When a cam grinder advertises "230 degrees of duration," that number is almost always measured at 0.050 inches of tappet lift, not at the moment the lobe first cracks the valve open. This convention exists because the ramp portions of a cam lobe — the "clearance ramps" and initial opening — barely flow any air and vary wildly between grinders. Duration at 0.050" gives a repeatable, apples-to-apples comparison.

Advertised duration vs. 0.050" duration: Advertised duration is typically measured at 0.006" lift (or sometimes when the lifter first starts to move). This includes all the slow-motion ramp area where the valve is barely off its seat. Advertised numbers are always larger — often by 50 to 70 degrees — and are essentially marketing fluff.

Rule of thumb for street vs. race:

• Stock cam: ~195–210° at 0.050"
• Mild street performance: 210–225° at 0.050"
• Aggressive street/strip: 225–245° at 0.050"
• Full race: 250°+ at 0.050"

Why 0.050" matters for airflow: Below about 0.050" lift, a valve barely flows any meaningful CFM — the port is essentially closed by the valve head shrouding the seat area. Real airflow begins once the valve is well off the seat, so duration at 0.050" correlates strongly with the amount of time the engine actually breathes.

Concrete example: A Comp Cams XE274H Chevy small-block cam has advertised duration of 274°/286° (intake/exhaust) but only 230°/236° at 0.050". That 44-degree difference is entirely opening and closing ramp — motion that moves the lifter but doesn't flow appreciable air. Two cams with identical advertised duration can have drastically different 0.050" numbers depending on ramp aggressiveness, and the one with more 0.050" duration will make more top-end power (at the cost of low-end torque and idle quality).

Simple calculation — cam "aggressiveness": Subtract 0.050" duration from advertised duration. A difference under 50° indicates an aggressive lobe with fast ramps (more area under the lift curve, more airflow for a given duration). A difference over 65° means a soft, slow-ramping lobe — often gentler on valvetrain but less efficient at making power.

This is why comparing cams by advertised duration is nearly useless — a "280-degree cam" from one grinder may behave like a "260-degree cam" from another. Always compare at 0.050".