I was looking up some info in my Helm manual and happened to open it at the "Cylinder Head , Camshaft Inspection" section and saw something that has me puzzled. I'm hoping that someone can enlighten me.

A cylinder has two intake valves that are operated by three cam lobes [Primary, Mid and Secondary] which have different profiles.

As I understand Vtec operation the "Mid" lobe [which has the highest profile by about 2.6mm ] is used for both intake valves when using Vtec and the Primary and Secondary lobes are used [one for each valve] in Non-Vtec operation.

The Primary and Secondary lobes have different profiles with the Secondary being about 0.28 mm higher. My question is why? What is Honda doing?

The physical dimensions of the two valves are identical which lead me to believe that they are placed symmetrically in the head. If they are then the Secondary valve will open further than the Primary and, perhaps -- but not necessarily--- stay open longer because of the higher profile.

Does the Secondary valve start to open first and close last? Or does the Primary start to open first followed by the Secondary? Or ......?????

Or have I missed something obvious?

The same questions also apply to the exhaust valves and cams.

 

I'm not an expert, but search on the term "swirl" and see if anything comes up. I think by opening one valve slightly earlier than the other, it causes a swirl in the chamber which apparently leads to better mixture and better combustion.