30 degree valve seats and 3 angle valve grid

[Don Malling <dmallin@attglobal.net>]

David Vizard and others on these lists recommend 30 degree valve seats
rather than 45 degree valve seats because they improve flow at low valve
lift. Rimflow and/or back cutting the valves also help during low lift. 

On page 207 of David Vizards "Tuning the A series Engine", he points out
that a single angle 30 degree valve seat reduces the port diameter as
compared to that of a single angle 45 degree seat. So while the single
angle 30 degree valve seat increases flow at low lift, the resulting
narrower port reduces flow at high lift. He then points out that a 15
degree sharp angle has the same flow characteristics as a smooth radius
bend. He then adds multiple 15 degree angles to the 30 degree valve seat
such that the port diameter approximates that of a single angle 45
degree valve seat.

Is this what a "3 angle valve grind" is, or is it something else??? 

Thanks,

Don Malling