Handy Cam Formulas

Last week I found myself doing a little research on cams for a certain application. As part of that project I ran into some specs that were not always published with each cam manufacturer. I decided to make a list of some cam formulas to help fill in some of the blanks. Some of these are self evident, and others not so much. These may have been published elsewhere, but I did not copy them. The last four listed here are of my own "invention" brought about by one particular cam grinder listing some of his cams with the duration @ .053 but not the specific opening and closing at that .053 event. If my equations look a little odd, it is because it was the best I could do to make them readable with "Blogger" formatting

INTAKE DURATION = 180 + INT OPENING + INT CLOSING

EXHAUST DURATION = 180 + EXH OPENING + EXHAUST CLOSING

INTAKE LOBE CENTER = (180 + INT CLOSING minus INT OPENING) divided by 2

EXHAUST LOBE CENTER = (180 + EXH OPENING minus EXH CLOSING) divided by 2

LOBE SEPARATION ANGLE = (INT LOBE CENTER + EXH LOBE CENTER) divided by 2

INTAKE OPENING = (INT DURATION divided by 2) minus LOBE CENTER

INTAKE CLOSING = DURATION minus 180 minus INTAKE OPENING

EXHAUST CLOSING = (EXH DURATION divided by 2) minus LOBE CENTER

EXHAUST OPENING = DURATION minus 180 minus EXHAUST CLOSING

You may notice a trend here, in that the constant "180" is used in many of these formulas. That is because most cam specs are given in degrees Before Top Dead Center / After Bottom Dead Center, etc. rather than using a full 360 degree scale. For example, an intake valve may be opened at 15 degrees before the piston reaches TDC on the "power" stroke. It will remain open for the full 180 degrees of the "intake" stroke, and then close at 31 degrees after BDC of that intake stroke. Add those three figures (15 + 180 + 31) and you get the 226 degrees of duration as listed for an Andrews EV13 cam.

note: if anyone has any corrections to any of these formulas, please let me know. I am working from a 40 year old high school education, and I often wish I had a couple of those Algebra and Geometry textbooks handy to refresh my memory.