Quote:
Originally Posted by reknapp52
In the past I have always measured .050 on both sides of max lift of the intake lobe to degree my cam. Using that method this cam is in the motor at 108 ATDC. The Crane specs on the cam call for an intake opening number of 42 degrees BTDC and closing 66 degrees ABDC (measured .050 off the base circle). Using this method I get a reading of 114 ATDC for the center of the intake profile. My gut tells me to use my old method and install it at 102.
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First lets decipher the cam, Crane "logic" says F-288/4134-6 means that at least the intake lobe is 288 degrees duration at .050. 4134 references the lobe lift (multiply that by rocker ratio for gross lift) and that little -6 at the end means the cam was ground with a lobe separation of 106 degrees.
I agree with your Gut, but lets go through your math.
Intake opening 42 BTDC, Intake closing 66 ABDC. There's 180 degrees between TCD and BDC. So...42+66=108...plus180=288. that matches the cam card number for .050 duration. The center of a lobe of 288 degrees is at 144 degrees from .050 opening. BUT we need the intake centerline in reference to TDC. SO....144-42=102 ATDC. That would be the intake centerline the cam card called for.
Most of the time it's perfectly acceptable to degree a cam "over the nose", that is .050 on both sides of max lift. But there are some instances where the opening and closing sides of a camshaft are not symmetrical. In those cases the nose of the cam lobe may not be perfectly centered between the opening and closing events. Since cylinder pressure is affected by the opening and closing timing (not the point of maximum lift), it's always good practice (to me) to start with the .050 numbers. AND...if you know the .050 numbers, and your balancer happens to show all 360 degrees, and you happen to run a belt drive timing setup. You can then simply pop the intake rocker off number one cylinder, qualify the .050 opening location, and verify if (and how much) belt stretch is affecting your original work.