no you loose strength most of the time... Helical gears have 2 or more teeth in mesh at any one time where as st8 cut only has 1 tooth at a time... however don't confuse a quafe st8 cut kit as being weak, it won't be - the material the gears are made out of and the hardness of the gears will be MUCH better than std.

The advantage is helical gears create a side thrust as the turn... where as st8 cut do not.

alex