Workshop geometry tool
Two-Stroke Port Timing Calculator
Convert measured port heights into crankshaft timing—or work backwards from target durations—using the exact stroke and connecting-rod geometry rather than a stroke-only approximation.
Calculated output
MeasuredTiming summary
Rod/stroke ratio —
Mean piston speed — m/s
Time per degree — µs
Exhaust opening piston speed — m/s
Live kinematics
0°–360°Piston travel vs. crank angle
Point to, focus, or tap a timing marker for exact crank angle, piston travel, and event details.
Port map
Cylinder reference
Event table
| Event | ATDC | Reference | Duration |
|---|---|---|---|
| Exhaust opens | — | — | — |
| Exhaust closes | — | — | |
| Transfers open | — | — | — |
| Transfers close | — | — |
Measurement reference matters.
Values assume the piston crown controls the top port edge and that events are symmetrical about BDC. Piston skirt-controlled inlet timing, asymmetric rotary/disc valves and power-valve motion require a different model.
Calculation method
Exact slider-crank formulas
The connecting rod changes piston position near TDC and BDC, so duration cannot be calculated accurately from stroke and port height alone. This tool uses crank radius r, rod length L, and crank angle θ.
1. Piston travel from TDC
x(θ) = r(1 − cos θ) + L − √(L² − r² sin² θ)
For measured ports, the opening angle is solved numerically where piston travel equals port-top distance minus TDC deck clearance.
2. Symmetrical duration
Duration = 360° − 2θopen
The same top edge closes at 360° − θopen when the piston rises, producing timing symmetrical around 180° BDC.
3. Target height
θopen = (360° − Duration) ÷ 2
The slider-crank position at that angle gives piston travel. TDC deck clearance is then added to report the required measurement from the cylinder deck.
4. Blowdown and time
Blowdown = θtransfer − θexhaust
Milliseconds = blowdown degrees ÷ (6 × rpm). This is the one-way interval between exhaust opening and transfer opening.
Workshop guidance
Port timing FAQ
Where do I measure port height?
Measure parallel to the bore axis from the cylinder deck to the highest point of the port roof. Use the same deck reference for exhaust and transfer measurements.
What is TDC deck clearance?
It is the piston-crown position relative to the cylinder deck at true TDC. Enter a positive value when the crown is below the deck and a negative value when it protrudes above it.
Why is connecting-rod length required?
Rod length changes piston motion at a given crank angle. Two engines with the same stroke and port height can have different timing if their rod lengths differ.
Does bore change the timing result?
No. Bore is recorded to identify the engine and report its geometry, but geometric opening angles depend on stroke, rod length, deck clearance and port-top position. Bore becomes important in area and time-area calculations, which are outside this tool.
What does blowdown mean here?
It is the crank angle from exhaust opening to transfer opening on the downstroke. The calculator also converts that interval to milliseconds at the selected RPM.
Does this calculate time-area?
No. Time-area also needs the effective port width, port shape, discharge coefficient, cylinder volume and intended output. This tool calculates geometric event timing only.
Can I use it for a power-valve cylinder?
Use the effective exhaust roof position for the selected valve state. The tool does not model valve travel or a changing exhaust roof automatically.
Does it handle piston-controlled inlet timing?
No. Inlet ports controlled by the piston skirt require skirt geometry and a different reference. Rotary and reed-valve inlet systems are also outside this model.
Are target heights safe machining dimensions?
Not automatically. Check ring travel, bridge support, chamfering, cylinder-wall thickness, casting limits, compression, pipe design and manufacturer specifications before machining.
How should I verify the result?
Find true TDC with a positive stop or dial indicator, use a degree wheel, and physically check opening and closing events. Treat calculated values as design and measurement guidance.