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2-Stroke Port Timing

Calculate precise exhaust and intake port open durations in crankshaft degrees by mathematically mapping the physical port height against stroke and rod length kinematics.

CRANKCASE GEOMETRY & CYLINDER MAPPING

Engine Stroke

Connecting Rod Length

Port Height (Drop from TDC)

Port Opens/Closes At

81.9°

Crank Degrees (ATDC / BTDC)

Total Port Duration

196.2°

Degrees of Crankshaft Rotation

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What is The Physics of PortTiming?

The technical foundation and mathematics behind the PortTimingCalc tool.

Mathematical Foundation

Crankcase Port Angle Kinematics

\theta_{port} = \arccos\left(\frac{r^2 + L^2 - R^2}{2rL}\right)

\theta_{port}

= Crankshaft angle when the piston pin perfectly aligns with the port opening (Radians).

r

= Crankshaft Throw Radius (Engine Stroke ÷ 2) (mm).

R

= Connecting Rod absolute center-to-center length (mm).

L

= Kinematic link: Distance from crank center to piston pin at the specified port drop height (mm).

Total Open Duration

Duration = (180^\circ - \theta_{degrees}) \times 2

Duration

= Total crankshaft degrees the specified port remains physically open.

Laws & Principles

The Symmetry Mandate: 2-stroke port timing is perfectly symmetrical around Bottom Dead Center (180°). If the exhaust port opens mechanically at 100° After Top Dead Center (ATDC) on the downstroke, the piston must travel 80° to reach BDC. It then travels upward, closing the port 80° later at 260° ATDC. Total open duration is identically 160° (80 * 2).
Rod Angle Geometry: Absolute port drop (millimeters measured from true TDC) does not linearly equal duration. Just like ignition timing, an engine with a long connecting rod maintains the piston higher in the bore for a longer rotational period. Moving a port 1mm up on a long-rod engine increases the duration significantly more than 1mm on a short-rod engine.

Step-by-Step Example Walkthrough

" A tuner measures the exhaust port roof drop on a 54mm stroke engine (110mm rod). Measuring straight down from true TDC, the port roof begins exactly 26.5mm low. "

1. Determine true TDC Pin Height: (Stroke 54 / 2 = 27) + Rod 110 = 137.0 mm Total.
2. Establish Kinematic L distance: 137.0 - 26.5mm Port Drop = 110.5 mm.
3. Apply Law of Cosines to find the angle at exactly 110.5mm drop: (27² + 110.5² - 110²) / (2 * 27 * 110.5).
4. Calculate Arc-Cos to obtain Radians, convert to degrees: 85.6° ATDC.
5. Calculate total downstroke travel remaining: 180° (BDC) - 85.6° = 94.4°.
6. Mirror for the upstroke: 94.4° * 2 = 188.8° Total Open Duration.

Final Result: A physical drop of 26.5mm on this specific rod/stroke geometry forces the exhaust port to open at 85.6° ATDC, resulting in an aggressive 188.8° total exhaust duration window.

Quick Answer: Why Calculate Port Timing from Distance?

Unlike a 4-stroke engine where valves are controlled by a camshaft, a 2-stroke engine uses the physical piston moving past holes (ports) in the cylinder wall to control air and exhaust flow. Tuners measure the physical distance of these holes from the top of the cylinder in millimeters. However, because of the geometry of the connecting rod pushing on the circular crankshaft, a 1-millimeter change in physical height does *not* equal a linear change in open time. You must convert that flat millimeter distance into rotational Crankshaft Degrees to understand how long the port stays open. Use the 2-Stroke Port Timing & Duration Calculator to instantly convert physical millimeter port layouts into exact rotational Degree Durations based on your engine's specific stroke and con-rod lengths.

Porting Geometry Failures

The Copycat Disaster

A scooter builder reads on a forum that raising the exhaust port to exactly "25 millimeters from Top Dead Center" produces massive top-end power. They grind their cylinder port roof exactly to 25mm. However, the forum post was for a Yamaha engine featuring a short 39.2mm stroke. The builder's scooter is a Piaggio with a longer 44mm stroke inside the same size cylinder. Because of the longer stroke and different connecting rod angles, that 25mm physical height creates a completely different degree duration on the Piaggio rotation. Instead of the intended 188° duration, they accidentally grind a massive 198° port, completely destroying all compression and ruining the cylinder block.

The Stroker Crank Correction

An experienced tuner installs a "+4mm Stroker Crankshaft" into their Yamaha Banshee to increase displacement. They know that pulling the piston 2mm further down the bore will completely change when the piston face uncovers the exhaust port. Before reassembling the engine, they enter the new 58mm stroke and the new long-rod length into the calculator, along with their existing physical port height measurements. The calculator reveals the new crankshaft has accidentally increased their exhaust duration by 6 degrees, ruining the blowdown time. Armed with this exact geometry, they mathematically calculate the spacer plate thickness needed to perfectly correct the port timing back to its stock powerband.

Typical 2-Stroke Exhaust Durations

Engine Application	Target Peak RPM	Typical Exhaust Duration	Dwell Characteristics
Trials / Rock Crawling	6,500 RPM	160° - 165°	Massive Low-End Compression
Enduro / Trail Bike	8,500 RPM	170° - 180°	Wide, Usable Mid-Range Torque
Motocross (125cc)	11,500 RPM	188° - 192°	Violent Top-End "Hit"
Shifter Kart / Road Race	13,000+ RPM	195° - 200°	Zero Low End, Maximum High-RPM HP

Advanced Note: Moving from 180° to 190° requires removing less physical cylinder material on a long-rod engine than on a short-rod engine. Always calculate the specific engine geometry before grinding.

Pro Tips for Port Modification

Do This

✓Measure from absolute Top Dead Center. When measuring port heights in millimeters, do not measure from the top edge of the cylinder block. Measure the depth from the exact position the piston crown reaches at Top Dead Center (which may be above or below the cylinder deck).
✓Calculate Blowdown simultaneously. Raising the exhaust port increases exhaust duration, but it also alters the "Blowdown" gap (the critical timing difference between when the exhaust opens and the transfers open). Always recalculate both sets of ports before committing to a roof grind.

Avoid This

✗Don't ignore base gasket thickness. A thicker or thinner paper base gasket placed under the cylinder block physically shifts the entire port layout up or down relative to the crankshaft. Swapping a 0.5mm gasket to a 1.0mm gasket will fundamentally change your exhaust and transfer durations.
✗Don't attempt linear math. Do not assume that "1 millimeter equals 3 degrees on this engine for the whole stroke." Connecting rod kinematics mean 1mm of port change near TDC causes far more degree change than 1mm of port change near BDC. You must calculate the arc geometry.

Frequently Asked Questions

Does changing connecting rod length change port timing?

Yes. Even if you don't change the stroke or the cylinder ports, installing a longer connecting rod alters the "dwell" time of the piston near Top Dead Center. This change in rod-angle sweeps the piston past the port at a slightly different rotational degree, modifying the final duration open-time.

Why use degrees instead of just millimeter heights?

Millimeters cannot be easily transferred to other engines. A 28mm port height on a 50cc moped behaves radically differently than a 28mm port height on a 300cc dirt bike due to differing stroke lengths. Rotational Degrees are the universal engine language; 190° behaves like 190° regardless of engine size.

What is "Blowdown" timing?

Blowdown is the critical period (in degrees) from when the Exhaust port opens to when the Transfer ports open. This specific window is where the high-pressure burning exhaust must escape the cylinder *before* the fresh fuel charge arrives to replace it. Raising the exhaust roof without raising the transfers dangerously increases blowdown.

Can I lower an exhaust roof if I cut too much?

Mechanically, no. Once you grind the roof of the port higher in the cylinder, the material is gone. However, some builders temporarily "repair" an overly raised exhaust port by having the cylinder base milled on a lathe, effectively dropping the entire cylinder block further down over the piston.