Free Tool
2-Stroke Port Timing Calculator
Enter your stroke, rod length and how far down the cylinder each port starts. The calculator gives the crank angle where the port opens, its duration in degrees and the blowdown between exhaust and transfers. Switch to reverse mode to turn a target duration into a port height. The section view redraws as you type.
Port Timing Calculator
Geometry only. Real port timing also depends on port shape, chamfers, gasket thickness and the exact deck height, so check with a degree wheel before cutting metal.
How The Port Timing Calculator Works
On a two-stroke the piston is the valve. A port opens when the top edge of the piston crown drops past the top edge of the port. So port timing is a geometry problem with three inputs: the stroke, the connecting rod length and how far below top dead center the port starts.
With crank radius r (half the stroke) and rod length L, the piston's drop from top dead center at crank angle θ is d = r + L − (r cos θ + √(L² − r² sin² θ)). The calculator finds the angle where that drop equals the port height minus the crown offset. The port closes at the same angle before top dead center on the way back up, so duration = 360 − 2 × opening angle.
Reverse mode runs the same formula backwards. Opening angle is 180 minus half the duration, and the port height is the piston drop at that angle plus the crown offset.
Exhaust Duration, Transfer Duration And Blowdown
The exhaust port sits higher than the transfers, so it opens first. The crank angle between the two openings is blowdown. During blowdown the cylinder pressure drops, so when the transfers open the fresh charge flows up into the cylinder instead of exhaust being pushed down into the crankcase. A taller exhaust port gives more blowdown, which suits high rpm, at the cost of a shorter effective power stroke.
Degrees are not the whole story. Time is what matters to the gas, and time shrinks as rpm rises. That is why the calculator also shows how many milliseconds the exhaust is open at the rpm you enter. Gordon Jennings built his 1964 Cycle World pipe method on exactly that conversion: time for a crank angle equals degrees divided by 360, divided by rpm over 60. Tuners also use "time-area," port window area multiplied by open time, which Gordon P. Blair develops in Design and Simulation of Two-Stroke Engines (SAE R-161).
| Port top below deck | Opens ATDC | Duration |
|---|---|---|
| 34 mm | - | - |
| 38 mm | - | - |
| 42 mm | - | - |
| 50 mm | - | - |
On this engine a millimetre of exhaust port height moves the opening point by about 1.5 degrees and the duration by about 3 degrees. That is why tuners cut ports in tenths of a millimetre.
How To Measure Your Ports
- Get the stroke from your manual. KTM's 2024 owner's manual lists 72 mm for both the 250 and the 300.
- Get the rod length, center to center, from the service manual or measure it with the crank out.
- With the cylinder off, measure from the top gasket face of the cylinder down to the top edge of each port with a depth gauge or vernier.
- Measure where the piston crown sits relative to the deck at top dead center and enter it as the crown offset. Head gaskets and base gaskets change it.
- Check the answer with a degree wheel on the crank. A real port has a chamfered, curved top edge, so the first crack of light may come a little before the number here.
On engines with a power valve, measure with the valve fully open. The valve lowers the effective port roof at low rpm; see how a power valve works.
Port Timing And The Pipe
Port timing and the expansion chamber are tuned together. The pipe's echo has to arrive back at the port while it is still open, so a longer exhaust duration gives the pipe a wider window and usually goes with a pipe tuned for higher rpm.
Track Your Top End Hours
Ports wear the piston and rings on every stroke. KTM's 2024 owner's manual changes the piston every 15 operating hours on its 250 SX motocross race bike, and KTM's 2026 300 XC-W enduro manual says every 45. MotoMind tracks engine hours and shows the top end and every other task as upcoming, due or overdue.
FAQ
How do you calculate two-stroke port timing?
Measure the stroke, the connecting rod length (center to center) and the distance from the top of the cylinder to the top edge of the port. With the piston crown height at top dead center, the slider-crank formula gives the crank angle where the crown uncovers the port. Duration is 360 minus twice that angle.
What exhaust duration does a 250 two-stroke have?
It varies by model and tune, and owner’s manuals rarely publish it. MotoMind’s figures use 190 degrees of exhaust and 125 degrees of transfer duration as illustrative values. Measure your own cylinder and use this calculator to get the real numbers.
What is blowdown in a two-stroke?
The crank angle between the exhaust port opening and the transfer ports opening. During blowdown the cylinder pressure falls so the fresh charge can come up the transfers instead of being pushed back into the crankcase.
Does rod length change port timing?
Yes, a little. A longer rod changes how far the piston drops at a given crank angle, so the same port height gives a slightly different opening angle. Use the real rod length from your service manual or measure it.
Does raising the exhaust port add power?
Raising the port opens it earlier and lengthens duration, which tends to move power up the rev range and narrow the powerband. It also shortens the effective power stroke. A power valve lets one engine change this with rpm.
What is the KTM 250 SX bore and stroke?
66.4 mm by 72 mm (249 cc), according to KTM’s 2024 250/300 SX and XC owner’s manual. The 300 uses a 72 mm bore with the same 72 mm stroke.
Sources
- KTM 250/300 SX and XC 2024 owner's manual (bore and stroke, PDF p. 132; service schedule p. 34)
- KTM 250 XC-W, 300 EXC, 300 XC-W 2026 owner's manual (enduro service schedule, PDF p. 55)
- Gordon H. Jennings, “Tuned Exhaust for the Two-Stroke,” Cycle World, November 1964
- Gordon P. Blair, Design and Simulation of Two-Stroke Engines, SAE R-161, 1996
- Wikipedia, Two-stroke engine (port timing and exhaust port width)
Know When Your Top End Is Due
MotoMind logs engine hours and shows the top end and every other service task as upcoming, due or overdue. Available to download on iOS.
