Stiffer valve springs do increase valve train performance. Too stiff springs will also rob horsepower and destroy components. Shimmed up springs can make parts fly faster than a jammed waste gate, when the spring bottoms out.
Two good ways to increase valve performance is to vary the pitch of a valve spring so it does not create a 'harmonic effect'. This varying of a spring's pitch keeps the shock waves from building on each other and bouncing the spring and valve.
Another way to increase valve performance is to change spring material. Most old MM valve springs were carbon steel. Reusing these springs is asking for trouble. Aside from performance issues; the chance of dropping a valve from a cracked spring becomes an issue with carbon springs. Carbon spring get brittle and will crack from age.
Chrome moly springs came to MM in the late fifties. They slowed spring tension loss and cut the chance of brittle spring fracture to almost nothing. The new AGCO springs all appear to be chrome moly. Even chrome moly springs should never be reused more than once. In use, Chrome moly springs lose tension slowly and they lose tension fast when they are used in a hot dry block engine.
Chrome silicon best valve springs are the way to get maximum performance in a spring that does not strain the valve train. Chrome silicon spring material never takes a set as long as some boob does not bottom out the spring with stiffiner shims. They tolerate heat better and the 'best' spec means they do not have fracture lines to allow a break.
Many fellows can spend an insane fortune for cam grinding to get half better performance from the valve train. The cam only opens the valve. The five buck spring in a 206, and the ten buck spring in a 403 is what pushes the valve shut.
Charlie at Moline Parts