8 wire stepper problem

Most unipolar stepper motors can be driven as bipolar motors by not connecting the center tap. In fact, as bipolar motors tend to have higher torques, many people do exactly this. Your motor does not have a center tap on the coils, but rather brings out the coil ends and you can do what you want with them.
8-wire steppers are the hardest to decode, but it can be done with trial and error. After you've separated the two pairs of four wires (you can usually do this visually), you then have to play with getting the polarity of each set-of-four correct. To drive the motor as a unipolar, you will need to join two of the wires to make a center tap, and try that combination to see if it works.
There are some Internet guides on decoding stepper wiring. The tutorial at is one place to start. See the section on bifilar windings. Note that just because the motor has 8 wires doesn't mean it's bifilar wound. There are a LOT of conbinations out there. If possible, find the manufacturer and model printed on the motor, and do a Google search to see if you can find anything on it.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza

After reading this i've tried it. A uni-polar stepper on a Velleman K8005 card. The velleman K8005 can be switched form uni-polar to bi-polar.
Then i connected a wire on the steppermotor axis. And taped a box on the ohter end of the wire. Then loaded the box with, among other thing, marbles. To the point where the stepper could not lift it anymore.
In bi-polar it lifted about 10 % more. Not that much. It also could be a test in-accuracy.
Or do you need to double the voltage ? And can steppermotors take this higher voltage ? Without worrying about burned out steppers ?
In short, should a 12 Volt uni-polar stepper be used as a 24 Volt bi-polar stepper ?

Not that much? 10% of added torque for spending no more money on the motor or drive circuit is pretty good. If you were a design engineer working for a company you might even get a raise -- you've increased motor torque by 10% without any extra cost in materials.
And yes, you can do even better with a chopper drive.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza

Is this the increase methode you meant ? What about increasing the voltage when using a uni-polar as bi-polar ?
I was thinking, using it a bi-polar places the coils in series. This doubles the resistance, which halves the current. To get the current back to uni-polar level you could double the voltage. Then you've got a real big torque increase. I wondered if this was what you meant.
Is this safe to do ? Or will this overheat, overload, burn out the stepper ? (Ok, i think it will overheat, but i want to know from someone who knows more about it then me :-) )
Thanks in advance :-))

If you want to step the "stepper" very fast you need a power source that is "constant current''. One way to do that is to use a high voltage and a series resister to limit the current. The reason for this is that the windings have inductance and the current for each pulse take time to build up to the steady state value. A high voltage speeds this process.
Bill K7NOM