A 3 ohm coil is fine, it will get warmer than the 4 or 5 ohm one that's all.
The box will also get a little warmer.
If you actually measure the current drawn and multiply by the typical
output stage volt drop (around 1.5 volts) you will get the watts dissipated
by the box, if running at 100% dwell. So typically.........
system volts = 14.5v when running and with good battery
coil =4 ohms @ 25 degs C
Voltage = (14.5 - 1.5) =13 Volts ( system voltage minus drop across output device.)
Current max = 13v/4 ohms = 3.25A
watts dissipated by coil max = 13 x 3.25 = 42.25w
watts dissipated by output device max = 3.25 x 1.5 = 4.875w
NOTE. Switching losses in the output device ie. time to switch on and time to switch off
increase the dissipation, most people would apply a multiplier to the static figure when
this is taken into account. Assume multiplier of min 2 for Fmax of 300HZ.
Dissipation is listed as constant, this is obviously not the case, the dwell duty cycle must
be taken into account for both the output stage and the coil.
If you do the sums for 3 ohms you will see the difference as the I max will change.
Also note that the volt drop across the output device will increase as it warms up.
Plus the dc resistance of the coil will increase as it warms.
It is very important to keep ignition coils as cool as possible for best performance.
A CDI type ignition utilises the coil as a transformer not as an energy storage device
the energy is stored in a capacitor charged to around 250-450v then switched onto
the primary side of the coil, the coil does not store energy as in an inductive setup.
E core type coils tend to work well with these systems as they have more suitable
coupled characteristics, but any standard coil will work to a point.
As i have stated before, there is more to a coils design that just it's static dc resistance,
many qualities make the coil suitable or unsuitable for purpose.
Allan, you can erase this note if you like, it will probably stir up a load of grief with
many people as they all know much more about it.