Thread: Cam Help Please

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According to info on the Sallee Chevrolet discussion board, here are the timing points at 0.004" tappet lift....

ZZ4: Seat-to-seat (@.004)
I: 24 BTDC, 66 ABDC
E: 75 BBDC, 28 ATDC

With this info, we can do a little calculating........

Adding 24 plus 66 plus 180 would yield an intake duration @ 0.004" of 270 degrees. Adding 75 and 28 and 180 would yield an exhaust duration @ 0.004" of 283 degrees.

Half the intake duration (135 degrees) added to the intake opening point would put the intake centerline at 111 degrees ATDC. You have to deduct the intake opening point of 24 from 135 to get to TDC, then it's 111 degrees to the centerline.

Half the exhaust duration (141.5 degrees) deducted from the exhaust closing point would put the exhaust centerline at 113.5 degrees BTDC. You have to deduct the exhaust closing point of 28 from 141.5 to get back to TDC, then it's 113.5 degrees to the centerline.

Adding the two centerlines together and dividing by two would yield an actual lobe separation angle of 112.25 degrees.

Now that you have the centerlines, you can also figure intake and exhaust opening and closing points @ 0.050" tappet lift.

Half the intake duration @ 0.050" is 104 degrees. Deducting 104 from 111 leaves the intake valve opening after top dead center at (7 degrees ATDC). 111 degrees ATDC is the same as 69 degrees BBDC, so we'll use up 69 getting to BDC, then go another 35 degrees past BDC to establish the 0.050" intake closing point at 35 degrees ABDC. 69 plus 35 equals the half intake duration of 104, so this proves the problem.

Half the exhaust duration @ 0.050" is 110.5 degrees. 113.5 degrees BTDC is the same as 66.5 degrees BBDC, so we figure back 66.5 of the 110.5 degrees to get to BDC. Going backwards another 44 degrees gets us to the exhaust opening point at 44 degrees BBDC @ 0.050". Adding 44 and 66.5 gets us back to the 110.5 to prove the problem. Going the other way from the exhaust centerline by 110.5 degrees puts us at 3 degrees before top dead center, so the exhaust valve closes at (3) BTDC.

To summarize 0.050" timing:
Intake opens (7) ATDC. Intake closes 35 ABDC.
Exhaust opens 44 BBDC. Exhaust closes (3) BTDC