large & efficient electro magnet

[therian]

Im building pulse motor, after construction small scale I decide to make large one, I will hand wound 2 km of thin wire to create electromagnet it a lot of wire to do manually so I want to make most efficient magnet from first time.
The question is what is more important , coil bobbin length or thickness , should my coil be tall or short and fat, I will use 1 inch diameter iron rod.

Formula from wiki
B = (N I)/L


* N is the number of turns of wire around the electromagnet
* I is the current in amperes
* L is the length of the magnetic circuit
* B is the magnetic field in teslas


show that fat coil will be more efficient (less L) but personal experiment show opposite result , long coil stop moves compass needle from 7 cm when thick coil twice shorter of same wire length stop move needle only from 4 cm away




Another question should I wind many wires together and then connect them (in series or parallel) or single wire is more efficient



maybe picture will explain better what I mean.