18-01-2019, 11:21 PM
Hi Alan
Worded primarily for others!
(Effects of valve clearance and follower radius ignored.)
The valves lift as soon as the cam departs from the base circle. As drawn the Williams book shows the flanks unaltered except between the new and original base circle surfaces. The two new short tangents (or steeper) from new to old base circle when extended represent a very small included angle (15 deg as drawn, and can be less, or negative!?) and hence a mighty long opening (opening in crank degrees is double(180 – cam included angle). Gives huge duration much with less than .050 lift. (Usually duration is the result of pursuit of generous opening; duration unaccompanied by significant lift is not normally an advantage, and certainly not in a road car. Too much and will barely run)
The new cam form should retain a tangent to the original nose circle, or at least to well up the flank, to avoid duration without significant lift.
For decades the Williams book was about the only info available for those outside UK Clubs. Still an interesting read but not ideal in the hands of many of todays relatively inexperienced newcomers. (It advocates Bowdenex brakes now generally dismissed. And convex big ends. Removal of rib from inlet port etc. Some time ago a whole page of ”errors” was published, and that did not include the few I had noticed!) It fosters the hot up mentality which is now not so appropriate for many cars.
Worded primarily for others!
(Effects of valve clearance and follower radius ignored.)
The valves lift as soon as the cam departs from the base circle. As drawn the Williams book shows the flanks unaltered except between the new and original base circle surfaces. The two new short tangents (or steeper) from new to old base circle when extended represent a very small included angle (15 deg as drawn, and can be less, or negative!?) and hence a mighty long opening (opening in crank degrees is double(180 – cam included angle). Gives huge duration much with less than .050 lift. (Usually duration is the result of pursuit of generous opening; duration unaccompanied by significant lift is not normally an advantage, and certainly not in a road car. Too much and will barely run)
The new cam form should retain a tangent to the original nose circle, or at least to well up the flank, to avoid duration without significant lift.
For decades the Williams book was about the only info available for those outside UK Clubs. Still an interesting read but not ideal in the hands of many of todays relatively inexperienced newcomers. (It advocates Bowdenex brakes now generally dismissed. And convex big ends. Removal of rib from inlet port etc. Some time ago a whole page of ”errors” was published, and that did not include the few I had noticed!) It fosters the hot up mentality which is now not so appropriate for many cars.