Everything the rest of this site does with bore and stroke stops working the moment you reach a rotary. A Wankel has no pistons, no bore and no stroke — a triangular rotor orbits inside an epitrochoid housing, and its displacement is defined an entirely different way. That is why this calculator asks not for bore and stroke but for the per-rotor chamber volume and the rotor count, the two figures a rotary actually has.
How rotary displacement is defined
Each rotor face sweeps a working chamber, and the maker's stated convention counts one chamber's swept volume per rotor to keep the figure comparable to a piston engine. Multiply by how many rotors the engine has and you have the quoted displacement — nothing squared, no stroke, no π/4.
The rotary line-up by rotor count
Because displacement scales directly with rotor count, the whole family reads off one per-rotor figure:
| Engine | Per-rotor cc | Rotors | Quoted displacement |
|---|---|---|---|
| Mazda 13B | 654 | 2 | 1,308 cc |
| Mazda 20B | 654 | 3 | 1,962 cc |
| Mazda 26B (race) | 654 | 4 | 2,616 cc |
| Small single-rotor | 654 | 1 | 654 cc |
Why a 1.3 L rotary is not a 1.3 L piston engine
On paper a twin-rotor 13B and a 1.3-litre four share a displacement, but they are not performance-equivalent. A rotary completes three combustion events per rotor revolution and breathes very differently, so motorsport bodies apply an equivalency factor — often somewhere around 1.5 to 2 times — rather than a straight one-to-one comparison. The calculator reports the honest cc and flags that the comparison needs that factor.
Why they rev the way they do
With no reciprocating pistons or valvetrain to limit engine speed, the rotating assembly spins smoothly and freely to high RPM, with far less of the vibration a piston engine fights. It is a genuinely different machine, and treating its displacement with the piston formula — as many generic calculators do — simply produces a wrong number.
What about compression?
A rotary does have a compression ratio, but it is set by rotor and housing geometry, not by a piston-clearance sum, so the site's piston compression-ratio calculator does not describe it either. If you need a rotary's compression figure, take it from the engine's specification rather than trying to build it from chamber cc the way you would for a piston engine.
References & standards
- SAE J1349 — Engine power test code — the standard behind 'SAE net' crankshaft ratings.
Frequently asked questions
Why can't I use bore and stroke for a rotary?
A Wankel rotary has no reciprocating piston, bore or stroke. Its displacement is defined per rotor by the chamber the rotor sweeps, so the piston formula simply does not apply.
What is the displacement of a Mazda 13B?
1308 cc under its stated convention — 654 cc per rotor multiplied by two rotors. This calculator reproduces that as per-rotor volume times rotor count.
How is rotary displacement stated?
Usually as per-rotor chamber volume × rotor count. A twin-rotor 13B is quoted as 654 cc × 2, and a three-rotor 20B as 654 cc × 3 = 1962 cc.
How many rotors do road rotaries have?
Most have two; some race and prototype engines use three or four. Total displacement scales directly with rotor count, which is the only multiplier here.
Is a 1.3 L rotary equal to a 1.3 L piston engine?
Not for performance. Rotaries complete power events differently, and racing bodies apply an equivalency factor — often around 1.5–2× — rather than a straight one-to-one comparison.
Why do rotaries rev so freely?
They have no reciprocating pistons or valvetrain to limit RPM, so the rotating assembly spins smoothly to high speed with far less vibration than a piston engine.
What is the per-rotor volume of a 13B?
654 cc. That figure is the volume swept by one face of the rotor over a working chamber, and it is the number you enter per rotor in this tool.
Does a rotary have a compression ratio?
Yes, but it is set by rotor and housing geometry rather than by a piston clearance sum, so the piston compression-ratio calculator does not describe it either.
Why does one convention give three chambers per rotor?
Each rotor face works a chamber, so a rotor completes three power pulses per rotor revolution. The stated displacement convention counts one chamber's swept volume per rotor to stay comparable.
Can I compare a rotary to a piston engine here?
You can compare the raw cc, but the tool flags that the comparison is not performance-equivalent, because racing equivalency factors exist for exactly that reason.
What is a 26B?
A four-rotor racing engine, famously in Mazda's Le Mans winner — 654 cc × 4 = 2616 cc under the same per-rotor convention.
Does the tool need bore or stroke at all?
No. It asks only for per-rotor chamber volume and rotor count, precisely because a Wankel has neither bore nor stroke to enter.
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