[911GT2]

It's hard to say.

Fairly low rpm engines (I'll use low rpm pushrods like the 3800 for example) don't need very much force because it'll never have to spring back very hard so they can use a relatively low sprign constant (measured in newtons of force/m or lb/ft for you imperial freaks).

High rpm engines need a much higher k (spring constant) because if it is too low it'll suffer from valvle float, when the camshaft pushes too hard and makes it accelerate (f=ma, remember folks) too hard and actually shoot off the cam lobe and create some overlap. If the valve float is too long it could also create some major problems as far as compression and combustion go.

In the end it doesn't create much friction because the cams and the springs on new cars are microfinished for the most part.