Two-stroke design tool
Expansion Chamber Designer
Enter your engine specifications to calculate a practical starting geometry, inspect the returning pressure wave, and print workshop dimensions.
Input
Engine specifications
Dynamic drawing
Live preview
Workshop sheet
Calculated dimensions
3-stage diffuserStarting geometry only—verify clearances, temperatures and dyno response.
Section lengths
- B1 — header 1
- mm
- B2 — header 2
- mm
- F1 — diffuser 1
- mm
- F2 — diffuser 2
- mm
- F3 — diffuser 3
- mm
- C — belly
- mm
- D — baffle cone
- mm
- E — stinger
- mm
Section diameters
- D1 — header
- mm
- DF1 — diffuser 1
- mm
- DF2 — diffuser 2
- mm
- D5 — belly
- mm
- D6 — stinger
- mm
Calculation method
Formulas and design logic
This calculator uses a simplified acoustic-return model and proportional cone allocation. It creates a defensible fabrication starting point, not a guaranteed final race pipe. Port shape, exhaust temperature, ignition, compression, silencer restriction and packaging still affect the result.
1. Engine displacement
Vd = π × (B ÷ 2)² × S × N ÷ 1000
B is bore in millimetres, S is stroke, and N is cylinder count. The result is total displacement in cubic centimetres.
2. Tuned acoustic length
L = Ed × c × 30 ÷ RPM
Ed is exhaust-open duration in degrees. The model uses an effective wave speed c of 520 m/s for petrol and 570 m/s for methanol.
3. Header and peak diameters
D1 = √(Vcyl ÷ 20) × 25.4 × 1.15
The belly diameter D5 is 2.5 × D1 and the stinger diameter D6 is 0.62 × D1. Intermediate diffuser diameters divide the taper evenly for the selected stage count.
4. Section allocation
B1+B2 = 0.12L · C = 0.12L · E = 0.13L
A three-stage diffuser receives 0.40L and a 0.23L baffle cone. A two-stage diffuser receives 0.36L and a 0.27L baffle cone. All sections sum to the tuned length.
5. BMEP estimate
BMEP = HP × 792,000 ÷ (Vd × RPM)
This is a design-load indicator in bar. Treat it as a comparison value rather than a measured dyno result.
6. Pressure-wave preview
θ = (2 × section length ÷ c) × RPM × 6
Travel time is converted to crank angle. The blue plot combines an estimated negative diffuser pulse and positive return pulse from the baffle region.
Workshop questions
Expansion chamber FAQ
What does an expansion chamber do?
It uses pressure waves created when the exhaust port opens. The diffuser helps produce a low-pressure wave that assists cylinder scavenging, while the baffle cone returns a positive wave intended to reduce fresh-charge loss near port closure.
How accurate are these dimensions?
They are a calculated baseline for comparison and prototyping. Final geometry normally requires accurate port maps, exhaust-gas temperature assumptions, packaging checks and dyno or track validation.
Should I choose a two-stage or three-stage diffuser?
A two-stage diffuser is simpler to fabricate and can give a broader, gentler transition. A three-stage diffuser offers finer control of taper progression and is commonly chosen for higher-specific-output designs.
Why are exhaust and transfer timings both requested?
Exhaust timing sets the simplified tuned length. Transfer timing is used to calculate the blowdown angle—half the difference between exhaust and transfer duration—and provides an important sanity check for the intended power range.
Can one chamber serve two cylinders?
The calculated header diameter is based on displacement per cylinder. Most high-performance two-stroke layouts use one tuned chamber per cylinder; shared systems require a different wave-interaction model.
Why does fuel type change tuned length?
The selector changes the assumed effective wave speed. Methanol uses a higher model value than petrol. Real wave speed depends primarily on exhaust-gas temperature, so verify the assumption for your engine.
Is the stinger diameter safety-critical?
Yes. An undersized or obstructed stinger can retain excessive heat and pressure, increasing seizure and component-damage risk. Confirm the outlet and silencer flow area before use.
What should I check before fabrication?
Confirm all diameters are internal dimensions, allow for material thickness, decide whether lengths are axial or centreline dimensions, provide mounting flexibility, and check frame, tyre, stand and ground clearance.