cam degree question
- Thread starter bwell
- Start date
Post the cam card numbers as written. Or take a picture of the card even better.
BubbletopMan
Well Known Member
bwell, if you advance your cam 4 degrees, it will shorten your intake centerline by 4 degrees (104 degrees), and lengthen your exhaust centerline by 4 degrees (112 degrees).
Last edited:
Skip FIx
Well Known Member
Advance 4degrees open at 16 close at 48. Another good thing to check especially since some cams are asymmetrical is find max lift and then go 0.050 down ech side to find the centerline of that lobe. I usually do both to double check all my math and the cam was ground right.
From a Car Craft article:
"Intake-Centerline MethodThe intake-centerline method does not require any special setup, but it does require some simple math, so go find your calculator. Begin by rotating the engine clockwise until the dial indicator reads max lift. Zero the dial indicator and rotate the engine counterclockwise until the dial indicator reads roughly 0.100 inch down from max lift. Slowly rotate the engine clockwise until the dial indicator reads 0.050 inch from max lift and record the number on the degree wheel. Next, continue to rotate the crankshaft clockwise until the dial indicator reads 0.050 inch on the closing side of the intake lobe from max lift and record that data.
With our big-block Chevy at 0.050 inch on the opening side of the lobe from max lift we recorded 62 degrees after top dead center (ATDC) and then 151 degrees ATDC at 0.050 inch from max lift on the closing side. Adding these two numbers together and dividing by two will give you the intake centerline. In our case: 62 + 151 = 213 / 2 = 106.5 degrees ATDC. The Comp Cams timing card supplied with our cam calls for an intake centerline of 106 ATDC, so we're within 11/42 degree of dead on. This told us that despite the numbers being off slightly at 0.015, the cam is installed correctly."
From a Car Craft article:
"Intake-Centerline MethodThe intake-centerline method does not require any special setup, but it does require some simple math, so go find your calculator. Begin by rotating the engine clockwise until the dial indicator reads max lift. Zero the dial indicator and rotate the engine counterclockwise until the dial indicator reads roughly 0.100 inch down from max lift. Slowly rotate the engine clockwise until the dial indicator reads 0.050 inch from max lift and record the number on the degree wheel. Next, continue to rotate the crankshaft clockwise until the dial indicator reads 0.050 inch on the closing side of the intake lobe from max lift and record that data.
With our big-block Chevy at 0.050 inch on the opening side of the lobe from max lift we recorded 62 degrees after top dead center (ATDC) and then 151 degrees ATDC at 0.050 inch from max lift on the closing side. Adding these two numbers together and dividing by two will give you the intake centerline. In our case: 62 + 151 = 213 / 2 = 106.5 degrees ATDC. The Comp Cams timing card supplied with our cam calls for an intake centerline of 106 ATDC, so we're within 11/42 degree of dead on. This told us that despite the numbers being off slightly at 0.015, the cam is installed correctly."
I'm confused. Everything checked out close to card specs when cam installed straight up. However when I advanced it 4* at the sprocket the numbers increased by 4 instead of decreasing. intake opening @ .050 went from @ 20 to 24 and intake at closing went from 52 to 55.
Just to check I retarded it 4* and opening went to 16 but the other numbers didn't make sense.
I'm pretty sure I did things correctly. Any thoughts or suggestions?
Just to check I retarded it 4* and opening went to 16 but the other numbers didn't make sense.
I'm pretty sure I did things correctly. Any thoughts or suggestions?
You need to note the "BTDC", "ABDC", etc. after the numbers. The degrees on the cam car are given in reference to top dead center and bottom dead center of the crankshaft. The intake lobe begins to open before top dead center (BTDC), which means that the number will get larger when you advance the camshaft. If the intake lobe opened after top dead center (ATDC), then that number would get smaller when the cam is advanced. Vice versa when you retard the camshaft.
The Comp Cams website has a pretty decent example image of how the event timing changes when you advance the camshaft.
For your camshaft, the timing would be:
As ground:
OPENS / CLOSES / LOBE CENTER / DURATION
Int.: 20° BTDC / 52° ABDC / 106° ATDC / 252°
Exh.: 62° BBDC / 14° ATDC / 114° BTDC / 256°
Advanced 4 degrees:
OPENS / CLOSES / LOBE CENTER / DURATION
Int.: 24° BTDC / 48° ABDC / 102° ATDC / 252°
Exh.: 66° BBDC / 10° ATDC / 118° BTDC / 256°