Camshaft specs explained
How to read a cam card — duration, lift, LSA and overlap, what each one does to the powerband and the idle, and why overlap is the personality number of any cam.
- Duration — how long the valve stays open, in degrees of crank rotation. More duration = more time to breathe = more power higher up.
- Lift — how far the valve opens (lobe lift × rocker ratio). More lift = more flow at every point in the cycle.
- LSA (lobe separation angle) — the spacing between the intake and exhaust lobe peaks. Together with duration it sets overlap: both valves open at once — the number that decides the idle.
Reading a cam card
A cam card is four families of numbers. Duration comes in two flavours: advertised (measured from nearly zero lift, where each brand picks its own point) and @ .050″ (measured once the valve is genuinely moving). Because advertised starting points differ between grinders, the .050 number is the honest one for comparisons.
Lift is quoted at the lobe and at the valve: valve lift = lobe lift × rocker ratio — a 8 mm lobe on 1.6 rockers gives 12.8 mm at the valve. LSA and the intake centreline place the lobes in time, and from them everything else falls out.
The 720° cycle and the four events
A four-stroke needs two full crank revolutions — 720° — for one cycle: power, exhaust, intake, compression. The cam turns at half engine speed, and its four events are timed against that 720° loop:
- Intake opens (before TDC) — starts filling the cylinder as the exhaust stroke ends.
- Intake closes (after BDC) — the big one for low-end torque: close it late and the rising piston pushes mixture back out, dropping cylinder pressure and softening the bottom end.
- Exhaust opens (before BDC) — trades a little expansion push for getting the cylinder empty cheaply.
- Exhaust closes (after TDC) — and with intake opening before TDC, there is a window around top-dead-centre where both are open at once.
Overlap: the personality number
That both-valves-open window is overlap, and it is the single best predictor of how a cam behaves. At rpm, the exiting exhaust pulse pulls the fresh charge in behind it — free scavenging that fills the cylinder better than the piston alone ever could. At idle there is no pulse to exploit: the same window lets exhaust dilute the intake and raw mixture escape, which is exactly the lope, the weak vacuum and the smell of an unburnt-fuel idle.
For a cam installed straight up, overlap ≈ average duration − 2 × LSA. Grow duration and both windows lengthen; tighten the LSA and the peaks squeeze together — both feed overlap, both move the personality the same way.
What overlap says about the cam
Advertised overlap, because it runs far wider than the .050″ figure:
| Advertised overlap | Idle | Powerband | Typical use |
|---|---|---|---|
| under 20° | smooth, strong vacuum | low-end and midrange | stock, towing, RV |
| 20° – 45° | noticeable street lope | broad, friendly street | street performance |
| 45° – 70° | lopey, weak vacuum | wakes up past ~3,000 rpm | street/strip — needs gears & converter |
| over 70° | race manners | high-rpm only | full race |
Advertised vs .050″ duration
Lobe ramp rates differ — a fast-ramp lobe and a gentle one can share an advertised number yet behave nothing alike. Measuring at .050″ of lift skips the ramps and compares the part of the cycle that actually flows air. As a rough ladder for the .050 figure on a V8: under 200° is stock/torque territory, 200–215° is street performance, 215–230° is street/strip (bring gears and a converter), 230–245° is strip-oriented, and beyond that you are racing.
Quick reference
Duration picks the rpm range, lift feeds flow, LSA shapes overlap, and overlap decides the idle. Match the cam to compression, converter and gears — then put the card through the calculator to see the events and the overlap for yourself.
FAQ
What is the difference between advertised and .050 duration?
Only the measuring point on the lobe. Advertised duration is measured from a nearly-closed lift point that each manufacturer chooses differently, so numbers are not comparable across brands. Duration at .050″ of lift skips the opening ramps and measures the part of the cycle that actually flows air — use it when comparing cams.
What does LSA do to idle and boost?
A wider LSA reduces overlap: smoother idle, stronger vacuum, broader powerband — and less exhaust backwash into the intake, which is why boosted engines usually run wider LSAs. A tighter LSA increases overlap for the same duration: more midrange punch on a naturally aspirated engine, but a rougher idle and weaker vacuum.
Do I need to check piston-to-valve clearance?
Yes, on anything beyond a mild stock-class cam — and always after milling heads, decking the block or changing head-gasket thickness. More duration, more lift and more overlap all move the valve closer to the piston around TDC. Check it with clay or a dial indicator during assembly, not with the starter motor.