The Cc To Hp Calculator estimates horsepower from engine displacement by converting cubic centimetres into intake airflow demand at a stated volumetric efficiency and engine speed.
Convert Engine Displacement to Estimated Horsepower Using Intake Airflow and RPM
This Cc To Hp Calculator estimates crank horsepower from engine displacement in cubic centimetres by routing the calculation through intake airflow demand, volumetric efficiency and target engine speed, and reverses the same chain to size displacement for a horsepower target. Engine builders, small-engine and powersports buyers, and anyone comparing swap candidates by specific output use it as a sizing check.
Setting Conversion Direction, Engine Technology, Displacement and RPM
Choose CC to Horsepower or Horsepower to CC, then pick one of four Engine Technology profiles: 4-stroke or 2-stroke, naturally aspirated or turbo/supercharged. The third field switches between Engine Displacement in CC and Target Power Output in HP. Enter Target Engine Speed in RPM. Displacement is entered metric; the airflow stage runs in imperial CFM and cubic inches.
How Displacement and RPM Become Intake Airflow in CFM
Displacement in CC is first converted to cubic inches at 16.387064 CC per cubic inch, then fed into the standard engine airflow equation published in Donaldson’s engine airflow calculation reference for air cleaner sizing, which gives separate divisors for four-cycle and two-cycle engines:
$$CFM = \frac{CID \times RPM \times VE}{1728 \times C}$$
Here $C = 2$ for a four-stroke and $C = 1$ for a two-stroke, so the combined constant is 3456 and 1728 respectively. The 1728 converts cubic inches to cubic feet; the second factor accounts for a four-stroke inducting a fresh charge only every other crankshaft revolution. The default case — 1000 CC, 6000 RPM, 4-stroke naturally aspirated at 85% VE — converts to 61.02 CID and returns 90.05 CFM, shown alongside 2,550 L/min at 28.3168 litres per cubic foot.
The common mistake here is entering redline rather than the RPM where peak power actually occurs; airflow scales linearly with RPM, so a 1,000 RPM overstatement inflates every downstream figure by the same proportion.
RPM is accepted from 100 upward and displacement from 0.1 CC, but the validation only rejects zero, negative and non-numeric entries — anything above zero calculates. Below roughly 1,000 RPM the linear airflow model loses meaning, because a real engine’s VE collapses at idle speeds rather than holding the fixed profile value the tool applies at every RPM.
Converting Airflow to Horsepower and Reversing to Required Displacement
The second stage divides airflow by a CFM-per-horsepower factor. This is an industry convention, not a standard — no SAE, EPA or DIN document defines a displacement-to-horsepower conversion, and none exists to cite.
The convention is documented in engine-building practice as roughly 1.5 to 1.6 CFM per horsepower for a naturally aspirated gasoline engine, tightening toward about 1.3 under forced induction. This tool applies 1.45 for a 4-stroke NA engine, 1.35 for 4-stroke boosted, 1.55 for 2-stroke NA and 1.45 for 2-stroke boosted:
$$HP = \frac{CFM}{CFM_{per\ HP}}$$
At the default inputs that returns 62.11 HP. Horsepower to CC runs the identical chain backwards: $CFM = HP \times CFM_{per\ HP}$, then displacement is solved from the airflow equation. Because the direction switch changes only the field label and unit badge, the number already typed stays in place — 1000 entered as CC becomes a 1000 HP target the moment the mode flips, which is the single most common input mistake on this tool.
The interaction worth understanding is what RPM does to the Estimated Torque card. Torque is derived from the result as $T = HP \times 5252 / RPM$, and in CC to HP mode horsepower itself rises linearly with RPM — so the two cancel and the torque figure stays fixed at 54.36 lb-ft regardless of whether you enter 3,000 or 9,000 RPM.
In Horsepower to CC mode the cancellation disappears, because horsepower is now your input rather than an RPM-dependent output, so raising RPM shrinks both required displacement and reported torque.
Torque is also converted at 1.355818 Nm and 0.138255 kgf·m per lb-ft, with power shown at 0.745699872 kW, 1.013869 PS and 2544.43 BTU/hr per HP. The BTU/hr figure is a mechanical power equivalent, not the engine’s heat rejection.
Where Displacement Feeds Into Airflow and Airflow Into Power
Published Volumetric Efficiency Values by Engine Type
Donaldson’s engine airflow reference lists these VE values for air cleaner sizing where a manufacturer figure is unavailable. The tool’s own profile presets do not match them: it applies 0.85 for a 4-stroke NA engine, above the gasoline range below, and 1.30 for 4-stroke boosted, below the turbo range.
| Engine type | Volumetric efficiency |
|---|---|
| 4-cycle gas, naturally aspirated | 0.70 – 0.80 |
| 2- and 4-cycle diesel, naturally aspirated | 0.90 |
| 2- and 4-cycle diesel, turbocharged | 1.50 – 3.00 |
Input Mistakes That Distort the Displacement or Power Estimate
Entering displacement in litres rather than CC — 1.6 instead of 1600 — which the field accepts without complaint and which understates output by a factor of a thousand.
Selecting a turbo/supercharged profile to model a planned boost upgrade: the profile only swaps the VE and CFM-per-HP constants and has no boost pressure input, so it cannot represent a specific PSI or bar target.
Entering a wheel horsepower figure as the Target Power Output in Horsepower to CC mode, which undersizes the required displacement because the airflow convention is referenced to crank power.
Questions About Estimating Horsepower from Engine CC
Why does the same CC give different horsepower per engine type?
Each profile carries its own volumetric efficiency and CFM-per-horsepower constants. Switching from 4-stroke NA to 4-stroke boosted changes both at once, so displacement and RPM held constant, the estimate rises by roughly 64%.
Is the output crank or wheel horsepower?
Crank. The airflow-to-power convention is referenced to engine output before the transmission, and no drivetrain loss factor is applied anywhere in the calculation.
Why does a 2-stroke show more power at the same displacement?
A two-stroke inducts a charge every crankshaft revolution rather than every second one, so the airflow divisor is 1728 instead of 3456 — doubling calculated CFM even though the profile’s VE is lower.
Can I enter my own volumetric efficiency?
No. VE is fixed by the Engine Technology dropdown and displayed on the Intake Airflow card. A build with known dyno-measured VE outside those four presets will not be represented accurately.
Why is the torque figure unchanged when I adjust RPM?
In CC to Horsepower mode, horsepower rises proportionally with RPM while torque is derived by dividing by RPM, so the two effects cancel. Torque only responds to RPM in Horsepower to CC mode.