[Fot] roller rockers and head flow

[roadandtrack at bigpond.com]

I stand correcting......... for any given camshaft, the average valve velocity will change with a rocker ratio change. For example, if changing the rocker ratio increases valve lift to say 16mm instead of 13mm, then the average velocity increase is 16/13 or 12%.  this is because the lift occurs in the same time (cam/crank moves the same amount).
What I really should have said originally, is, the biggest useful influence over the rate of valve opening, is the camshaft, because flow testing suggests you need a big increase in rate of lift early in the cycle, not just averaged. 
I will take some measurements of cam rates on various Triumph cams and report back.
To the various folk who emailed me about lifters, I understand various sized lifters can be fitted---its just a case of boring out the 12 lifter bores (tedious job).  Cam grinders will specify the minimum cam follower diameter their cam will successfuly work on. The faster the acceleration rate of the cam, the larger the lifter diameter required. If you dont get the lifter diamter correct, the cam will edge ride. This means the lower edge of the lifter will try to dig into the cam, rather than ride up and over it.
I would be deligthed to ehar form anyone who has such cam data, or even what is the most agreeive cam people have found to work on stock lifters.
Terry O'Beirne