Chainsaw 2-Stroke Port Timing & Duration Calculator

Engineering Mechanics of 2-Stroke Port Timing

For sawbuilders, engine builders, and competition hot-saw enthusiasts, port timing modification ("cylinder porting") is the single most effective way to unlock substantial horsepower gains from a 2-stroke chainsaw engine.

In a 2-stroke engine, ports (exhaust, main transfers, boost transfers, intake) are opened and closed directly by the top and bottom edges of the piston crown as it moves up and down inside the cylinder bore. Port timing is expressed in Crankshaft Duration Degrees ($^\circ\text{BTDC}$ and $^\circ\text{ATDC}$).

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Mathematical Formula for Port Duration Degrees

The exact position of the piston relative to Top Dead Center ($TDC$) at any crank angle ($\theta$) is given by the trigonometric slider-crank equation:

Engineering Formula: x(theta) = R left(1 - costhetaright) + L left(1 - sqrt{1 - left({R}{L} sinthetaright)^2}right)

Where:

• $R$ = Crankshaft throw radius ($0.5 \times \text{Stroke}$).
• $L$ = Connecting rod center-to-center length.
• $x(\theta)$ = Distance down from TDC to the port opening roof.

From the opening angle ($\theta_{\text{open}}$ ATDC), total duration is calculated as:

Engineering Formula: {Exhaust Duration (Degrees)} = 2 × (180^circ - theta_{{open}})

Engineering Formula: {Blowdown (Degrees)} = {{Exhaust Duration} - {Transfer Duration}}{2}

Typical Port Timing Specs Across Saw Builds

Engine Build Target	Exhaust Duration	Transfer Duration	Blowdown Period	Intake Duration
Stock Work Saw	152° – 158°	116° – 120°	18° – 19°	145° – 152°
Mild Woods Port Saw	162° – 168°	122° – 124°	20° – 22°	158° – 162°
Full Race Hot Saw	172° – 180°	128° – 132°	22° – 25°	168° – 175°

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Understanding the Blowdown Period

Blowdown is the duration in crankshaft degrees between when the exhaust port roof opens and when the transfer port roofs open.

• Purpose of Blowdown: Allows high-pressure combustion exhaust gases ($60+\text{ PSI}$) to expand out through the exhaust port before the transfer ports open.
• Critical Blowdown Target: If blowdown is too short ($<17^\circ$), hot exhaust gas will backfeed down into the crankcase, heating the incoming fresh fuel-air charge and causing power loss. For woods ported saws, a blowdown period of 20° to 22° delivers maximum torque under heavy log cuts.

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Base Gasket Delete & Squish Clearance Measurement

Before grinding cylinder ports with a carbide burr, sawbuilders perform a base gasket delete to lower the entire cylinder, increasing static compression and advancing intake duration:

1. Measure Stock Squish: Place solder wire across the wrist pin axis on top of the piston crown. Bolt the cylinder down and rotate the flywheel past TDC. Measure squish thickness with a digital micrometer.
2. Safe Minimum Squish: Professional saws require a minimum squish gap of .018" to .022" (0.45mm – 0.55mm) to prevent piston-to-cylinder-roof collision at $14,000\text{ RPM}$ due to rod stretch.
3. Sealant: If eliminating the paper base gasket, seal the cylinder mating flange with a thin layer of fuel-resistant sealant like Permatex Motoseal or ThreeBond 1194.