FOS Pipe Calc

What this tool does

A two-stroke engine has no exhaust valve — the piston uncovers a port in the cylinder wall, and the port stays open for a fixed number of crankshaft degrees. That makes the exhaust system far more than a muffler: a correctly shaped expansion chamber (a "tuned pipe") uses the engine's own pressure waves to scavenge burnt gas and shove escaping fresh mixture back into the cylinder, adding a large slice of power across a chosen rev range.

The hard part is the geometry. This calculator takes a handful of engine numbers and produces the full pipe: each cone's diameters, lengths and angles, the exhaust-port area, and a printable drawing. It also computes the flat-pattern development of every cone, so you can mark sheet metal, cut, roll and weld the real thing — or hand the numbers to a fabricator. Everything runs locally in your browser; nothing is uploaded.

How a two-stroke expansion chamber works

When the exhaust port opens, a sharp positive pressure pulse races down the pipe. The chamber is shaped to turn that single pulse into two useful reflected waves whose timing is matched to engine speed:

• The diverging diffuser cone reflects a negative (suction) wave back toward the cylinder. Arriving while the port is open, it helps pull spent gas out and draw a fresh charge up through the transfers — a strong scavenging effect.
• The converging baffle (rear cone) then reflects a positive (compression) wave. Timed to return just before the port closes, it pushes the fresh mixture that had spilled into the header back into the cylinder. This "plugging" pulse is where much of the extra power comes from.

Because both reflections travel at the speed of sound in hot exhaust gas, the whole system is tuned to a specific RPM band — the rev range where the engine comes "on the pipe." Move away from that band and the waves arrive at the wrong moment, which is why a hard-hitting two-stroke feels peaky.

The five sections

Header

The lead-in from the cylinder. Its length sets where the returning waves start their journey.

Diffuser

The diverging cone(s) that generate the scavenging suction wave. A gentler taper widens the power band; a steeper one sharpens it.

Belly

The wide, constant-diameter mid section — the chamber's volume.

Baffle

The converging rear cone that fires the plugging compression wave back at the port.

Stinger

The narrow tailpipe. Its diameter sets back-pressure and how strongly the pipe holds the pulse; its length is unimportant once the diameter is right.

Using the calculator

Open the calculator and fill in what you know about the engine. Sensible defaults are pre-filled, and the drawing updates live as you type.

What you get back

• Recommended exhaust-port area and diameter, displacement and BMEP.
• A dimensioned cross-section drawing with every diameter, length and cone half-angle, downloadable as SVG or hi-res PNG.
• A cone-development table (inner/outer radii and sweep angle for each cone) plus a CSV export for the workshop.

Building the pipe

Each cone unrolls into a flat ring segment. For every cone the calculator gives an inner radius, an outer radius and a sweep angle: scribe those two arcs on flat sheet, cut the segment, and roll it into the cone. Straight sections (the belly and stinger) roll from a simple rectangle measuring length × π·Ø. The original site distributed .con files for the Cone Layout program; since that format is no longer available, this tool gives the development data directly so any sheet-metal worker can lay it out.