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Port Timing

From $1

Table of contents

1. 1. The Variables

2. 2. The Calculations

1. 2.1. Port Distance in Relation to Crank Centre

2. 2.2. Calculating New Exhaust Timings

1. 2.2.1. Open ATDC

2. 2.2.2. Close ABDC

3. 2.3. Calculating Transfer Timings

1. 2.3.1. Open ATDC

2. 2.3.2. Closing ABDC

3. 3. Vespa KDX Port Timings1.

1. 3.1.1. Exhaust:

2. 3.1.2. Transfer:

This shows the conversion between standard KDX 200 port timing and the Vespa KDX port

timing.

The Variables

1997 KDX 200 Specs (Note: This was information given to me and I am not sure howaccurate it is)

Exhaust:

Open ATDC: 86 deg

Close ABDC: 94 deg

Total duration: 188 deg

Transfer:

Open ATDC: 117 deg

Close ABDC: 62 deg

Total duration: 125 deg

 ATDC (After Top Dead Centre)

 ABDC (After Bottom Dead Centre)

Here are the differences:

KDX 200 Stroke = 58mm

Vespa KDX Stroke = 60mm (cylinder raised 1mm to compensate)

KDX 200 Conrod Length = 109mmVespa KDX Stroke = 110mm

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The Calculations

To calculate the port timing, we can treat the problem as a triangle and use a calculator such

as Irregular Triangle Calculator to work out the new values.

Here is what the diagram below represents:

a = conrod length

 b = stroke radius (i.e. half stroke)

c = distance from port to crank centre (our reference point)

A = angle (e.g. ATDC)

Port Distance in Relation to Crank Centre

To do the calculations we first need to work out the unknown for the KDX 200, which is "c"

the distance from the port to the crank centre.

Exhaust Open ATDC: 86 deg

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A = 86 deg

a = 109mm

 b = 29mm

c = 107.11mm (calculated using Irregular Triangle Calculator )

Transfer Open ATDC: 117 deg

c = 92.72mm (calculated using Irregular Triangle Calculator )

Transfer Close ABDC: 62 deg

A = 118 deg (i.e. 180 deg - 62 deg)

c = 92.34mm (calculated using Irregular Triangle Calculator )

Then for the Vespa KDX we can adjust the parameters and calculate stuff...

Calculating New Exhaust Timings

Open ATDC

a = 110mm (Vespa conrod length)

 b = 30mm (longer stroke radius)

c = 108.11mm (new distance from crank centre: old distance +1mm)

A = 86 deg (calculated using Irregular Triangle Calculator 85.69 rounded up)

Close ABDC

The exhaust port is symetrical, so the close will be a mirror image of the open and (in

this case) 4 degrees above the horizontal.

A = 94 deg

Calculating Transfer Timings

Open ATDC

a = 110mm (Vespa conrod length)

 b = 30mm (longer stroke radius)

c = 93.72mm (new distance from crank centre: old distance +1mm)

A = 115 deg (calculated using Irregular Triangle Calculator 115.45 rounded down)

Closing ABDC

a = 110mm (Vespa conrod length)

 b = 30mm (longer stroke radius)

c = 93.34mm (new distance from crank centre: old distance +1mm)

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A = 64 deg (calculated using Irregular Triangle Calculator 116.37 rounded down -

180)

Vespa KDX Port Timings

Exhaust:

Open ATDC: 86 deg

Close ABDC: 94 deg

Total duration: 188 deg

So for the exhaust port timing the Vespa KDX is the same as standard KDX 200.

Transfer:

Open ATDC: 115 degClose ABDC: 64 deg

Total duration: 129 deg

So for the transfer port timing the Vespa KDX is 4 degrees more