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As a follow up of this thread about cams I would very much like to hear all your opinions on cams, profiles, valve lift, cam wear, lifter shape, etc on the B18/20 pushrod engines.
I made some drawings on the way a lifter responds to cam shape:
The blue curved line shows the cam profile (as if measured with a micro meter). The red line show the actual profile the lifter would follow. It's clear that they differ.
When plotting graphs of lobe profile and actual valve lift you can again see that they differ. The blue and red line again correspond to lobe shape and valve action. They are basically the same lines as in the figure above, only plotted in a graph.
You can see that the actual valve action starts earlier then you would expect based on cam duration numbers. Naturally, with a large vale lash this effect gets smaller. With an extrem valve lash it would theoretically look like this:
What one would want for a better valve action is more lift at low duration. You want the area under the curve to be as large as possible:
So to get to this valve action, would would the lobe shape look like? You would a 'fatter' start/bottom of the lobe, so that the valves open further, more quickly, like the cam in Case B below. However, when you would use the same CAM-duration, this would yield in a very large increase in actual VALVE duration. A lot more then allready was the case with the stock cam in Case A.
To eliminate this effect of strongly increased Valve duration, one could create a lobe with less overall duration, and create something like the cam below, in Case C. This lobe would be quite good imo. One could maintain a low overall duration, but the valves would open quicker so the area under the curve would be increased. However, the side load on the lifter would be relative high, which would result in increased cam wear.
IMO, the only way to solve this is to create a new lifter shape: a lifter with a smaller bottom. That way you could use the cam of Case B, but not have the effect of increased valve duration.
So, what do you guys think if this?
Cheers, Ben
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P131, '65, B20B+M47. P131, '69, B20E+AW71L+LSD. (www.tinustechniek.tk)
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