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- Nov 26, 2009
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- 3,273
Everyone knows about the PW3 cam. Its probably the most popular race cam and many use it on cafe racers as well. Everyone has been happy with this cam but it has recently been noted that the ramps are very abrupt (more than others) to the point that the lifter comes slamming down so hard that it causes valve bounce (as demonstrated by Jim Comstock with his spintron). This cam is fine until you get it above 7500 RPM and then bounce can become evident depending on the springs used etc. For most people this is fine but Mike (Brooking 850) has been using a JS2 cam which is modeled after the PW3. It revs a little higher than the PW3 because of the lighter BSA lifters but Mike requested a wider margin of safety. The short story is that many months went by and a lot of blood sweat tears AND $ was spent to design longer softer ramps for this popular cam. The results are shown below. Snotzo analyzed this new cam (with BSA lifters and Beehive springs and alloy pushrods).
"I’ve run a simulation with the above as an intake cam, up to engine speeds in excess of 9000 rpm.
With your beehive spring set to have 0.040” clearance from coil bound at full lift, the BSA 1.125” radius lifter of 38 g weight, and a steel valve weighing 66 grams, there is no component separation and no bounce.
I tested the profile in both an intake and an exhaust position, and both simulations ran to 9000 plus with nothing indication a potential problem by so doing. What I cannot simulate is the bending effect of the camshaft occasioned by cams of the adjacent cylinder also adding load, but with a solid steel shaft I believe the effect would be minimal............................................
The valve spring can be set to give coil bind clearance of .080 to.090 thou, and should be perfectly satisfactory. There is a trace of bounce, but of a minute order of less than one thou’, which is a common side effect of a great many installations."
Snotzo mentions that the Beehive springs are under a lot of stress at such high RPMs but if there has not been any failures (and there hasn't), then "there is no reason that you should have a problem now.
The valve spring can be set to give coil bind clearance of .080 to.090 thou, and should be perfectly satisfactory. There is a trace of bounce, but of a minute order of less than one thou’, which is a common side effect of a great many installations."
There is concern about buckling of alloy pushrods with this elevated stress but so far they have always held up. I've been using stiffer high strength aircraft alloy for more strength.
See the cam profile comparison between the closing ramps of the PW3 and the custom JS2 below. There is quite a big difference.
"I’ve run a simulation with the above as an intake cam, up to engine speeds in excess of 9000 rpm.
With your beehive spring set to have 0.040” clearance from coil bound at full lift, the BSA 1.125” radius lifter of 38 g weight, and a steel valve weighing 66 grams, there is no component separation and no bounce.
I tested the profile in both an intake and an exhaust position, and both simulations ran to 9000 plus with nothing indication a potential problem by so doing. What I cannot simulate is the bending effect of the camshaft occasioned by cams of the adjacent cylinder also adding load, but with a solid steel shaft I believe the effect would be minimal............................................
The valve spring can be set to give coil bind clearance of .080 to.090 thou, and should be perfectly satisfactory. There is a trace of bounce, but of a minute order of less than one thou’, which is a common side effect of a great many installations."
Snotzo mentions that the Beehive springs are under a lot of stress at such high RPMs but if there has not been any failures (and there hasn't), then "there is no reason that you should have a problem now.
The valve spring can be set to give coil bind clearance of .080 to.090 thou, and should be perfectly satisfactory. There is a trace of bounce, but of a minute order of less than one thou’, which is a common side effect of a great many installations."
There is concern about buckling of alloy pushrods with this elevated stress but so far they have always held up. I've been using stiffer high strength aircraft alloy for more strength.
See the cam profile comparison between the closing ramps of the PW3 and the custom JS2 below. There is quite a big difference.
