Cams
Camshafts are the engine's breathing schedule, deciding when valves open, how far, and for how long.
By Anthony LimpertASE Master Automobile Technician- Larger cams move more air, increasing peak power (usually at higher RPM). Trade-off: rougher idle, less low-end torque, and possibly emissions issues.
- Duration = crank degrees the valve is off the seat past a lift point (typically 0.050" for hot-rod cams, or 0.006" advertised).
Performance cams reshape that schedule to move more air, with trade-offs every builder needs to explain honestly to customers.
Lift, Duration, and the Trade-offs
Aftermarket performance camshafts make power by increasing valve lift (how far the valve opens), duration (how long it stays open), and/or overlap (the window when intake and exhaust valves are open together). All three changes serve one purpose: improving airflow through the engine at the target RPM range. More air in and out per cycle means more power, usually concentrated at higher RPM where the extra breathing capacity gets used. Cams do not make power by reducing lift or by cooling anything, and the sound is a side effect, not the mechanism.
The trade-offs are real. A big cam's long duration and overlap wreck low-RPM cylinder filling, so the engine gives up low-end torque and idles rough, the classic lopey idle. Overlap also lets unburned mixture escape into the exhaust at low speed, which can create emissions problems and make a street car fail testing. Cam selection is always about matching the profile to the engine's intended operating range, not just buying the biggest grind in the catalog.
Duration has a precise definition worth knowing: it is measured in degrees of crankshaft rotation during which the valve is open beyond a specified lift point, not in inches or millimeters. Because the lift checkpoint matters, two conventions exist: advertised duration is typically measured at 0.006 inch of lift, while the more comparable industry standard measures at 0.050 inch. Comparing one cam's advertised number against another's 0.050-inch number is comparing apples to oranges, always compare at the same checking height.
| Spec | Definition | What it changes |
|---|---|---|
| Lift | How far the valve opens | Peak flow potential |
| Duration | Crank degrees the valve stays open past a lift checkpoint | Where the powerband lives — more duration moves power up the RPM range |
| Advertised duration | Measured at ~0.006 in lift | Flatters every cam — not comparable between brands |
| Duration @ 0.050 in | The industry comparison standard | The number to compare cams with |
| Overlap / LSA | Window both valves are open / lobe separation angle | Idle quality, low-end torque, emissions behavior |
Always compare cams at the same checking height — advertised versus 0.050-inch numbers are apples to oranges. Bigger numbers are not better; matched-to-use numbers are.
- Define the engine's real operating range and use: daily street, tow, strip. The cam must match where the engine actually lives, and this answer disqualifies most of the catalog immediately.
- Compare candidates by 0.050-inch duration and LSA, never advertised numbers. Same checking height or the comparison is meaningless.
- Check the street realities of overlap: lopey idle, weak low-end, and unburned mixture into the exhaust that can fail emissions testing. A street car in a testing state has a legal ceiling on cam size.
- Verify the supporting cast: valve spring pressure and coil-bind clearance for the lift, piston-to-valve clearance for the duration and overlap. The cam that hits a piston makes zero power.
- Confirm the tune, torque converter, and gearing match the new powerband. A big cam with a stock stall speed is a package mismatch the customer feels at every stoplight.
- Buying the biggest grind in the catalog. Long duration and overlap wreck low-RPM cylinder filling — the street car loses the bottom-end torque it uses daily to gain top-end it visits twice a month.
- Comparing one cam's advertised duration against another's 0.050-inch figure. Advertised specs are measured so early in the lift curve they flatter every cam; cross-standard comparison picks the wrong part with confidence.
- Installing a cam without checking piston-to-valve clearance and spring requirements. More lift and duration move the valves closer to the pistons at overlap — assumptions here are how engines eat their own valves on the first start.
A customer's new big cam idles rough and lost torque below 3,000 RPM, exactly as the old forum post promised power. What happened?
The cam is working as designed — for a different use. Long duration and overlap improve breathing at high RPM at the direct cost of low-speed cylinder filling: at low RPM the overlap lets charge escape and reversion pollute the intake, so torque drops and the idle lopes. The engine now makes its power where the customer rarely drives. Matching profile to operating range was the skipped step.
Why do two cams that both claim '280 degrees duration' behave completely differently?
Different checking heights. One brand's 280 is advertised duration measured near 0.006-inch lift; the other's is at 0.050. The 0.050 cam is enormously bigger in real terms — the valve spends far longer meaningfully open. Duration numbers only compare at the same checkpoint, which is why the 0.050-inch standard exists.
A customer's new big cam idles rough and lost torque below 3,000 RPM, exactly as the old forum post promised power. What happened?
Missed one? The reasoning above comes straight from the PRF exam bank, so this is the standard you will be held to.