Step 11: Once you've properly located the valve's closing point, reverse direction exactly 0.100-inch. You might be tempted to reverse the engine exactly 0.050-inch and record that value, but don't. An engine's tensioner will only synchronize its cams and crank appropriately when spinning forward, which means that any measurements taken when spinning backward will be inaccurate. Reversing its direction 0.100-inch (initial 0.050-inch + additional 0.050-inch) will give us an exact reference point and will make finding our next figure much easier. It'll also ensure accuracy since we'll once again change direction, rotating the crank 0.050-inch of valve movement, before arriving at our final reading.
Step 12: Once you've arrived at the 0.100-inch mark, rotate the crank once again in its normal operating direction exactly 0.050-inch. As mentioned, this additional rotation in the correct direction will once again synch up the tensioner with its cams and crank. Record the value shown on the degree wheel. This will typically occur ABDC.
Note: both of these numbers—0.100-inch and 0.050-inch—are generally accepted values to use when degreeing cams. We could have just as easily used .030-inch, even .075-inch, so long as we used identical numbers after the valve opens and before it closes.
Step 13: Take the two recorded values (open number and closed number), add them together, and add 180. The result is the cam's duration at 0.050-inch lift. Divide that figure by two. Next, subtract the opening value recorded at 0.050-inch of lift from this figure. The result is the current centerline. For example:
24° (open BTDC) + 52° (close ABDC) +180° (distance from TDC to BDC) = 256° (total duration at 0.050-inch lift) / 2 = 128°
128° – 24° = 104° centerline
Note: This formula applies only to engines with intake valves that open BTDC. Some engines feature low-overlap cams with intake valves that open ATDC.
Step 14: To set your cam to your manufacturer's recommended centerline, note the difference between your current centerline and the manufacturer's centerline and adjust the cam appropriately. Loosen the adjustable cam gear bolts and rotate the cam in the appropriate direction to achieve the correct amount of advance or retard. For example, if you've recorded a 104-degree centerline on a cam that's supposed to have a 98-degree centerline, it's clear that there's a six-degree difference. Since an engine's cams spin at half the speed of the crank, the cam must be advanced three degrees, not six. If you've recorded a 96-degree centerline, the cam should be retarded one degree, which is equal to two degrees of crankshaft rotation.
|To set your cam to your manufacturer's recommended centerline, record the difference between your current centerline and the manufacturer's centerline and adjust the cam appropriately, giving it the appropriate amount of advance or retard.|
Step 15: Tighten the adjustable cam gear bolts and repeat the process to ensure that you've achieved your cam's advertised centerline. If you haven't, start over.
Step 16: If you've made any sort of adjustments, be sure to re-mark your adjustable cam gears to reflect their new “0” reference points. For example, if you've retarded your intake cam two degrees, then “–2” degrees is your new “0.”
Step 17: Repeat the process for the exhaust cam using your cam card's exhaust centerline specifications. Although the math is the same, keep in mind that the exhaust valves open BBDC and close ATDC.
Note: This formula applies only to engines with exhaust valves that open BBDC. Some engines feature low-overlap cams with exhaust valves that open BTDC.
|Most forced induction and high-output naturally aspirated engines will respond positively to small overlap adjustments, even after being degreed. Even so, cam degreeing allows for a sound baseline in terms of centerline position, essentially speeding up the tuning process.|
Step 18: Although degreeing ensures that your cam centerlines are in their recommended locations, that doesn't always mean that there isn't room for improvement. Most forced induction and high-output naturally aspirated engines will respond well to incremental overlap adjustments. Once degreed, and with the appropriate valve clearance information, more power can often be found with the help of a dyno.