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PostPosted: Sat Apr 13, 2019 4:45 am 
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Joined: Mon Oct 21, 2002 11:08 am
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Location: Blacksburg, VA
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Hi Folks,

I had a discussion over PMs about finding intake closing angle from other cam specs. Here is a procedure to do this and some reasoning:

1) Take the intake centerline (you set this by degreeing the cam), which is the degrees after top dead center (ATDC) when the valve is at its maximum opening (lift). Usually the cam companies grind this into the cam around 104 to 108 deg ATDC, but I usually set mine around 100 for a 225, which means I advance the cam using cam gear bushings (or crank gear keyways or whatever) while I am degreeing the cam. Let's say this number is 100 degrees, for example.

2) Now divide the advertised intake duration by 2. The cam card or lobe specs will have this number, and it usually means the duration at 0.020" valve lift. From the peak opening point to the valve closing, you have half of the intake duration to go until the valve closes. For example, if we have 270 advertised duration, then we have 135 degrees from the intake centerline until the valve is closed.

3) Add the two numbers above to get the closing point. 100 + 135 = 235 degrees ATDC. This is the same as 235 - 180 = 55 degrees after bottom dead center (ABDC), and this is usually the number given on the cam card.

You can also do the closing point @ 0.050" lift, and this would just be the intake centerline plus half of the duration @ 0.050". For 100 deg centerline and 220 duration @ 0.050" we would have 100 + 110 = 210 deg ATDC or 30 deg ABDC.

Hope this helps someone!


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