Thread: Cam Help Please

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Hey Denny, thanks for the numbers. I have all of these. I need the intake lobe centerline.

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

Here’s the specifications from the “Chevy Power” Manual

GM Performance 10185071 Hydraulic Roller Tappet

“This hydraulic roller tappet is used on the 5.7-liter H.O. 350 ZZ3 & 4 engines. The duration at lash point in degrees (intake/exhaust) is 275/280; duration at .050" tappet lift (intake/exhaust) is 208/221; and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 474/501. Valve lash is zero/zero and lobe centerline is 112 degrees.”

Remember that the Intake Lobe Centerline is an imaginary line that passes through the camshaft rotation axis at the point of maximum lift of the intake lobe.

Intake Lobe Centerline, or the point of intake maximum lift, refers to the distance in crankshaft degrees from the cylinder's Top Dead Center point to the maximum lift point of the intake lobe. This is usually measured as degrees after Top Dead Center. This figure changes when the cam is advanced or retarded. As you advance the cam, this number will get smaller, as you are opening it fewer degrees after Top Dead Center. Retarding the cam will make this number larger, as you are opening it more degrees after Top Dead Center.

Exhaust lobe centerline, or the point of exhaust maximum lift, is typically expressed in crankshaft degrees before Top Dead Center. As you advance the cam, this number will get larger, since you are opening it more degrees before Top Dead Center. Retarding the cam will make this number smaller.

The average of the intake lobe centerline and the exhaust lobe centerline should equal your lobe separation. 112 degrees is your intake lobe centerline on this camshaft.

Hope this helps,
Glenn