The Camshaft Horsepower Calculator takes a current and a proposed duration at 0.050-inch lift, then returns an estimated peak power figure, an RPM shift, and an idle vacuum change.
Estimate Peak Horsepower and Powerband Shift From a Camshaft Duration Swap
This camshaft horsepower calculator compares a current and a proposed duration at 0.050-inch tappet lift and returns an estimated new peak power figure, a powerband RPM shift, an airflow support number, and an idle vacuum change. Engine builders, cam shoppers, and street/strip tuners use it to sanity-check a cam step before committing to a grind.
What to Enter From the Cam Card and the Dyno Sheet
Choose Imperial (CID, HP) or Metric (L, kW) — switching resets all four fields to that system’s defaults. Enter displacement, current peak power, and both durations in crankshaft degrees at 0.050 inch. RPM and degree outputs are unit-free; airflow reads CFM or L/s and vacuum reads inHg or kPa.
How the Duration Change Becomes a Peak Power and RPM Estimate
The tool first takes the duration delta, $\Delta D = D_{new} – D_{old}$, then applies a fixed linear multiplier of 0.35 percent of peak power per degree and a fixed 40 RPM per degree band shift:
$$P_{new} = P_{old} \times (1 + 0.0035 \, \Delta D) \qquad \Delta RPM = 40 \, \Delta D$$
The 0.35 percent per degree multiplier could not be traced to any published standard or named engineering reference, so treat the power figure as the tool’s own linear convention rather than sourced engineering.
The 40 RPM per degree figure is a more conservative version of a documented shop convention — not a standard — published by DSPORT Magazine, which puts a 10-degree duration increase at roughly 500 RPM of torque-peak and redline movement, against this tool’s 400 RPM.
Enter duration at 0.050 inch, not advertised duration: advertised figures on the same lobe can run 50 to 60 degrees higher depending on ramp design, which inflates every output on the page.
One quirk worth knowing before you read the results: displacement never enters the power math at all, it only divides into the result to produce the HP/CID (or kW/L) specific-output figure, so entering a 350 or a 454 with the same power and duration returns identical horsepower.
Both duration fields accept 150 to 300 degrees; anything outside that, or below 10 CID and 10 HP in Imperial (0.1 L and 1 kW in Metric), stops the calculation and returns the halted warning instead of a result.
Because the power model is linear with no ceiling, the extreme case — 150 degrees stepped to 300 — returns a 52.5 percent power gain that no cam swap alone produces, so the estimate is only defensible across roughly a 20 to 30 degree step, which is also where the risk card flips from Moderate to High. A zero delta returns zeros across every card, since lift, lobe separation angle, and installed centerline are not inputs here.
Reading the Powerband Shift Before You Order the Grind
How the Airflow Support and Idle Vacuum Numbers Are Derived
The airflow card multiplies the calculated power change by an induction-sizing convention — commonly quoted as 1.5 to 2.0 CFM per horsepower in carburetor and throttle-body sizing guidance, not a standard from any standards body — while the vacuum card applies a fixed 0.15 inHg per degree that, like the power multiplier, has no traceable published source:
$$Q_{support} = 1.5 \, \Delta P \qquad \Delta V = 0.15 \, \Delta D$$
Metric mode swaps in 1.2 L/s per kW, which is not a conversion of the Imperial rule (1.5 CFM/HP works out closer to 0.95 L/s per kW), so the two measurement systems will not return matching airflow figures from the same engine — the vacuum output, by contrast, is a true conversion at 3.386389 kPa per inHg.
Enter flywheel power rather than a chassis-dyno number, or the airflow support figure will understate what the induction actually has to flow. The idle character label keys off the absolute value of the new duration rather than the change, crossing into Noticeable Lope above 215 degrees, Rough/Choppy above 225, and Aggressive Race above 240, which means stepping down from 260 to 245 degrees still returns Aggressive Race even though the cam got smaller.
Idle vacuum in a real engine tracks overlap and lobe separation angle far more closely than duration alone, so a duration-only estimate can be off by several inches on a wide- or tight-LSA grind; if the projected vacuum lands anywhere near what a vacuum-assisted brake booster needs, confirm with a gauge on the running engine before driving the car, since real vacuum also varies with compression, altitude, and tune.
Input Mistakes That Skew the Duration Comparison
Comparing an intake duration in one field against an exhaust duration in the other, when cam cards list splits such as 224/230 — use intake for both, or exhaust for both.
Typing metric figures while the tool is set to Imperial, such as 5.7 for displacement, which falls under the 10 CID floor and halts the calculation.
Entering a target or advertised power figure as the current peak power, which scales every downstream output off a baseline the engine never made.
Camshaft Duration and Horsepower Questions Builders Ask
Should I use intake or exhaust duration in both fields?
Use intake duration in both fields, since intake duration is what most closely tracks where peak torque and peak power land. Mixing intake against exhaust produces a delta that reflects the cam’s split, not the size step.
Why doesn’t changing displacement change the horsepower result?
Displacement is used only for the specific output figure in HP/CID or kW/L. The power estimate scales the peak power you entered, which already reflects the engine’s size, so displacement would double-count if it also fed the multiplier.
Why is the RPM shift lower than the 500 RPM per 10 degrees rule?
This tool uses 40 RPM per degree, or 400 RPM per 10 degrees, against the 500 RPM figure DSPORT Magazine publishes. Both are conventions, and actual peak movement depends on head flow, intake, and exhaust as much as on duration.
What power figure should I enter as the baseline?
A flywheel figure corrected to SAE J1349 gives the most comparable baseline, since that standard defines the net power correction most engine and dyno data is reported against. A wheel-horsepower number will understate the airflow support estimate.
Does this apply to a turbocharged or supercharged engine?
The linear model assumes a naturally aspirated engine. Boosted engines respond differently to added duration, where extra overlap can bleed boost and hurt spool, so a duration-only estimate is unreliable for forced induction.