stock b18b plug gap is .044 or 1.1mm correct..?

what difference would it make if it was gapped at .040 or up .050?

or in other words what happens when under gapped or over gapped?

thanks

 

You do not want to over gap it, it wont be able to produce strong enough spark.

If you under gap it, you need to be able to compensate for it, hence with fuel and tuning. So it might actually hurt your car too. Keep it at the optimal factory settings.