Measure from the cylinder intake port opening to the filter mouth — split at the carb. Switch between reed valve and piston port equations: both use Jennings N×L = 42,000 for the post-carb stack; pre-carb uses Helmholtz (reed) or open-period wave return (piston-port). Live SVG diagrams scale as you adjust lengths.

Preset Intake type Changes pre-carb equations — presets set this automatically Intake open duration (°) Piston-port only — crank degrees the skirt uncovers the port (~120–160° stock mopeds) Target RPM

RPM to tune ram effect — pipe peak or dyno peak

L_pre — port to carb center (mm) Cylinder intake port roof to carb venturi center along centerline L_post — carb center to filter mouth (mm) Velocity stack / clamp-on filter horn — primary Jennings ram length Intake tract ID (mm) Manifold inner diameter at cylinder intake flange Carb venturi (mm) Sha / Bing / Dellorto size — post-carb pipe ID is often matched Carb body length (mm) Flange-to-flange carb body (and spacers) — scales the diagram; small path-length correction Crankcase volume (cc) For Helmholtz pre-carb calculation — ~170–210 cc on 50cc reed engines Intake air temperature (°C) Sound speed: c ≈ 331.4 + 0.606×T — warmer = faster waves

Jennings 2-stroke ram (both types): N × L_in = 42,000 → L_mm = 1,066,800 / N

Reed pre-carb — Helmholtz: f₀ = (c / 2π) √(A / (L · V))

Piston-port pre-carb — open period: L = c · θ / (12 · N) (round-trip while port is open)

Quarter-wave ref: L_λ/4 = c / (4f)

Why intake length matters on a 2-stroke

Unlike a 4-stroke with unlimited intake flow through valve overlap, a crankcase-scavenged 2-stroke has a finite charge trapped below the piston. Every cubic centimeter of mixture that escapes or fails to transfer is lost power. Exhaust tuning dominates because preserving that charge at the exhaust port is critical — but the intake tract still shapes how fast the crankcase fills, and whether a reflected pressure wave arrives in phase with intake closing.

Gordon Jennings explains this in Two-Stroke Tuner's Handbook (1973) under crankcase pumping and resonance: intake tuning on 2-strokes is subtler than exhaust tuning, but not zero. The handbook stresses experimentation — formulas get you in the ballpark, then you verify on the road or dyno.

Reed valve vs piston port — why the equations differ

Both induction types share the same post-carb ram physics (Jennings N×L), but the pre-carb tract couples to the engine differently:

Reed valve: Petals open and close with pressure difference, not a fixed crank angle. The case stays partially “soft-coupled” through the reed cage. Pre-carb length is modeled as a Helmholtz resonator — crankcase volume V plus tract area and length set the natural frequency that should land near your target pulse rate (f ≈ N/60).
Piston port: The skirt hard-opens and hard-seals the port at fixed crank degrees. There is a single open window per revolution for a pressure wave to leave and return. Pre-carb targets use that open-period round-trip (duration θ in degrees): L = c·θ/(12·N). Helmholtz is not applied — the piston wall is an acoustic hard gate, not a flexible reed.

Stock examples: Puch E50, Motobécane AV7, Minarelli V1 are piston-port; Tomos A35, Peugeot 103 are reed. Aftermarket reed kits on a piston-port cases (e.g. Polini on Puch) should use reed mode.

The carb is a partition — two tracts, not one

Jennings devotes attention to carburetor location because the carb splits the intake into two acoustically distinct sections:

Pre-carb (port → carb): Reed bikes: Helmholtz + reed dynamics. Piston-port bikes: timed open window and case fill. Too long softens throttle (extra tract volume); too short can lose mid-range coupling.
Post-carb (carb → filter): The classic ram tube / velocity stack. A rarefaction wave runs to the filter mouth and returns as compression — same idea Bud Connolly describes for ITB stacks on Challengers101, adapted to 2-stroke timing.

That is why L_post matters as much as L_pre: shortening the stack raises the RPM where the ram pulse returns (Jennings N×L rule); lengthening it biases torque lower.

Flow, pressure, and what you actually feel

Flow: Longer tracts add volume and surface area — slightly more resistance, but on mopeds the dominant feel is inertia of the air column, not carb CFM limits. Everything2Stroke forum builds (e.g. inlet tuning threads) consistently report: shorter = snappier throttle; longer = lazier response but sometimes stronger mid pull.

Pressure waves: When the intake opens (reed crack or piston uncovers the port), a rarefaction wave runs toward the filter; its reflection returns as compression. If that positive pulse aligns with intake closing, crankcase pressure is higher at transfer — more charge available. Mis-timed pulses waste less than exhaust mis-tuning but still show up as flat spots.

Spit-back at low RPM: Challengers101 notes 2-strokes can spit fuel back out the carb at low RPM when intake pulses are out of phase — a symptom of tract length and intake-timing mismatch, not jetting alone.

Formulas this calculator uses

Model	Formula	Applies to
Jennings ram (2-stroke)	N × L_inches = 42,000	Post-carb stack for both reed and piston-port (4-stroke ITB uses 84,000)
Helmholtz resonance	f₀ = (c/2π) √(A / (L·V))	Reed only — pre-carb vs crankcase volume V at f = N/60
Open-period round-trip	L = c · θ / (12 · N)	Piston-port only — total path for wave leave/return while intake is open θ°
Quarter-wave reference	L = c / (4f)	Acoustic reference only — often longer than practical moped stacks; shown for comparison
Sound speed	c ≈ 331.4 + 0.606·T_°C m/s	Intake air temperature correction for wave timing

Example: at 8,400 RPM, Jennings gives L = 42,000/8,400 = 5.0 in (127 mm) for the ram section — a value repeated in Moped Army / Jennings discussions. For a piston-port engine at the same RPM with θ = 140°, open-period total path ≈ c·140/(12·8400) ≈ 480 mm (pre + post + carb share) — use as a total-system check, not a filter-stack length alone.

Rules of thumb (real-world, not gospel)

Shorter post-carb stack → higher RPM peak, snappier throttle, less mid-range ram (Everything2Stroke / forum consensus).
Longer post-carb stack → torque bias lower in the band, softer hit, sometimes better with long intake manifolds on kitted bikes.
Keep pre-carb smooth — sharp bends and collapsible rubber boots destroy wave timing and flow more than 10 mm of length.
Match pipe and intake — a pipe that peaks at 9,500 RPM wants a shorter stack than a 6,500 RPM commuter setup.
Verify by testing — Jennings and Graham Motzing (Moped Army) both warn that displacement, port area, and reed stiffness (or intake duration on piston-port) move the optimum; cut incrementally (5–10 mm) and re-test.
Pick the right mode — reed kits on formerly piston-port cases need reed equations; stock Puch/AV7/V1 stay piston-port.

How to measure your bike

Port reference: Reed cage opening, or cylinder intake port roof on piston-port engines — use the same point every time.
Carb center: Venturi center along the intake centerline (not flange gasket face).
Carb body length: Flange-to-flange length of the carb (plus spacers if any) — can exceed 60 mm on long bodies or stacked adapters.
Filter mouth: Open end of foam/pod filter or end of velocity stack — not outer filter diameter.
Centerline path: Follow the air path, not hose outer length around bends.
Intake duration (piston-port): Measure with a degree wheel if possible; stock mopeds are often ~120–160° open.

Related tools

Carb jetting guide — intake changes need jet verification
Exhaust port timing calculator — port duration sets RPM band
Expansion chamber designer — match pipe peak to intake ram target
Exhaust porting guide — full cylinder tuning context

References & further reading

Gordon Jennings — Two-Stroke Tuner's Handbook (1973): crankcase pumping, resonance, carb location, inertia vs wave dynamics. 2strokeengine.net mirror
Everything2Stroke / forum threads — Inlet Tuning It Can Be Done and related: practical stack-length experiments, dyno anecdotes, shorter-vs-longer rules of thumb.
Bud Connolly — Challengers101 Exhaust and Intake Tuning: pressure wave principles, ram tuning, 2-stroke vs 4-stroke intake behavior.
Moped Army — Calculating intake length: N×L=42,000 derivation and Helmholtz links.

Disclaimer: Torque curve and peak-RPM shift graphics are illustrative models — not measured dyno data. Use calculated lengths as starting points; confirm with plug chop, EGT, and lap times.

2-Stroke Intake Length Tuning