs for his Focus. When he cut his springs he possibly stumbled into the correct rate for more stiffness for the Focus. Something tells me he probably guessed at what spring rate he wanted to go to in order to improve the suspension by increasing the stiffness. Removing coils from a spring can increase the spring rate of a spring if it is measured in pound per inch of spring deflection. There's an equation to figure this out:
pounds per inch of spring deflection = torsional modulus of elasticity x (diameter of spring wire)to the 4th power / 8 x number of coils x (diameter of spring coil)to the 3rd power
Good luck with that. Cutting springs will require that you are going for a particular rate and that you know the beginning rate that you have to start with. You have to be very exact about how you cut the spring and about how you measure so make sure you do the same thing all the way around, this is very hard to do without the correct tools.
The other problem you have with cutting coils off a spring is that springs are designed to be closed ended so they seat properly
at the top and bottom. Cutting the spring will ruin the ability of the spring to do this properly. This can be especially critical under harsh driving conditions which can be assumed since you are trying to achieve better performance from your suspension.
The question you should ask yourself is what is it that you don't like about the ride you currently have. Is there too much body roll? Is there too much understeer? too much oversteer? Is it too soft or too harsh? Not enough lateral grip?
Start with a problem then do things to fix that problem. Cutting the springs is almost never the answer to that problem.
It's very possible that whatever the problem is can be solved by playing with tire pressure or changing the tires to a different compound or design. Or maybe by adding or removing a sway or strut bar. Or adding some camber adjustment. Or possible changing the springs to a different spring rate or changing the shocks. But changing springs is better done by buying a set of springs with a specific rate that is manufatured to strict tolerances that is designed and built to properly fit in your vehicle. Or getting some sort of adjustable set up like the ground controls which allow you to play with the rates.
If you're just going for "look" then cut away! Just don't expect better performance. Remember that MP3 suspensions have been dialed in pretty good from the factory and the shock/spring combo was evaluated by factory engineers to work properly and with higher performance.
