With a M90, depending on your setup you won't need a pump until somewhere around a 2.7" pulley. I ran totally out of fuel pump with the 2.7 and from 4500 to 6500 the afr's would climb from 11.5 to 13.0 no matter how much harder i ran the injectors. Lean damage is obvious and well known but too rich for a long time can do damage also, it will wipe out the rings and dilute your oil which could potentially cause bearing failure.