underpower driving is harmfull for engine as well as transmission ( prticularly for clutch plates ),
knocking wont appears in cng case , then wat ? no harm at any speed & at any gear .........
its always best to drive car 1500 rmp and above, its like the demand of engine , its his choice of gears, also as per required by the reponse from the driver .
my suggestion never run underpower, its also not gud for economy , its eats more when u drive under-power , also in case of desiel engines, they are more vulnerable for underpower driving.
thats my exp so far, i hope senior members put more better light over this topic.
cheerz