12-11-2020, 11:50 PM
(12-11-2020, 09:28 PM)dickie65 Wrote: The other problem's with high lift are hitting the cylinder head and valve shrouding. Both are curable at a loss of compression.
I could not use a Whatmough Hewitt head on my car without having to do major work to allow full lift and to stop the valve edge hitting the head as it lifted.
I am now using a Ricardo Replica head.
Its difficult to get high compression along with high lift and good gas transfer.
Its all to do with the infernal triangle of side valve tuning.
I have seen Side Valve Ford blocks with a sunken inlet valve and a trench from the inlet valve to the cylinder.
I suspect the top ring was deeper down the bore than on a seven to allow this.
The general rule of thumb for unblown engines is that valve lift doesn't need to exceed 25% of valve diameter. For a 29mm A7 valve that equates to 285 thou lift. You may argue that this doesn't strictly apply to a side valve with the shrouding on the rear of the valve but as Richard says, the limitations around CR and the transfer area between valve and piston are the significant.
C