There's probably a reason not to, but in all the four stroke motorcycle
engines I ever ported I cut the guide on the intake side flush with the port
roof. Never had a problem with intake valves getting cooled enough no matter
what I did. The guides won't last as long I suspect, but that was never
really a problem with racing engines. We did this as a first step, before we
even tried anything else to the head. You get a big bump in flow right away,
and everything you do after that can be done more conservatively. In a two
valve head the highest pressure area (measured using a probe) is always in
the area of the valve and seat, even at max lift. The best performing heads
had a gentle reduction in area all the way from carb to valve seat. I never
came up with a satisfactory reason why that would be important, but it
always worked on the dyno. The obstruction of the valve stem was usually
compensated for with light troughs next to the guide, or occassionally with
light grinding on the opposite side to create a bulge.
Of course then again I never honed guides--always reamed them--and I hear
(and read in most engine prep books) that reaming is a real no-no.
|